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Electrolytes

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)

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Electrolytes

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  1. Electrolytes

  2. 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

  3. 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

  4. 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

  5. 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

  6. Sweat Chloride for Cystic Fibrosishttp://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

  7. CO2 • Primarily bicarbonate • Keep sample capped to prevent loss of CO2 • Dissolved CO2 escapes rapidly once the sample is opened

  8. CO2 Specimen • Serum or heparinized plasma (venous blood)

  9. 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+)

  10. CO2 Clinical Significance • Normal Range (23-30 mmol/L) • Increased Metabolic Alkalosis, Compensated respiratory acidosis, Emphysema • Decreased Metabolic Acidosis, Compensated respiratory alkalosis, Hyperventilation

  11. 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

  12. 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)

  13. 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

  14. Clinical Significance: Sodium • Hypernatremia • Water deficiency • Excessive sweating • Fever • Burns • Hyperventilation • Diabetes insipidus • Diarrhea and vomiting

  15. 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

  16. 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

  17. 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.

  18. 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

  19. 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

  20. 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!

  21. 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

  22. 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

  23. Correlations

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