Use of Laboratory Use of Laboratory Tests in Kidney Tests in Kidney

DiseaseDisease

OverviewOverview

Review functions of the kidney and related testsReview functions of the kidney and related tests

Discuss specific tests and issues relating to Discuss specific tests and issues relating to interpretationinterpretation

Tests of kidney functionTests of kidney function

What does a kidney do?What does a kidney do?

Blood flow to kidney is about 1.2 L/min (1/5 of Blood flow to kidney is about 1.2 L/min (1/5 of Cardiac output)Cardiac output)

About 10% of blood flow is filtered across the About 10% of blood flow is filtered across the glomerular membrane (100 – 120 ml/min/1.73mglomerular membrane (100 – 120 ml/min/1.73m22

Tests: urea, creatinine, creatinine clearance, eGFR, Tests: urea, creatinine, creatinine clearance, eGFR, Cystatin CCystatin C

GlomerulusGlomerulus

Glomerulus MicroscopicGlomerulus Microscopic

Tests of kidney functionTests of kidney function

Kidney Functions – cont’dKidney Functions – cont’d

Selectively secretes into or re-absorbs from the Selectively secretes into or re-absorbs from the filtrate to maintainfiltrate to maintain

Salt BalanceSalt Balance Tests: Tests: NaNa++, Cl, Cl--, K, K+ + Aldosterone, ReninAldosterone, Renin

Acid Base BalanceAcid Base Balance

Tests: pH, HCOTests: pH, HCO33--, NH, NH44

++ Acid loading, Urinary Anion Acid loading, Urinary Anion GapGap

Kidney Functions – cont’dKidney Functions – cont’d

Selectively secretes into or re-absorbs from the Selectively secretes into or re-absorbs from the filtrate to maintainfiltrate to maintain

Water BalanceWater Balance Tests: Tests: specific gravity, osmolarity, water deprivation specific gravity, osmolarity, water deprivation

testing, Antidiuretic hormonetesting, Antidiuretic hormone Retention of nutrientsRetention of nutrients

Tests: Tests: proteins, sugar, amino acids, phosphateproteins, sugar, amino acids, phosphate Secretes waste productsSecretes waste products

Tests: urate, oxalate, bile saltsTests: urate, oxalate, bile salts

Kidney Function – cont’dKidney Function – cont’dEndocrine FunctionEndocrine Function

Target organTarget organ Parathyroid hormone (CaParathyroid hormone (Ca++++, Mg, Mg++++)) Aldosterone (salt balance)Aldosterone (salt balance) ADH (water balance)ADH (water balance)

ProductionProduction ErythropoietinErythropoietin 1, 25 dihydroxycholecalciferol1, 25 dihydroxycholecalciferol

Calcium MetabolismCalcium Metabolism

Renin Angiotensin SystemRenin Angiotensin System

AldosteroneAldosterone

ADHADH

Tests that predict kidney Tests that predict kidney diseasedisease

eGFReGFR Albumin Creatinine Ratio Albumin Creatinine Ratio

(aka ACR or Microalbumin) (aka ACR or Microalbumin)

Tests of Glomerular Filtration Tests of Glomerular Filtration RateRate

UreaUrea CreatinineCreatinine Creatinine ClearanceCreatinine Clearance eGFReGFR Cystatin CCystatin C

Glomerular Filtration Rate (GFR) Glomerular Filtration Rate (GFR)

Volume of blood filtered across glomerulus per unit Volume of blood filtered across glomerulus per unit timetime

Best single measure of kidney functionBest single measure of kidney function

Glomerular Filtration Rate Glomerular Filtration Rate (GFR) – cont’d(GFR) – cont’d

Patient’s remain asymptomatic until there has Patient’s remain asymptomatic until there has been a significant decline in GFRbeen a significant decline in GFR

Can be very accurately measured using “gold-Can be very accurately measured using “gold-standard” techniquestandard” technique

Glomerular Filtration Rate Glomerular Filtration Rate (GFR) – cont’d(GFR) – cont’d

Ideal MarkerIdeal Marker Produced endogenously at a constant rateProduced endogenously at a constant rate Filtered across glomerular membraneFiltered across glomerular membrane Neither re-absorbed nor excreted into the urineNeither re-absorbed nor excreted into the urine

UreaUrea

Used historically as marker of GFRUsed historically as marker of GFR Freely filtered but both re-absorbed and excreted Freely filtered but both re-absorbed and excreted

into the urineinto the urine Re-absorption into blood increased with volume Re-absorption into blood increased with volume

depletion; therefore GFR underestimateddepletion; therefore GFR underestimated Diet, drugs, disease all significantly effect Urea Diet, drugs, disease all significantly effect Urea

productionproduction

Urea Urea IncreaseIncrease DecreaseDecrease

Volume depletionVolume depletion Volume Volume ExpansionExpansion

Dietary proteinDietary protein Liver diseaseLiver disease

CorticosteroidsCorticosteroids Severe Severe malnutritionmalnutrition

TetracyclinesTetracyclines

Blood in G-I tractBlood in G-I tract

CreatinineCreatinine

Product of muscle metabolismProduct of muscle metabolism Some creatinine is of dietary originSome creatinine is of dietary origin Freely filtered, but also actively secreted into urineFreely filtered, but also actively secreted into urine Secretion is affected by several drugsSecretion is affected by several drugs

Serum Creatinine Serum Creatinine

IncreaseIncrease DecreaseDecrease

MaleMale AgeAge

Meat in dietMeat in diet FemaleFemale

Muscular body typeMuscular body type MalnutritionMalnutrition

Cimetidine & some Cimetidine & some Muscle wastingMuscle wasting

other medications other medications AmputationAmputation

Creatinine vs. Inulin ClearanceCreatinine vs. Inulin Clearance

Creatinine Clearance Creatinine Clearance

Measure serum and urine creatinine levels and Measure serum and urine creatinine levels and urine volume and calculate serum volume cleared urine volume and calculate serum volume cleared of creatinineof creatinine

Same issues as with serum creatinine, except Same issues as with serum creatinine, except muscle massmuscle mass

Requirements for 24 hour urine collection adds Requirements for 24 hour urine collection adds variability and inconveniencevariability and inconvenience

Cystatin CCystatin C

Cystatin C is a 13 KD protein produced by all cells Cystatin C is a 13 KD protein produced by all cells at a constant rateat a constant rate

Freely filteredFreely filtered Re-absorbed and catabolized by the kidney and Re-absorbed and catabolized by the kidney and

does not appear in the urinedoes not appear in the urine

eGFReGFR

Increasing requirements for dialysis and Increasing requirements for dialysis and transplant (8 – 10% per year)transplant (8 – 10% per year)

Shortage of transplantable kidneysShortage of transplantable kidneys

Large number at riskLarge number at risk

StageStage DescriptionDescription GFR GFR ML/min/1.173mML/min/1.173m22 PrevalencePrevalence33

11 Kidney Damage with Normal or Kidney Damage with Normal or ↑ ↑ GFRGFR >90>90 478,500478,500

22 Kidney Damage with Mild Kidney Damage with Mild ↓ ↓ GFRGFR 60 – 8960 – 89 435,000435,000

33 Moderate Moderate ↓↓ GFR GFR 30 – 5930 – 59 623,500623,500

44 Severe Severe ↓↓ GFR GFR 15 – 2915 – 29 29,00029,000

55 Kidney FailureKidney Failure <15 or dialysis<15 or dialysis 14,50014,500

eGFR – cont’deGFR – cont’d

Cumulative 8-year mortality rate, depending on Cumulative 8-year mortality rate, depending on serum creatinine level at baseline, in the serum creatinine level at baseline, in the

Hypertension Detection and Follow-up ProgramHypertension Detection and Follow-up Program

Serum creatinine Serum creatinine

mg/dL (mg/dL (µmol/L)µmol/L)

MortalityMortality

Rate (%)Rate (%)

0.8 - 0.99 (71 - 88)0.8 - 0.99 (71 - 88) 1010

1.1 - 1.29 (97 – 114)1.1 - 1.29 (97 – 114) 1212

1.3 – 1.49 (115 – 132)1.3 – 1.49 (115 – 132) 1616

1.5 – 1.69 (133 – 149)1.5 – 1.69 (133 – 149) 2222

1.7 – 1.99 (150 – 176)1.7 – 1.99 (150 – 176) 3030

2.0 – 2.49 (177 – 220)2.0 – 2.49 (177 – 220) 4141

≥≥2.5 (≥221)2.5 (≥221) 5454

Data from Shulman et al.Data from Shulman et al.99

eGFR – cont’deGFR – cont’d

The Old Standard: Serum The Old Standard: Serum CreatinineCreatinine

ProblemProblem

Need an easy test to screen for early decreases in Need an easy test to screen for early decreases in GFR that you can apply to a large, at-risk GFR that you can apply to a large, at-risk populationpopulation

Can serum creatinine be made more sensitive by Can serum creatinine be made more sensitive by adding more information?adding more information?

eGFR by MDRD FormulaeGFR by MDRD Formula

Mathematically modified serum creatinine with Mathematically modified serum creatinine with additional information from patients age, sex and additional information from patients age, sex and ethnicityethnicity

eGFR = 30849.2 x (serum creatinine)eGFR = 30849.2 x (serum creatinine)-1.154-1.154 x (age) x (age)-0.203-0.203

(if female x (0.742))(if female x (0.742))

Screening Test – cont’dScreening Test – cont’d

The ResultsThe Results

eGFR – cont’deGFR – cont’d

eGFR calculation has been recommended by eGFR calculation has been recommended by National Kidney Foundation whenever a serum National Kidney Foundation whenever a serum creatinine is performed in adultscreatinine is performed in adults

Guidelines & ProtocolsGuidelines & ProtocolsAdvisory CommitteeAdvisory Committee

Identification, Evaluation and Management of Identification, Evaluation and Management of Patients with Chronic Kidney DiseasePatients with Chronic Kidney Disease

Recommendations for:Recommendations for: Risk group identificationRisk group identification ScreeningScreening Evaluation of positive screenEvaluation of positive screen Follow-upFollow-up

Identify High Risk GroupsIdentify High Risk Groups

DiabetesDiabetes HypertensionHypertension Heart DiseaseHeart Disease Family HistoryFamily History High Risk Ethnic GroupHigh Risk Ethnic Group Age > 60 yearsAge > 60 years

Screen High Risk GroupsScreen High Risk Groups

eGFReGFR UrinalysisUrinalysis Albumin / Creatinine RatioAlbumin / Creatinine Ratio

Follow-up based on Screen ResultsFollow-up based on Screen Results

Kidney UltrasoundKidney Ultrasound Specialist ReferralSpecialist Referral Cardiovascular Risk AssessmentCardiovascular Risk Assessment Diabetes ControlDiabetes Control Smoking cessationSmoking cessation Hepatitis / Influenza ManagementHepatitis / Influenza Management

Creatinine Standardization in Creatinine Standardization in British ColumbiaBritish Columbia

Based on Isotope dilution /Based on Isotope dilution /mass spectrometry mass spectrometry measurements of creatinine standardsmeasurements of creatinine standards

Permits estimation and correction of creatinine Permits estimation and correction of creatinine and eGFR bias at the laboratory level.and eGFR bias at the laboratory level.

Importance of StandardizationImportance of Standardization

Low bias creatinine:Low bias creatinine: Causes inappropriately increased eGFRCauses inappropriately increased eGFR Patients will not receive the benefits of more intensive Patients will not receive the benefits of more intensive

investigation of treatment.investigation of treatment.

High bias creatinine:High bias creatinine: Causes inappropriately decreased eGFRCauses inappropriately decreased eGFR Patients receive investigations and treatment which is Patients receive investigations and treatment which is

not required. Wastes time, resources and increases not required. Wastes time, resources and increases anxiety.anxiety.

High 143.3

Low 116.0

Mean 124.6

Poor Creatinine PrecisionPoor Creatinine Precision

Incorrect categorization of patients with both Incorrect categorization of patients with both “normal” and decreased eGFR.“normal” and decreased eGFR.

Total ErrorTotal Error

TE = % bias + 1.96 CVTE = % bias + 1.96 CV Goal is <10%Goal is <10%

(requires bias (requires bias ≤ 4% and CV ≤ 3%)≤ 4% and CV ≤ 3%)

ProteinuriaProteinuria

In health:In health: High molecular weight proteins are retained in the High molecular weight proteins are retained in the

circulation by the glomerular filter (Albumin, circulation by the glomerular filter (Albumin, Immunoglobulins)Immunoglobulins)

Low molecular weight proteins are filtered then Low molecular weight proteins are filtered then reabsorbed by renal tubular cellsreabsorbed by renal tubular cells

Proteinuria – cont’dProteinuria – cont’d

Glomerular:Glomerular: Mostly albumin, because of its high concentration and Mostly albumin, because of its high concentration and

therefore high filtered loadtherefore high filtered load

Tubular:Tubular: Low molecular weight proteins not reabsorbed by tubular Low molecular weight proteins not reabsorbed by tubular

cells (e.g. alpha-1 microglobulin)cells (e.g. alpha-1 microglobulin)

Overflow:Overflow: Excessive filtration of one protein exceeds reabsorbtive Excessive filtration of one protein exceeds reabsorbtive

capacity (Bence-Jones, myoglobin)capacity (Bence-Jones, myoglobin)

Albumin Creatinine Ratio Albumin Creatinine Ratio (Microalbumin)(Microalbumin)

Normal albumin moleculeNormal albumin molecule In health, there is very little or no albumin in the In health, there is very little or no albumin in the

urineurine Most dip sticks report albumin at greater than Most dip sticks report albumin at greater than

150 mg/L150 mg/L

Urinary Albumin – cont’dUrinary Albumin – cont’d

Detection of low levels of albumin (even if below Detection of low levels of albumin (even if below dipstick cut-off) is predictive of future kidney dipstick cut-off) is predictive of future kidney disease with diabetesdisease with diabetes

Very significant biologic variation usually requires Very significant biologic variation usually requires repeat collectionsrepeat collections

Treatment usually based on timed urine albumin Treatment usually based on timed urine albumin collectionscollections

UrinalysisUrinalysis

DipstickDipstick ProteinProtein

• Useful screening testUseful screening test

• Dipstick more sensitive to albumin than other Dipstick more sensitive to albumin than other proteinsproteins

• Large biologic variationLarge biologic variation

Urinalysis – cont’dUrinalysis – cont’d

Dipstick – cont’dDipstick – cont’d HemoglobinHemoglobin

• Glomerular, tubular or post-renal sourceGlomerular, tubular or post-renal source

• Reasonably sensitiveReasonably sensitive

• Positive dipstick and negative microscopy with lysed Positive dipstick and negative microscopy with lysed red cellsred cells

Urinalysis – cont’dUrinalysis – cont’d

Dipstick – cont’dDipstick – cont’d GlucoseGlucose

• Reasonable technically, however screening and Reasonable technically, however screening and monitoring programs for diabetes are now done by monitoring programs for diabetes are now done by blood and Point-of-Care devicesblood and Point-of-Care devices

Specific GravitySpecific Gravity

Approximate onlyApproximate only Measurement of osmolarity preferred when Measurement of osmolarity preferred when

concentrating ability being assessedconcentrating ability being assessed

pHpH

pH changes with time in a collected urinepH changes with time in a collected urine Calculations to determine urine ammonium levels Calculations to determine urine ammonium levels

and response to acid-loading generally required to and response to acid-loading generally required to assess for renal tubular acidosisassess for renal tubular acidosis

Microscopic UrinalysisMicroscopic Urinalysis

Epithelial CellsEpithelial CellsSquamous, Transitional, RenalSquamous, Transitional, Renal

All may be present in small numbersAll may be present in small numbers Important to recognize possible malignancyImportant to recognize possible malignancy Comment on unusual numbersComment on unusual numbers

Renal Tubular EpithelialRenal Tubular Epithelial

Red CellsRed Cells

May originate in any part of the urinary tractMay originate in any part of the urinary tract Small numbers may be normalSmall numbers may be normal There is provincial protocol for the investigation There is provincial protocol for the investigation

of persistent hematuriaof persistent hematuria

Red CellsRed Cells

White Blood CellsWhite Blood Cells

Neutrophils often present in small numbersNeutrophils often present in small numbers Lymphocytes and moncytes less oftenLymphocytes and moncytes less often Marker for infection or inflammationMarker for infection or inflammation

NeutrophilsNeutrophils

CastsCasts

Hyaline and granular casts not always pathologic, Hyaline and granular casts not always pathologic, clinical correlation requiredclinical correlation required

Red cell casts always significant, usually Red cell casts always significant, usually glomerular injury glomerular injury

WBC casts also always significant, usually WBC casts also always significant, usually infection, sometimes inflammationinfection, sometimes inflammation

Bacterial casts only found in pyelonephritisBacterial casts only found in pyelonephritis Waxy casts found in significant kidney diseaseWaxy casts found in significant kidney disease

Hyaline CastHyaline Cast

Granular CastGranular Cast

White Cell CastWhite Cell Cast

Red Cell CastRed Cell Cast

Waxy CastWaxy Cast

Tests for Renal Tubular AcidosisTests for Renal Tubular Acidosis

Urinary Anion GapUrinary Anion Gap

(Na(Na+ + + K+ K++) – Cl) – Cl--

In acidosis the kidney should excrete NHIn acidosis the kidney should excrete NH44++ and and

the gap will be negativethe gap will be negative

RTA – cont’dRTA – cont’d

If If NHNH44++ is not present (or if HCO is not present (or if HCO33

-- is present) the is present) the

gap will be neutral or positive, implying impaired gap will be neutral or positive, implying impaired kidney handling of acid load. kidney handling of acid load.

Urine Anion Gap = (NaUrine Anion Gap = (Na++ + K + K++) –Cl) –Cl--

RTA – cont’dRTA – cont’d

Ammonium Chloride LoadingAmmonium Chloride Loading Load with ammonium chlorideLoad with ammonium chloride Hourly measurements of urine pHHourly measurements of urine pH Normal at least one pH below 5.5Normal at least one pH below 5.5

Tests of Kidney Concentrating Tests of Kidney Concentrating AbilityAbility

To differentiateTo differentiate Psychogenic polydipsiaPsychogenic polydipsia Central diabetes insipidusCentral diabetes insipidus Nephrogenic Nephrogenic diabetes insipidus diabetes insipidus

Overnight Water Deprivation Overnight Water Deprivation TestingTesting

(Serum osmolarity <295 monitor patient (Serum osmolarity <295 monitor patient weight hourly)weight hourly)

Collect urine hourly from 0600 for osmolarityCollect urine hourly from 0600 for osmolarity Baseline serum osmolarity, NaBaseline serum osmolarity, Na++, ADH , ADH When osmolarity plateaus repeat above tests and When osmolarity plateaus repeat above tests and

administer ADHadminister ADH

InterpretationInterpretation

If urine concentrates (osmolarity >600 and If urine concentrates (osmolarity >600 and serum osmolarity below <295)serum osmolarity below <295)

Normal physiology (? psychogenic polydipsia)Normal physiology (? psychogenic polydipsia)

No Urine ConcentrationNo Urine ConcentrationNo Response to ADHNo Response to ADH

Nephrogenic diabetes insipidusNephrogenic diabetes insipidus

No Urine ConcentrationNo Urine Concentration

Positive response to ADHPositive response to ADHCentral diabetes insipidusCentral diabetes insipidus

QuestionsQuestions