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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