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Abstract
Cardiac biomarkers can be helpful in differentiating cardiac from non-cardiac disease
in dyspnoeic patients, in detecting occult heart disease and in determining prognosis
in patients with both cardiac and some non-cardiac diseases. Cardiac troponin I and
N-terminal proB-type natriuretic peptide are the most widely used in clinical practice
and can easily be measured from a blood sample. However, there are limitations in
their use and appropriate interpretation of results is important. This article will
discuss the physiology of these biomarkers and the evidence available for their use
in dogs and cats.
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Introduction
Biological markers (biomarkers) have been defined by the Biomarkers Definitions
Working Group (2001), as “a characteristic that is objectively measured and
evaluated as an indicator of normal biological processes, pathogenic processes, or
pharmacologic responses to a therapeutic intervention”. Biomarkers are widely used
in medicine and veterinary medicine to aid in diagnosis, prognostication and
monitoring of therapeutic intervention. A good biomarker will be specific to the organ
and/or disease process and should be quantifiable, with a change in magnitude
proportional to disease severity. In theory, biomarkers can include physical
examination findings, such as heart rate and respiratory rate, as well as blood and
tissue-based markers. In this review, we will focus on cardiac biomarkers that can be
measured in blood.
Use of cardiac biomarkers in veterinary practice has only recently become
commonplace, compared to more routinely measured biochemical parameters. The
two commercially available cardiac biomarkers, and the ones which will be discussed
further in this article, are the natriuretic peptides and cardiac troponins. This article
aims to briefly describe the physiology of both of these biomarkers, and to guide the
reader on their use and interpretation based on recent literature.
Natriuretic Peptides
Physiology
The natriuretic peptides share a central 17 amino acid loop, with a variable carboxyl
terminal (C-terminal) and amine terminal (N-terminal) (Prošek and Ettinger, 2010).
There are several forms of natriuretic peptide; BNP (brain or B-type natriuretic
peptide) and ANP (atrial natriuretic peptide) are the most clinically relevant in dogs
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and cats. BNP is produced by both atrial and ventricular myocytes and its synthesis
and release is increased in response to myocardial stress caused by volume
overload, pressure overload or ischaemia (Nakagawa et al. 1995; Biondo et al. 2003;
Goetze et al. 2004). The ventricles become the main source of BNP production in
disease. Stored ANP is released from granules in the atrial myocytes when the atria
are stretched (Ledsome et al. 1985), but can also be released from the ventricles in
disease. Secretion of both ANP and BNP can also be stimulated neurohormonally by
angiotension II, endothelin and catecholamines (Erne et al. 1987; Fu et al. 1992).
CNP (C-type natriuretic peptide) is released from the endothelium and acts through
the NPR-B receptor to cause vasodilation. DNP (dendroaspis natriuretic peptide) is
released in the venom of the green mamba snake and VNP (ventricular natriuretic
peptide) is produced by primitive myocytes, mainly in fish. Urodilatin is a natriuretic
peptide produced by the distal renal tubule (Prošek and Ettinger, 2010).
The natriuretic peptides are formed as preprohormones and are processed to
prohormones. Once released, proANP and proBNP are quickly cleaved to an active
C-terminal (C-BNP, C-ANP) and inactive N-terminal (NT-proBNP, NT-proANP). The
main function of the active C-terminal natriuretic peptides is to counteract the renin-
angiotensin-aldosterone system. They generally act to decrease blood volume and
blood pressure through natriuresis, diuresis, vasodilation and direct inhibition of renin
and aldosterone. They exert these actions mainly through interaction with the NPR-A
receptor (Potter 2011). Clearance of the natriuretic peptides occurs through cleavage
by membrane-bound neutral endopeptidases, internalization and lysosomal
degradation via the NPR-C receptor, or through excretion in the urine or bile (Prošek
and Ettinger, 2010). The half-life of ANP is shorter than BNP in humans as it has a
higher affinity for the NPR-C receptor (Suga et al. 1992). The half-life of the N-
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terminal is longer than the active C-terminal, which only lasts a few minutes in
circulation, as the N-terminal depends more on excretion by the liver and kidney
(Potter 2011). In the dog, the half-life of C terminal BNP is shorter than in humans,
and is more similar to ANP (Thomas and Woods 2003). The half-life of NT-proBNP
has not been reported in dogs. Nevertheless, based on human data, NT-proBNP is
assumed to be relatively more stable and therefore, to be a more attractive
biomarker.
Natriuretic peptide assays
C-terminal ANP
Homology between feline, canine and human C-terminal ANP is high (Biondo et al.
2002; Hori et al. 2008a), and so previous veterinary studies measuring this
biomarker have successfully used a human assay (Haggstrom et al. 2000; Hori et al.
2008b). However, given that this test is not readily available, there are a limited
number of veterinary studies measuring C-terminal ANP, and results are sometimes
conflicting, it is not recommended to routinely measure this biomarker.
N-terminal proANP
Veterinary studies measuring NT-proANP using a human assay have demonstrated
its use in differentiating cardiac and respiratory disease, and in determining
prognosis of mitral valve disease (Prosek et al. 2007; Connolly et al. 2008; Eriksson
et al. 2014). However, it is not readily available to veterinary practitioners and so its
routine use is not currently recommended.
C-terminal BNP
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Studies measuring C-terminal BNP in dogs have used a canine specific
radioimmunoassay. These studies have demonstrated a possible use for C-terminal
BNP in differentiating cardiac from respiratory disease (DeFrancesco et al. 2007;
Prosek et al. 2007), in predicting mortality in myxomatous mitral valve disease
(MacDonald et al. 2003) and in detection of occult dilated cardiomyopathy (Oyama et
al. 2007). The same canine assay used in cats was of limited use clinically
(MacDonald et al. 2006). There is a commercially available assay for C-terminal BNP
available in the USA (ANTECHTM Cardio-BNP canine), but it is not currently available
in Europe.
N-terminal BNP
This is the most familiar, most widely used and most stable of the natriuretic peptide
biomarkers. First generation NT-proBNP assays required shipment of samples
frozen or in a tube containing a protease inhibitor in order to prevent degradation of
the sample. However, more recently a second-generation assay (Canine and Feline
Cardiopet® proBNP Assay IDEXX Laboratories Inc., Westbrook, Maine, USA) has
been developed which allows shipment of EDTA plasma in a plain tube at room
temperature, although shipping on ice is recommended if a delay of more than 48
hours is expected. Plain serum (without protease inhibitor or EDTA) is not suitable
for shipment at room temperature (Hezzell et al. 2015; Mainville et al. 2015). This
second-generation ELISA targets epitopes at a more stable region of the NT proBNP
molecule than the first-generation assay and has been validated for use in both dogs
(Cahill et al. 2015) and cats (Mainville et al. 2015). The canine second-generation
assay has a higher upper detection limit of 10,000 pmol/l, compared to the previous
limit of 3000 pmol/l with the first-generation assay. The feline assay has a range of
detection of 24-1500pmol/l.
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A point-of-care SNAP test for cats is available (SNAP® Feline proBNP, IDEXX
Laboratories Inc., Westbrook, Maine, USA). This ELISA SNAP test uses serum or
EDTA plasma and provides a positive or negative result within 10 minutes. The test
uses the same antibodies as the second generation quantitative ELISA and
becomes positive somewhere between 100-200pmol/l (SNAP® Feline proBNP data
sheet, Hezzell et al. 2016).
There are several limitations of the NT-proBNP assay that should be taken into
account when interpreting results. Firstly, there is large biologic variability in NT-
proBNP between individuals, as well as moderate variability within the individual
when repeated measures are taken over days to weeks (Kellihan et al. 2009; Harris
et al. 2017; Winter et al. 2017). This suggests that subject-based reference intervals
and “monitoring the trend” may be more relevant than population based intervals. A
change of 70.8% is required in a healthy dog to be considered a significant change,
and 58.2% in a dog with myxomatous mitral valve disease (Winter et al. 2017). A
change of 39.8% between days and 60.5% between weekly measurements is
required to be considered significant in the cat (Harris et al. 2017). There also
appears to be considerable interbreed variation in NT-proBNP, with healthy
Labradors and Newfoundlands having the highest concentrations, and Dachshunds
having the lowest (Sjӧstrand et al. 2014). Plasma NT-proBNP in healthy Labradors is
frequently above current laboratory reference ranges (Borgeat et al. 2017) but breed
specific reference ranges are yet to be published.
Secondly, NT-proBNP can be affected by comorbid conditions. Renal dysfunction
can result in a significant increase in NT-proBNP in dogs (Raffan et al. 2009,
Schmidt et al. 2009) and cats (Lalor et al. 2009). It was previously thought that this
was due to passive accumulation secondary to decreased renal clearance. However,
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this hypothesis has recently been challenged by a paper in dogs showing no
correlation between glomerular filtration rate and NT-proBNP (Pelander et al. 2017).
The increase in NT-proBNP seen with renal dysfunction may instead be due to
increased production. Additionally, Bijman et al 2017 found an increase in NT-
proBNP in hypertensive CKD cats but not in those with CKD but no hypertension,
suggesting that hypertension may be the cauchronic kidney disease than non-
hypertensive cats with chronic kidney diseaseHypertension and uncontrolled
hyperthyroidism can be associated with an increase in NT-proBNP in cats (Lalor et
al. 2009, Sangster et al. 2014). False positive results can also occur due to the
presence of pulmonary hypertension secondary to primary respiratory disease
having an effect on the heart (Oyama et al. 2009).
Uses of NT-proBNP in clinical practice
1.) Differentiation of cardiac from non-cardiac causes of respiratory signs
Dogs: There are several studies demonstrating the utility of NT-proBNP
measurement in diagnosing congestive heart failure (CHF) in dogs presenting with
respiratory distress. Studies using the first-generation assay identified various cut-off
values of >1158 pmol/l (Oyama et al. 2009), >1400pmol/l (Fine et al. 2008) and
>1725pmol/l (Oyama et al. 2008) as suggestive of CHF. One study by Boswood et
al. (2008) identified a cut-off value of >210pmol/l as being relatively sensitive and
specific for CHF. It is unknown why this value is so much lower than other studies.
Fox et al. (2015) have more recently performed a similar study using the second-
generation NT-proBNP ELISA. They identified an optimal cut-off of >2447pmol/l as
81.1% sensitive and 73.1% specific to discriminate between cardiac and non-cardiac
causes of respiratory distress. A lower cut-off would increase the sensitivity but at
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the expense of specificity, and a higher cut-off vice versa. The current
recommendation from IDEXX is that a value of >1800pmol/l is highly suggestive of
CHF in a patient presenting with appropriate signs (Table 1). Unfortunately, the
practical use of NT-proBNP measurement to differentiate congestive heart failure
from other causes of respiratory distress is limited in dogs, as the sample must be
sent to a laboratory and so results are not immediate. The suspected diagnosis must
therefore be made through other methods such as physical examination, thoracic
radiography and echocardiography. A value >1800pmol/l may support your
diagnosis, but usually results will not be available until treatment has started. A value
<900pmol/l makes CHF very unlikely.
Cats: The accuracy of NT-proBNP for differentiating cardiac from respiratory causes
of dyspnoea in cats appears to be even higher than for dogs. Initial studies using the
first-generation ELISA all identified similar cut-offs of > 220pmol/l (Connolly et al.
2009), >277pmol/l (Wess et al. 2008), >265pmol/l (Fox et al. 2009) and >214.3pmol/l
(Humm et al. 2013) for diagnosing CHF. A more recent study by Hezzell et al. (2016)
using the second-generation ELISA identified a cut-off of >199pmol/l to have a
sensitivity of 95.2% and specificity of 82.4% for diagnosing CHF in cats with pleural
effusion. IDEXX suggests a cut-off of >270pmol/l as suggestive of CHF in cats with
appropriate clinical signs and <100pmol/l as very unlikely in CHF (Table 1). The
point-of-care SNAP test also has good diagnostic accuracy for identifying CHF, with
a positive test having a sensitivity of 95.2% and specificity of 87.5% in cats with
pleural effusion (Hezzell et al. 2016). The point-of-care test is likely the most
practical in this situation, as results are available immediately. The point-of-care test
has a high negative predictive value and so is recommended mainly as a rule-out
test, meaning that if the result is negative then congestive heart failure is most likely
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ruled out but if it is positive then congestive heart failure could be the cause of
dyspnoea, but another cause is also possible.
Two studies have measured NT-proBNP in pleural fluid. Using the first-generation
assay, a cut off of >322.3pmol/L had 100% sensitivity and 94.4% specificity for
identifying a cardiogenic cause of pleural effusion (Humm et al. 2013). The second
study, using the second-generation assay, identified a cut-off of >240pmol/l in pleural
fluid to have a sensitivity of 100% but poorer specificity of 76.5%, suggesting that
false positive results are possible (Hezzell et al. 2016). A positive result on the point-
of-care SNAP test using pleural fluid had good sensitivity for detection of CHF but
specificity was poor at 64.7%. Although no recommendations have been made by
the manufacturer for measurement of NT-proBNP in pleural fluid, point-of-care SNAP
testing may be a practical and quick way to rule-out CHF as a cause of pleural
effusion in patients for which blood sampling is too distressing, and thoracic
radiography and/or echocardiography is not available or possible.
2.) Detecting occult heart disease
Dog: NT-proBNP is relatively accurate for predicting echocardiographic changes
associated with occult dilated cardiomyopathy (DCM) in Doberman Pinschers, using
a cut-off of 400pmol/l (Wess et al. 2011) or 457pmol/l (Singletary et al. 2012).
However, its sensitivity is much poorer for detecting occult DCM in dogs with solely
arrhythmogenic changes. A combination of NT-proBNP and 24 hour Holter monitor
provided improved accuracy. IDEXX recommends a cut-off of >735pmol/l for
detecting occult dilated cardiomyopathy in Doberman Pinschers based on a 2013
abstract (Gordon et al. 2013) (Table 1), although a 2015 abstract by the same group
(Gordon et al 2015) suggested a cut-off of 548pmol/l. Measurement of NT-proBNP is
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not recommended to replace echocardiography and 24 hour Holter monitoring, which
is considered the gold standard for detection of occult DCM. The European Society
of Veterinary Cardiology screening guidelines for dilated cardiomyopathy in
Doberman Pinschers (2017) suggest that it may be reasonable to screen Doberman
Pinschers over the age of 3-4 years with NT-proBNP if there are financial concerns
which prevent gold standard screening. However, a positive result should be
followed up with confirmatory tests before making a diagnosis and initiating
treatment. Wess et al, 2011 found that NT-proBNP was increased in dogs 1.5 years
before development of echocardiographic or arrhythmogenic signs of DCM, but
further validation of these results would be required before recommendations can be
made.
Cat: Cut-offs of 100pmol/l in two studies using the second-generation NT-proBNP
assay resulted in relatively good sensitivity and specificity (Machen et al. 2014;
Harris et al. 2017) for detecting occult heart disease in cats with history or physical
exam findings suggestive of cardiac disease. Two studies looking at the POC test
were slightly conflicting in their results, Machen et al, 2014 finding more false
positive results (positive predictive value (PPV) of 62% and negative predictive value
(NPV) of 93.8%) and Harris et al, 2017 finding more false negative results (PPV
100%, NPV 87.1%). This may be in part because the prevalence of disease was
lower in the first study than the second (25% vs. 50%). In a general practice
population, it is likely the prevalence would be even lower, which would further
decrease the PPV but increase the NPV, therefore making false positives more likely
but false negatives less likely. The quantitative and POC assays appear to be
insensitive for mild disease, but better for moderate or severe disease (Hsu,
Kittleson and Paling, 2009; Machen et al, 2014). As the assays have only been
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investigated in cats with a suggestion of heart disease, indiscriminate testing of
apparently healthy cats, particularly younger cats, is not recommended as this will
increase the risk of a false positive.
Between 16-44% of cats have murmurs (Cote et al. 2004; Paige et al. 2009; Wagner
et al. 2010), and between 16-77% of these have occult heart disease (Paige et al.
2009; Dirven et al. 2010; Wagner et al. 2010; Nakamura et al. 2011) the rest being
physiologic or innocent murmurs. Current recommendations are that adult cats with
≥ grade III/VI murmurs, gallop rhythms or arrhythmias should undergo
echocardiography to identify occult heart disease (Cote et al. 2015). NT-proBNP may
be used to aid in compliance in these cats. In other words, NT-proBNP >100pmol/l or
a positive NT-proBNP POC assay identifies the cats at most risk of occult heart
disease and so may encourage owners to proceed with echocardiography. However,
NT-proBNP should not be used alone to rule in or out cardiac disease in these cats.
In cats with ≤ grade II murmur, echocardiography is less strongly recommended,
although is still the most sensitive way to detect occult heart disease and even cats
in heart failure may not have a murmur. NT-proBNP can be measured in these cats
and if >100pmol/l then echocardiography more strongly recommended. If NT-
proBNP is <50pmol/l then significant heart disease is less likely, but keep in mind
that heart disease can develop over time so annual re-assessment of the patient is
advised (Cote et al. 2015).
3.) Use of NT-proBNP in determining prognosis
Dogs: NT-proBNP is correlated with the severity of MMVD (Oyama et al. 2008,
Chetboul et al. 2009). A cut-off value of >1500pmol/l has been determined as an
independent risk-factor for the development of congestive heart failure within 3-6
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months (Reynolds et al. 2012) in dogs with asymptomatic MMVD and as predictive
of mortality within 6 months in dogs with MMVD and CHF (Serres et al. 2009). In two
other studies, lower values of >466pmol/l or >740pmol/l have been predictive of
progression of disease or mortality in dogs with MMVD (Chetboul et al. 2009;
Moonarmart et al. 2010). Dogs that continue to have elevated NT-proBNP
>965pmol/l 7-30 days after starting treatment for CHF have a shorter survival time
(Wolf et al. 2012). NT-proBNP >900pmol/l was predictive of all-cause mortality in
Doberman Pinschers with occult dilated cardiomyopathy (Singletary et al. 2012). A
value <2,966pmol/l has a sensitivity of 71% and specificity of 88% for detecting
resolution of CHF in dogs with MMVD, but sleeping respiratory rate remains more
sensitive and specific for this purpose (Schober 2011). NT-proBNP >900pmol/l was
predictive of all-cause mortality in Doberman Pinschers with occult dilated
cardiomyopathy (Singletary et al. 2012). Unfortunately, the above studies all used
the first-generation assay and so the cut-offs mentioned likely do not apply when
using the current, second-generation assay.
Cats: NT-proBNP also appears to be correlated with severity of cardiomyopathy in
cats (Fox et al. 2011; Machen et al. 2014). Cats with a larger reduction in NT-
proBNP from admission to discharge have a longer survival than those with a
smaller reduction, but further research is required to identify exact cut-offs and
percent decreases (Pierce et al. 2017). Other factors such as left atrial size and the
presence of CHF are likely more reliable prognostic indicators in cats.
Cardiac troponins
Physiology
The cardiac troponins are proteins that mediate the interaction of actin with myosin.
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There are three forms of cardiac troponin; cardiac troponin C, I and T (Figure 1).
Cardiac troponin I (cTnI) inhibits hydrolysis of ATP required for actin and myosin
interaction (Langhorn and Willesen 2016). When cardiac troponin C (cTnC) binds
Ca2+, it loosens the bond between cTnI and actin, removing the inhibitory effects of
cTnI and allowing tropomyosin to be removed from the myosin binding site of actin.
Cardiac troponin T (cTnT) binds tropomyosin and the troponin complex to actin
(Gomes et al. 2002; Katz 2011). When myocardial cell death or damage occurs,
troponin proteins are released quickly from a cytosolic pool and more slowly from the
bound complex (Langhorn and Wellesen 2016). cTnI and cTnT have specific cardiac
isoforms and so are very specific for myocardial damage. Cardiac and skeletal
troponin C are structurally the same and so it is not a clinically useful cardiac
biomarker. cTnI is more sensitive for myocardial damage than cTnT as it is more
easily released, possibly because cTnT is more tightly bound (Schober et al. 1999).
Therefore, cTnI is the more popular cardiac biomarker. An increase in cTnI in the
blood can be detected 2-7hrs after damage occurs, and peaks within 18-48 hours.
The blood levels then fall over the following days to weeks (Prošek and Ettinger
2010; Langhorn and Willesen 2016), depending on if there is ongoing myocardial
damage. The method of elimination has not been confirmed, although proteolysis,
reticuloendothelial degradation and renal excretion likely all play a role (Langhorn
and Willesen 2016).
Cardiac troponin assays:
There are currently at least 15 different assays for cardiac troponin I on the market,
all with antibodies directed towards different epitopes, meaning that comparisons
cannot be made between measurements from different assays. The majority are
human assays, but homology between human, canine and feline cTnI is very high.
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Only two have been validated in dogs and one in cats (Oyama and Salter 2005,
Langhorn et al. 2013). The more recently developed assays are “high-sensitivity”
assays and so can detect cTnI at increasingly lower levels in the blood. There is a
point-of-care assay available (i-STAT® Cardiac Troponin I (cTnI), Abbott Point of
Care, Princeton, NJ) meaning that results can be available to the clinician within 10
minutes (Figure 2).
There is a single assay available for cTnT, which has not been validated for dogs
and cats. However, it has been used successfully in several studies in dogs
(DeFrancesco et al. 2002; Tarducci et al 2004; Shaw et al. 2004) and one study in
cats (Langhorn et al. 2014).
Measurement of cTnI has several limitations. Renal disease can result in an increase
in circulating cTnI, as at least some of the degradation products of cTnI are renally
excreted (Porciello et al. 2008; Sharkey et al. 2009). Other extra-cardiac diseases
can also result in an elevation in cTnI by causing secondary myocardial damage,
through release of inflammatory cytokines, oxidative damage, hypoxia and other
mechanisms. Therefore, elevated cTnI is seen with systemic disease such as
anaemia (Lalor et al. 2014), immune-mediated haemolytic anaemia (Cartwright et al.
2015), gastric dilation and volvulus (Schober et al. 2002), systemic inflammatory
response syndrome (Langhorn et al. 2013; Langhorn et al. 2014, Hamacher et al.
2015), hyperthyroidism (Connolly et al. 2005) and systemic and pulmonary
hypertension (Bijmans et al. 2017; Guglielmini et al. 2010), among others. cTnI also
increases with age (Oyama et al. 2004; Ljungvall et al. 2010) and is higher in certain
breeds such as Greyhounds (LaVecchio et al. 2009) and Boxers (Baumwart et al.
2007).
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Mishandling of the sample can also result in false cTnI results. It has been reported
that serum samples are not stable at room temperature, 4˚C or -20˚C (O’Brien et al.
2006). However, some laboratories still allow shipment of refrigerated samples, so it
is important to check the guidelines of the individual laboratory before shipping.
Point-of-care tests should be run immediately after collection for the most accurate
results. Lipaemia or haemolysis of the sample can also result in false elevation
(Langhorn and Wellesen 2016).
Uses of cTnI in clinical practice
1.) Differentiation of cardiac from non-cardiac causes of respiratory signs
Dogs: cTnI does not appear to be useful in differentiating cardiac from primary
respiratory causes of dyspnoea (Prošek et al. 2007; Payne et al. 2011). Although
circulating cTnI levels were higher in cardiogenic cases of respiratory distress in one
study using the point-of-care iSTAT analyser (Payne et al. 2011), there was
significant overlap meaning that the specificity of the test was very poor. This is likely
due to increases in cTnI due to hypoxic damage to the myocardium.
Cats: cTnI may be slightly more useful in cats to differentiate cardiac from non-
cardiac causes of dyspnoea, although significant overlap still does occur (Connolly et
al. 2009). The most recent study using the point-of-care iSTAT analyser identified a
cut-off of 0.24ng/ml to have 100% sensitivity and a cut-off of 0.66ng/ml to have 100%
specificity for cardiogenic dyspnea (Wells et al. 2014). The range of values between
these cut-offs represents a grey-area. These results are similar to a previous study
by Herndon et al (2008) who found a lower cut-off of 0.19ng/ml and an upper cut-off
of 1.42ng/ml to have 100% sensitivity and specificity respectively. Cats with HCM
and congestive heart failure have higher cTnI than cats with HCM but without CHF
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(Herndon et al. 2002). As with NT-proBNP, this test should only be used as an aid in
diagnosis and other confirmatory tests such as thoracic radiography and
echocardiography should be performed, particularly because concurrent renal
disease is a common comorbidity in cats with CHF and can result in elevation of
cTnI.
2.) Detecting occult heart disease
Dogs: An earlier study by Oyama et al. (2007) did not find cTnI useful in detecting
occult DCM. However, a 2010 study by Wess et al. found that a cut-off of
>0.22ng/mL had a sensitivity of 79.5% and specificity of 84.4% for detecting all forms
of cardiomyopathy in Dobermans. The sensitivity was slightly better for
echocardiographic changes (86.6%), and slightly worse for arrhythmogenic changes
(70.5%). Similar to NT-proBNP, The European Society of Veterinary Cardiology
screening guidelines for dilated cardiomyopathy in Doberman Pinschers (2017)
suggest that cTnI may be suitable as a screening tool when the gold standard is
unavailable, but a positive result should be followed up with echocardiography and
Holter monitoring before a diagnosis is made. Equally, a negative result cannot rule-
out the presence of occult DCM. Just as with NT-proBNP, cTnI was shown by Wess
et al (2010) to be elevated 1.5 years before the development of echocardiographic or
arrhythmogenic signs of DCM but further investigation is necessary before additional
recommendations can be made.
Cats: Two studies have shown higher levels of cTnI in cats with moderate to severe
HCM compared to normal cats (Herndon et al. 2002; Connolly et al. 2003). However,
the HCM groups in these studies contained cats with both occult HCM and CHF.
Connolly et al. (2003) described a cut-off of 0.2ng/ml to have a sensitivity of 87% and
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specificity of 84% for the presence of HCM, which was the lower limit of detection of
the assay used at the time. Further studies are required before recommendations
can be made for the use of cTnI in detection of occult cardiomyopathy in cats.
3.) Use of cTnI in determining prognosis
Dogs: In general, cTnI levels are associated with the severity of cardiac disease in
dogs (Oyama et al. 2004; Spratt et al. 2005; Ljungvall et al. 2010; Polizopoulou et al
2014). Dogs with cardiomyopathy and cTnI >0.2ng/ml had a shorter survival time
than those with cTnI <0.2ng/ml (Oyama et al. 2004). Similarly, Kluser et al (2016)
found that cTnI >0.34ng/ml predicted an increased risk of sudden death in
Doberman Pinschers with dilated cardiomyopathy but in this study, other factors
such as heart size, NT-proBNP and the presence of ventricular tachycardia were
more important predictors. Linklater et al. (2007) suggested that in dogs with MMVD
presenting with CHF, an undetectable cTnI value using an assay with a lower
detection limit of 0.1ng/ml might have a longer survival time than those with
detectable cTnI. Hezzell et al. (2012) used a higher sensitivity assay and identified a
value of >0.025ng/ml to be associated with a 1.9 times increased risk of death in
dogs with MMVD. A more rapid rate of increase in cTnI was also associated with an
increased risk of mortality in this study, although the sudden increase in cTnI
occurred late in life. Fonfara et al. (2010) looked at dogs with cardiac disease of any
cause and found that dogs with cTnI <0.15ng/ml had a better survival than those
with presenting values between 0.151ng/ml and 1ng/ml. Dogs with cTnI >1ng/ml had
an even worse prognosis. In the group of dogs with cTnI >1ng/ml at presentation,
persistent elevation of cTnI at a 2 month recheck was associated with a shorter
survival time.
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Cats: Two 2014 studies have looked at the value of cardiac troponins as prognostic
indicators in cats with HCM. Langhorn et al. (2014) used only purebred cats and
identified a cTnI value of > 0.14ng/ml at admission to have a sensitivity of 80% but a
poor specificity of 61.5% for cardiac death during the follow-up period. cTnT of
>13ng/mL at admission had a lower sensitivity (60%) but higher specificity (84.6%),
and the final cTnT value before death also had prognostic potential, overall making
cTnT a better prognostic indicator than cTnI. Borgeat et al. (2014) found that cats
with HCM and a cTnI value of >0.7ng/ml had a significantly shorter survival time,
although significance was lost if the regional left ventricular wall hypokinesis was
detected on echocardiography. Overall, the low sensitivities and specificities of cTnI
and cTnT make them unsuitable for use as sole prognostic indicators and other
prognostic factors such as left atrial size, left ventricular thickness and systolic
function and the presence of CHF should be taken into account. They are probably
most useful when echocardiography is unavailable.
4.) Other uses for cTnI
Myocarditis: Some of the most dramatic increases in cTnI are seen in dogs and cats
with myocarditis, with values often hundreds of times greater than the upper
reference value. Values are generally expected to be higher than with other cardiac
diseases, due to the acute, diffuse myocardial cell death and damage that occurs.
However, myocarditis is not often proven in veterinary medicine as endomyocardial
biopsy is rarely undertaken and cTnI levels, along with the clinical picture, are
instead used to make a presumptive diagnosis.
Pericardial effusion: cTnI is elevated in dogs with pericardial effusion, and is higher
in dogs with haemangiosarcoma than idiopathic pericardial effusion (Shaw et al.
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2004). A cTnI value >0.25ng/ml suggests that pericardial effusion is neoplastic in
origin, with high sensitivity (81%) and specificity (100%) (Chun et al. 2010).
Collapse: cTnI values are higher in dogs with cardiogenic causes of collapse than
neurological or vaso-vagal causes. However, its use as a diagnostic test in this
sense is limited as there is significant overlap between groups (Dutton et al. 2017).
Systemic inflammatory response syndrome (SIRS): cTnI is prognostic for short-term
survival in patients with SIRS, with a value <0.24ng/ml being an excellent predictor of
short-term survival. However, values >0.24ng/ml are not necessarily suggestive of
death (Langhorn et al. 2013). cTnI and cTnT have poor sensitivity and specificity to
determine long-term (1 year) outcome (Langhorn et al. 2014).
Conclusions
Cardiac biomarkers have proven to be a useful addition to clinical practice for
diagnosis and prognostication in cardiac and some non-cardiac diseases. However,
they must be used and interpreted appropriately to have the most benefit.
Indiscriminate measurement will result in an increased number of false positive
results so testing is only recommended if there is already a suspicion of disease
based on signalment, history, physical examination and/or other diagnostic findings
and results should be interpreted in light of these other findings. It is important to
remember that biomarkers are not diagnostic for any one disease, and positive
results should always be followed up with appropriate confirmatory diagnostic tests.
Key Words
Biochemical markers; troponin; natriuretic peptides; heart; canine; feline
Key-points
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The natriuretic peptides are released in response to myocardial stretch. The
NT-proBNP assay is the most studied and widely available of the natriuretic
peptide family.
The cardiac troponins are released in response to myocardial cell damage or
death. Cardiac troponin I is the most studied and widely available assay.
Measurement of NT-proBNP in dogs and particularly in cats can aid in the
differentiation of respiratory signs due to cardiac versus non-cardiac disease.
cTnI is not as useful for this purpose.
NT-proBNP and cTnI measurement may be useful in screening Doberman
Pinschers for DCM when the gold standard (echocardiography and Holter
monitoring) is not available. Their sensitivity is lower in dogs with solely
arrhythmogenic changes than those with structural changes.
NT-proBNP is relatively sensitive for the detection of moderate to severe
occult heart disease in cats when there is a clinical suspicion of heart disease.
cTnI is less well studied.
NT-proBNP and cTnI levels are generally associated with severity of disease
and may be helpful in determining prognosis, but other clinical and
echocardiographic findings are likely to be more useful for this purpose.
Indiscriminate testing is not recommended as this increases the risk of false
positive results.
All positive results must be followed up with confirmatory diagnostic tests
before a definitive diagnosis can be made.
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Table 1. Cut-off values recommend by IDEXX using the Canine and Feline
Cardiopet® proBNP Assay IDEXX Laboratories Inc., Westbrook, Maine, USA.
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For Dogs Suspected of Heart Disease (murmur or at risk breed)
<900 pmol/l Not compatible with increased stretch and stress on the myocardium. Clinically significant heart disease is unlikely at this time.
>900 pmol/l Compatible with increased stretch and stress on the myocardium. Clinically significant heart disease is likely. Additional diagnostics are recommended to diagnose and assess severity of the disease.
> 735 pmol/l (Doberman)
Increased risk of dilated cardiomyopathy.
>1500pmol/l(<20kg with MMVD)
Increased risk of heart failure in the coming 12 months. Thoracic radiographs and vertebral heart score, at a minimum, are required.
For Dogs with a Murmur and Clinical Signs Consistent with Cardiac Disease
<900 pmol/l Likelihood that clinical signs (respiratory and/or exercise intolerance) are due to heart failure is low.
900-1800pmol/l
Increased stretch and stress on the myocardium. However, results in this range do not allow reliable differentiation between clinical signs due to heart failure versus those from other causes. Additional diagnostics are recommended.
>1800pmol/l Increased stretch and stress on the myocardium. The likelihood that clinical signs (i.e. respiratory and/or exercise intolerance) are due to heart failure is high. Additional diagnostics are recommended to diagnose and assess severity of the disease.
Recommendations for cats
<100 pmol/l Clinically significant cardiomyopathy is unlikely
100-270
pmol/l
Clinically significant cardiomyopathy is unlikely but early disease may be present. Consider repeating NT-proBNP in 3-6 months or an echocardiogram.
>270 pmol/l Clinically significant CM is highly likely. Further cardiac work-up including an echo is recommended.
Figure 1. Cardiac troponin complex
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Figure 2. VetScan i-STAT® 1 Handheld Analyzer and VetScan i-STAT® Cardiac
Troponin I (cTnI) cartridge, Abaxis, Union City, CA.
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