electrolytes

What body fluid compartment contains the largest volume of water in the human body?

Intracellular space (about 25 liters)

What is the primary physiological function of sodium (Na+)?

Major extracellular cation responsible for regulating water distribution and maintaining osmotic pressure

Which hormone promotes renal sodium reabsorption and potassium excretion, and what disease results from its deficiency?

Aldosterone; deficiency causes Addison’s disease (hyponatremia and hyperkalemia)

Which tube anticoagulant must be avoided when collecting blood specimens for sodium testing?

Sodium heparin (it falsely elevates sodium values)

What effect does severe hemolysis have on sodium and potassium lab measurements?

Falsely lowers sodium (dilutional effect) and falsely elevates potassium (intracellular leakage)

What is the reference range for serum sodium?

136 to 145 mEq/L

What is the primary intracellular cation and its main clinical function?

Potassium (K+); critical for heart and muscle contraction

Why must serum or plasma for potassium testing be separated from red blood cells ASAP?

To prevent in vitro diffusion of potassium out of cells into the serum/plasma, which causes false elevations

What are the clinical symptoms and ECG/cardiac dangers of severe hyperkalemia?

Nausea, vomiting, muscle weakness, and life-threatening cardiac arrhythmias/arrest

What is the reference range for serum potassium?

3.5 to 5.1 mEq/L

What membrane material is used in Ion-Selective Electrodes (ISE) for measuring potassium?

Valinomycin liquid polymer membrane

What is the body distribution of chloride (Cl-)?

Two-thirds in plasma/serum and one-third inside red blood cells

Explain the "Chloride Shift" when red blood cells contain oxygen versus carbon dioxide.

Oxygenated RBCs: chloride shifts from RBC to plasma, bicarbonate enters RBC. CO2-rich RBCs: bicarbonate exits RBC, chloride shifts into RBC

What tube additive causes false elevations in chloride testing due to exogenous contamination?

Sodium fluoride

What is the reference range for serum chloride?

98 to 107 mEq/L

What are the three steps of a sweat chloride test for Cystic Fibrosis diagnosis?

1) Pilocarpine iontophoresis sweating stimulation, 2) 30-minute sweat collection, 3) Quantitative analysis via coulometric titration

What sweat chloride concentration is diagnostic for Cystic Fibrosis?

Greater than 60 mmol/L (40–60 mmol/L is borderline; <40 mmol/L is normal)

What fraction of total carbon dioxide does bicarbonate represent in extracellular fluids?

Bicarbonate is the major fraction, representing approximately 90%

How fast can carbon dioxide levels decrease if a blood specimen is left exposed to ambient air?

Carbon dioxide can decrease by up to 6 mmol/L per hour

What is the reference range for serum bicarbonate (total CO2)?

22 to 29 mmol/L

How is the Anion Gap calculated and what is its normal reference range?

[Na+ + K+] - [Cl- + HCO3-] (or [Na+] - [Cl- + HCO3-]); Normal range is 10 to 20 mmol/L

What causes an elevated anion gap (e.g., in diabetic ketoacidosis or renal failure)?

An increase in unmeasured anions (organic acids, ketones) causing a decrease in chloride and/or bicarbonate

What is the most common cause of a decreased or negative anion gap?

Hypoalbuminemia or instrument/laboratory error

What is the primary advantage of Direct (undiluted) ISE over Indirect (diluted) ISE?

Direct ISE is unaffected by hyperlipidemia or hyperproteinemia (volume exclusion effect)

How does a coulometric-amperometric chloridometer measure chloride?

Silver ions (Ag+) are generated at the anode to react with chloride to form AgCl precipitate; when chloride runs out, free Ag+ is sensed to stop the timer

How does a pCO2 gas-sensing electrode measure carbon dioxide?

CO2 diffuses across a gas-permeable membrane, reacts with water to form H2CO3, and the resulting change in internal buffer pH is measured potentiometrically