Electrolytes

Chloride

• Major Extracellular anion (~103 mEq/L)• Maintains hydration, osmotic pressure,

ionic balance• Changes parallel changes in Na• ISE Silver Chloride/silver sulfide sensing

element• Also colorimetric and coulometric-

amperometric (Ag+ + Cl- AgCl)• Sweat Chloride Cystic Fibrosis

Chloride Metabolism

• Obtained from the diet and completely absorbed in the gut

• Excreted through the GI tract, skin, urine

• Reabsorbed by the proximal tubule and the Loop of Henle

Chloride Clinical Significance

• Normal Range (98-109 mmol/L)

• Increased Hyperparathyroidism, renal tubular

disease, diarrhea, dehydration, Chronic Heart Failure CHF

• Decreased Salt losing renal disease,

overhydration, prolonged vomiting, burns

Sweat Chloride

• 17th Century Saying

• “Woe to that child who when kissed on the forehead taste salty. He/She is bewitched and soon must die”

• Pilocarpine nitrate A stimulant which causes localized sweating so that sweat may be collected and analyzed

Sweat Chloride for Cystic Fibrosis

http://www.cff.org/aboutcf/testing/sweattest/• Gauze soaked in pilocarpine nitrate and

potassium sulfate reagents• Gauze is placed on the arm and

connected to the electrodes• Sweat is then analyzed for chloride• Ranges

– Normal = 0 – 35 mmol/L– Ambiguous = 35-60 mmol/L– Cystic Fibrosis = >60 mmol/L

CO2

• Primarily bicarbonate• Keep sample capped to prevent loss of

CO2

– Dissolved CO2 escapes rapidly once the sample is opened

CO2 Specimen

• Serum or heparinized plasma (venous blood)

CO2 Measurement

• Sample must be acidified or alkalinized

• Acidification converts various forms of CO2 to gaseous CO2

• Alkalinizing converts all CO2 to HCO3-

• Measurements involve electrode-based or enzymatic methods– Electrodes – use PCO2 electrode

– Enzymatic – convert to bicarbonate HCO3- react with

phosphoenolpyruvate, measure a decrease in absorbance at 340nm (NADH + H+ NAD+)

CO2 Clinical Significance

• Normal Range (23-30 mmol/L)

• IncreasedMetabolic Alkalosis, Compensated

respiratory acidosis, Emphysema

• DecreasedMetabolic Acidosis, Compensated

respiratory alkalosis, Hyperventilation

Sodium

• Major extracellular cation (serum/plasma concentration 135-148 mEq/L, urine concentration 40-217 mEq/24hr)

• Functions in maintaining osmotic pressure in the ECF

• Highly regulated by the kidneys– 70%-80% reabsorbed in the proximal tubules– 20%-25% reabsorbed in the loop of Henle

Sodium Specimen

• Serum or heparinized plasma (no sodium-containing anticoagulants)– Must be centrifuged in <30 min from collection

• Serum/plasma may be stored at 2-4°C• Urine collected unpreserved• Hemolysis DOES NOT cause significant errors• Lipemic samples should be measured by direct

ion-selective electrode• Avoid IV line draws (draw below IV)

Sodium Measurement

• Atomic Absorption Spectra (AAS)• Flame Emission Spectra (FES)• Ion-selective Electrode (IES)

– Sodium electrode with a glass membrane– Potentiometric method– Indirect = sample is diluted with a high ionic strength

buffer– Direct = no dilution– Subject to error by lack of selectivity, protein coating,

and salt-bridge competition with the selected ion

Clinical Significance: Sodium

• Hypernatremia – Water deficiency– Excessive sweating– Fever– Burns– Hyperventilation– Diabetes insipidus– Diarrhea and vomiting

Clinical Significance: Sodium

• Hyponatremia– Water excess (dilutional hyponatermia)– Heart failure, liver disease, nephrotic syndrome, renal

failure– Inappropriate ADH– Sodium deficit > water deficit – vomiting diarrhea, GI

obstruction, burns, diuretics, hypoaldosterone– ECF to ICF– Psuedohyponatremia – hyperglycemia,

hyperlipidemia, hyperglobulinemia

Potassium

• Major intracellular cation (serum/plasma concentration of 3.5-5.3 mEq/L, urine concentration 30-90 mEq/24hr)

• Highly reabsorbed in the proximal tubules • Secreted by the distal tubules for Na+

exchange when influenced by aldosterone• Potassium is required for muscle

irritability, respiration, and myocardial function

Potassium Specimen

• MUST avoid hemolysis• Levels in plasma and whole blood are 0.1-

0.7 mEq/L lower than serum (due to platelet release of K+ in serum)

• CANNOT refrigerate whole blood sample– Falsely increased due to poor K-ATPase

pump regulation = leaking

• CANNOT store unseparated at room temp– Glycolysis occurs and shifts K+ to ICF

Therefore, collect the sample between 25-37 C, and centrifuge within 30 min.

Potassium Measurement

• Atomic Absorption Spectra (AAS)• Flame Emission Spectra (FES)• Ion-selective Electrode (IES)

– Potassium electrode with liquid ion-exchange membranes which incorporate valinomycin

– Potentiometric method– Indirect = sample is diluted with a high ionic strength

buffer– Direct = no dilution– Subject to error by lack of selectivity, protein coating,

and salt-bridge competition with the selected ion

Clinical Significance: Potassium

• Hyperkalemia (Addison’s, Acidosis, Cardiac Arrest)– Pseudohyperkalemia – hemolysis,

leukocytosis– High intake/Decreased excretion – renal

failure, hyperalsodteronism, diuretics– SYMPTOMS: changes in EKG, arrhythmia,

muscle weakness, paresthesias, cardiac arrest

Clinical Significance: Potassium

• Hypokalemia (Cushings, Alkalosis, Arrhythmias) – ECF to ICF due to alkalosis, increased insulin– Decreased intake– Increased GI loss – vomiting, diarrhea, malabsorption,

laxatives– Increased urinary loss – increased aldosterone, renal

disease, tubular acidosis, Fanconi syndrome– SYMPTOMS: nausea, vomiting, abdominal

distension, muscle cramps, EKG changes, lethargy, confusion

No renal threshold for potassium!

Electrolyte Exclusion Principle

• The exclusion of electrolytes from the fraction of plasma which is occupied by solids

• Solids occupy 7% of plasma (93% is water)

• Therefore, 145 x (100/93) = 156 mEq/L

• Becomes a problem during hyperlipidemia or hyperproteinemia

Anion GAP

• (Na + K) – (Cl+ CO2) (10 -20)

• Or• Na – (Cl +CO2) (8-16)

• Difference between unmeasured anions and unmeasured cations

• Increased Renal failure, diabetic acidosis, lactic acidosis, drugs or toxins or lab error

• Decreased QC Check Can’t be a negative number• Analytical error, such as false elevated Cl or low Na• Lipemia

Correlations

Correlation Coefficient Regression Plot

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