Sodium, potassium and chloride

19 DOH Abu Dhabi (HAAD) lab licensing practice questions on Sodium, potassium and chloride, part of Clinical Chemistry. The first 19 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Hard

   A well outpatient's serum has visible fibrin strands. Indirect ISE on the main analyzer gives sodium 121, potassium 3.4 and chloride 88 mmol/L, and the glucose, urea and total protein from the same run are each about 13% below his results last month. Run from the same tube on the blood gas analyzer, direct ISE gives sodium 139, potassium 3.9 and chloride 101 mmol/L. Measured osmolality is 289 mOsm/kg (reference 275-295) and the serum is clear, not lipemic. QC on both analyzers is within limits. What is the most likely cause of the low results on the main analyzer?

   1. ATrue hyponatremia from water retention
   2. BContamination of the sample with IV fluid
   3. CElectrolyte exclusion by lipid or protein
   4. DShort sampling by a probe blocked by fibrin

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   Answer: D · Short sampling by a probe blocked by fibrin

   Every result from the main analyzer, electrolytes and photometric tests alike, is low by the same proportion: 121/139, 3.4/3.9 and 88/101 are all about 0.87. That points to less sample than intended being aspirated, which happens when fibrin partly blocks the probe without triggering the clot detector. Electrolyte exclusion needs very high lipid or protein, but the serum is clear and the protein is low, not high. Contamination with IV fluid or true hyponatremia would change the sample itself, so the direct ISE sodium and the osmolality would be low too. Remove the fibrin, re-centrifuge and rerun the sample, and check the probe.

   • A: True hyponatremia would give a low direct ISE sodium and a low osmolality.
   • B: IV fluid would dilute the whole tube, so direct ISE would also be low.
   • C: Exclusion needs high lipid or protein; the serum is clear and the protein is not raised.

2. Question 2 · Hard

   A 72-year-old man becomes confused and nauseated near the end of a 90-minute transurethral resection of the prostate using 1.5% glycine irrigation. Results: sodium 118 mmol/L by indirect ISE and 117 mmol/L by direct ISE, potassium 4.3 mmol/L, glucose 6.0 mmol/L (108 mg/dL), urea 5.0 mmol/L (BUN 14 mg/dL), total protein 58 g/L, clear serum, and measured osmolality 272 mOsm/kg (reference 275-295). Calculate osmolality as 2 × Na + glucose + urea, using the indirect ISE sodium; the osmolal gap reference is below 10 mOsm/kg. These results are best explained by:

   1. APseudohyponatremia from electrolyte exclusion
   2. BWater shift from cells caused by hyperglycemia
   3. CAbsorption of the glycine irrigating fluid
   4. DSIADH triggered by surgical stress

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   Answer: C · Absorption of the glycine irrigating fluid

   Direct and indirect ISE agree, so the low sodium is real and not pseudohyponatremia from excess lipid or protein. Calculated osmolality is 2 × 118 + 6.0 + 5.0 = 247 mOsm/kg, giving an osmolal gap of 272 − 247 = 25 mOsm/kg, so an unmeasured solute is present. Glycine absorbed from the irrigating fluid through opened prostatic veins dilutes the sodium while adding osmoles of its own, which is why the measured osmolality falls much less than the sodium. SIADH lowers measured osmolality in step with sodium and leaves no gap, and the glucose is normal. Sodium and osmolality should be followed as the glycine is metabolized.

   • A: Direct and indirect ISE agree, so the hyponatremia is real.
   • B: Glucose is 6.0 mmol/L, far too low to draw water out of cells.
   • D: SIADH lowers osmolality in step with sodium and leaves no osmolal gap.

3. Question 3 · Hard

   A patient with acute myeloid leukemia (WBC 190 x 10^9/L) has potassium 2.4 mmol/L in a heparin tube that waited 5 hours, unspun, in a warm courier box. A whole-blood potassium measured at the bedside right after collection was 4.0 mmol/L. What explains the low laboratory result?

   1. ALeukemic cells lysed and released potassium during transport
   2. BHeparin bound potassium during the delay
   3. CTrue hypokalemia that the bedside analyzer missed
   4. DLeukemic cells took up potassium while the sample waited

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   Answer: D · Leukemic cells took up potassium while the sample waited

   Large numbers of metabolically active blasts keep pumping potassium into cells with the Na+/K+-ATPase, and warmth speeds this up. Over a long delay, plasma potassium falls: pseudohypokalemia. The bedside value measured at once reflects the patient. Such samples should be separated promptly or measured immediately on whole blood, so that potassium is not given needlessly.

   • A: Cell lysis would raise potassium, not lower it.
   • B: Heparin does not bind potassium.
   • C: The bedside result was measured at once and reflects the patient.

4. Question 4 · Medium

   Several members of one family have potassium of 6.0-6.5 mmol/L when samples wait some hours at room temperature, but normal values when plasma is separated within 30 minutes at 37 °C. ECGs and renal function are normal. What is the most likely explanation?

   1. AHyperkalemic periodic paralysis
   2. BFamilial pseudohyperkalemia
   3. CGordon syndrome (pseudohypoaldosteronism type 2)
   4. DType 4 renal tubular acidosis

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   Answer: B · Familial pseudohyperkalemia

   In familial pseudohyperkalemia, an inherited red cell membrane trait lets potassium leak out of cells below body temperature. Samples that wait or cool show a high potassium, while the patient's true plasma potassium is normal. Prompt separation at 37 °C, or an immediate whole-blood potassium on a blood gas analyzer, shows the true value. Recognizing it avoids needless treatment.

   • A: Periodic paralysis causes true, episodic hyperkalemia with weakness.
   • C: Gordon syndrome causes true hyperkalemia with hypertension, not a sample artifact.
   • D: Type 4 RTA causes true hyperkalemia that does not depend on sample handling.

5. Question 5 · Medium

   The electrolyte exclusion effect falsely lowers sodium in very lipemic samples measured by:

   1. AIndirect ISE, but not flame photometry
   2. BDirect ISE only
   3. CFlame photometry and indirect ISE
   4. DDirect ISE and blood gas analyzers

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   Answer: C · Flame photometry and indirect ISE

   Flame photometry and indirect ISE both measure sodium in a fixed, diluted volume of whole plasma and assume it is about 93% water. When lipid or protein takes up part of that volume, the result is falsely low. Direct ISE measures sodium activity in plasma water and is not affected. A suspicious low sodium in a lipemic or high-protein sample should be checked by direct ISE.

   • A: Flame photometry also measures a diluted total-plasma volume, so it is affected too.
   • B: Direct ISE is the method that is not affected.
   • D: Blood gas analyzers use direct ISE and are not affected.

6. Question 6 · Medium

   A sample has albumin 14 g/L (1.4 g/dL) and total protein 38 g/L. Sodium is 145 mmol/L on the main analyzer (indirect ISE) and 140 mmol/L on the blood gas analyzer (direct ISE), both run on the same heparinized sample. What explains the difference?

   1. ALow protein raises plasma water, so indirect ISE reads high
   2. BHeparin in the blood gas syringe has diluted the sample
   3. CDirect ISE reads low when protein binds less sodium
   4. DLow protein lowers plasma water, so indirect ISE reads high

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7. Question 7 · Medium

   A visibly lipemic sample is drawn from the arm opposite an intravenous lipid emulsion. Sodium by indirect ISE is 126 mmol/L, and measured osmolality is 290 mOsm/kg (reference 275-295). The same sample on a direct ISE blood gas analyzer would most likely give a sodium of about:

   1. A150 mmol/L
   2. B126 mmol/L
   3. C116 mmol/L
   4. D138 mmol/L

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8. Question 8 · Easy

   An unseparated blood sample for electrolytes is stored at 4 °C overnight. How will its potassium change?

   1. AIt rises, because cold stops the Na+/K+-ATPase pump
   2. BIt falls, because cold drives potassium into cells
   3. CIt rises, because cold activates red cell glycolysis
   4. DIt stays stable, because cells are inactive in cold

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9. Question 9 · Hard

   An inpatient's results, with yesterday's in brackets: sodium 145 (139) mmol/L, chloride 122 (101) mmol/L, potassium 2.5 (4.1) mmol/L, bicarbonate 15 (25) mmol/L, glucose 3.6 (6.0) mmol/L, calcium 1.40 (2.30) mmol/L, total protein 42 (70) g/L. The patient is clinically unchanged. What is the most likely cause?

   1. AContamination with 0.9% saline from an IV line
   2. BContamination with 5% dextrose from a drip
   3. CContamination with K-EDTA from another tube
   4. DA new hyperchloremic metabolic acidosis

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10. Question 10 · Easy

    The relationship between an ion-selective electrode's potential and the logarithm of the ion's activity is described by the:

    1. ANernst equation
    2. BHenderson-Hasselbalch equation
    3. CBeer-Lambert law
    4. DMichaelis-Menten equation

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11. Question 11 · Medium

    During calibration, an analyzer's sodium electrode responds slowly and shows a slope of 49 mV per tenfold change in activity; the manufacturer's acceptable range is 52-64 mV at 37 °C. What is the most likely cause?

    1. AMeasuring cell temperature above 37 °C
    2. BProtein build-up or ageing of the sensing membrane
    3. CCorrect performance for a monovalent ion
    4. DHypertriglyceridemia in the previous sample

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12. Question 12 · Hard

    A patient with chronic lymphocytic leukemia (WBC 280 x 10^9/L) has potassium 7.6 mmol/L in lithium heparin plasma sent by pneumatic tube; calcium, creatinine and the ECG are normal. Serum from a tube carried by hand gives 4.6 mmol/L. What is the most likely explanation?

    1. APotassium carried over from an EDTA tube
    2. BTrue hyperkalemia from tumor lysis syndrome
    3. CPlatelets released potassium during clotting
    4. DLeukemic cells lysed in the heparin tube in transit

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13. Question 13 · Hard

    Sodium is 128 mmol/L and glucose 44.8 mmol/L (807 mg/dL). Using a correction of 1.6 mmol/L of sodium for every 5.6 mmol/L (100 mg/dL) of glucose above 5.6 mmol/L, what is the corrected sodium?

    1. A141 mmol/L
    2. B139 mmol/L
    3. C145 mmol/L
    4. D117 mmol/L

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14. Question 14 · Medium

    An unspun sample reaches the laboratory 6 hours after collection. Potassium is 12.8 mmol/L, calcium 0.85 mmol/L (3.4 mg/dL) and glucose 5.2 mmol/L, the same as a meter reading taken at collection. The patient is well. Which tube was most likely used?

    1. ALithium heparin
    2. BFluoride/oxalate
    3. CPotassium EDTA
    4. DSodium citrate

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15. Question 15 · Hard

    A 58-year-old with primary biliary cholangitis has deep jaundice, ALP 1,450 U/L and total cholesterol 28 mmol/L (1,080 mg/dL); the serum is not turbid. Sodium is 125 mmol/L by indirect ISE and 138 mmol/L by direct ISE, and measured osmolality is 289 mOsm/kg (reference 275-295). The low indirect sodium is best explained by:

    1. AHypertriglyceridemia from cholestasis
    2. BBilirubin interfering with the sodium electrode
    3. CHypotonic hyponatremia from liver disease
    4. DLipoprotein X occupying plasma volume

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16. Question 16 · Hard

    A 6-year-old boy with known G6PD deficiency is admitted with dark urine two days after eating fava beans. Every sample, including one drawn carefully by free flow, is hemolyzed; potassium is 6.2 mmol/L, haptoglobin is undetectable and the urine is positive for blood with no red cells. How should the laboratory handle the potassium?

    1. AReport it as falsely raised by in vitro hemolysis
    2. BCorrect it using the free hemoglobin level
    3. CReport it with a comment that hemolysis is likely in vivo
    4. DReject it and keep requesting recollection

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17. Question 17 · Easy

    Serum potassium is normally slightly higher than heparin plasma potassium from the same draw because:

    1. ARed cells take up potassium in heparin tubes
    2. BHeparin binds potassium in the plasma
    3. CPlatelets release potassium during clotting
    4. DClot activator contains potassium salts

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18. Question 18 · Hard

    A sample with severe hypertriglyceridemia gives a falsely low sodium. Which method causes this?

    1. AA blood gas analyzer using a direct electrode
    2. BOsmometry
    3. CIndirect ion-selective electrode (diluted sample)
    4. DDirect ion-selective electrode (undiluted sample)

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19. Question 19 · Medium

    Which of these commonly causes a falsely high potassium?

    1. ADelayed separation, chilling or fist clenching
    2. BInsulin treatment for hyperglycemia
    3. CLong-term treatment with loop or thiazide diuretics
    4. DMetabolic alkalosis from vomiting

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