MLT GlobeChemistry: Electrolytes & osmolality – page 2
77 Chemistry MCQs on Electrolytes & osmolality with answers and explanations.
Where is most of the body's magnesium stored?
About 50–60% of body magnesium is in bone and most of the rest is inside soft tissue cells; only about 1% is extracellular. This is why serum Mg can be normal while body stores are low.
What is the approximate reference interval for serum osmolality in adults?
Serum osmolality is normally about 275–295 mOsm/kg, controlled by thirst and ADH. Values above 300 mOsm/kg indicate hypertonicity.
Osmolality differs from osmolarity because osmolality is expressed per:
Osmolality is osmoles per kilogram of water and is what osmometers measure; osmolarity is per litre of solution. The difference matters when plasma water is reduced, e.g. by lipids.
Which property of a solution is NOT a colligative property?
Colligative properties depend only on the number of dissolved particles: freezing point depression, boiling point rise, vapor pressure lowering and osmotic pressure. Viscosity depends on particle size and shape.
Blood for electrolytes was collected in a sodium heparin tube. Which result will be falsely affected?
Sodium heparin adds sodium to the sample, so sodium is falsely increased. Lithium heparin is the preferred heparin salt for electrolyte testing.
Which ion is the main cation of the extracellular fluid?
Sodium is the dominant extracellular cation (about 135–145 mmol/L in plasma). Potassium is the main intracellular cation.
Which ion is the main anion of the extracellular fluid?
Chloride is the most abundant extracellular anion, followed by bicarbonate. Phosphate and proteins are the main intracellular anions.
Which hormone increases water reabsorption in the renal collecting ducts?
ADH inserts aquaporin water channels in collecting duct cells, increasing water reabsorption and concentrating urine. Aldosterone acts mainly on sodium and potassium.
In adults, hypokalemia is usually defined as a serum potassium below about:
The adult reference range for serum potassium is about 3.5–5.0 (to 5.1) mmol/L, so values below 3.5 mmol/L are hypokalemia.
In adults, hyponatremia is usually defined as a serum sodium below about:
The adult reference interval for sodium is about 135–145 mmol/L, so values below 135 mmol/L are hyponatremia.
The most dangerous effect of severe hyperkalemia is on the:
High potassium changes cardiac membrane excitability and can cause life-threatening arrhythmias and cardiac arrest, so critical potassium values are phoned immediately.
Which ion is needed for blood clotting and muscle contraction, and is regulated mainly by parathyroid hormone?
Calcium is essential for coagulation and muscle contraction. PTH raises blood calcium by acting on bone, kidney and (via vitamin D) the intestine.
A patient has raised serum calcium, low phosphate and raised parathyroid hormone. The most likely diagnosis is:
Excess PTH raises calcium and increases phosphate excretion. Osteomalacia and rickets cause low or normal calcium with secondary PTH rise; hypoparathyroidism causes hypocalcaemia.
Measured serum osmolality is 320 mOsm/kg and calculated osmolality is 290 mOsm/kg. This osmolal gap most suggests:
The osmolal gap (measured minus calculated) is 30, well above the usual limit of about 10. It points to an osmotically active substance not in the formula, most often alcohols or ethylene glycol.
A patient with severe hyperlipidemia has sodium 128 mmol/L by indirect ISE and 139 mmol/L by direct ISE. The discrepancy is due to:
Indirect ISE dilutes the sample and assumes normal plasma water; excess lipid or protein reduces water fraction, giving pseudohyponatremia. Direct ISE measures activity in plasma water and is unaffected.
Which condition can cause pseudohyponatremia when sodium is measured by indirect ISE?
High paraprotein reduces the plasma water fraction and causes a falsely low sodium by indirect ISE. Hyperglycemia causes true (translocational) hyponatremia, not a measurement artefact.
A patient with a platelet count of 1200 × 10^9/L has serum K 6.4 mmol/L but no ECG changes. What is the best next step?
Platelets release potassium during clotting, causing pseudohyperkalemia in serum when counts are very high. Plasma (heparin) potassium avoids clotting and shows the true value.
Serum K is 8.5 mmol/L and calcium is 0.4 mmol/L (1.6 mg/dL) in an outpatient with no symptoms. The most likely cause is:
K2EDTA adds potassium and chelates calcium, giving very high K with very low Ca, often with low Mg and ALP. This occurs with wrong order of draw or tube decanting.
A sample drawn above an IV line infusing 0.9% saline would show:
Contamination with normal saline increases chloride (and may keep or raise sodium) while diluting other analytes such as potassium, glucose, calcium and protein. Glucose would rise with dextrose infusion.
A patient has glucose 30 mmol/L (540 mg/dL), urea 5 mmol/L (BUN 14 mg/dL) and Na 135 mmol/L. What is the calculated osmolality using 2Na + glucose + urea (mmol/L)?
2 × 135 + 30 + 5 = 305 mmol/kg. In conventional units, 2Na + glucose/18 + BUN/2.8 gives about 305 mOsm/kg as well.