A 47-year-old man with alcohol use disorder is admitted with acute pancreatitis, and his serum is milky. Main chemistry analyzer (indirect ISE): sodium 121 mmol/L, potassium 3.6 mmol/L, glucose 112 mg/dL [6.2 mmol/L], BUN 6 mg/dL [2.1 mmol/L], triglycerides 2,600 mg/dL [29.4 mmol/L], total protein normal. A heparinized whole-blood sample drawn at the same time gives sodium 125 mmol/L on the blood gas analyzer (direct ISE). Measured serum osmolality by freezing point is 259 mOsm/kg (reference 275-295). How should the sodium result be interpreted?

Translocational hyponatremia from glucose

Pseudohyponatremia from excess protein

Pseudohyponatremia from lipemia

Electrolytes: sodium, potassium, chloride, bicarbonate

15 ASCP MLS practice questions on Electrolytes: sodium, potassium, chloride, bicarbonate, part of Chemistry. The first 15 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Hard
A 47-year-old man with alcohol use disorder is admitted with acute pancreatitis, and his serum is milky. Main chemistry analyzer (indirect ISE): sodium 121 mmol/L, potassium 3.6 mmol/L, glucose 112 mg/dL [6.2 mmol/L], BUN 6 mg/dL [2.1 mmol/L], triglycerides 2,600 mg/dL [29.4 mmol/L], total protein normal. A heparinized whole-blood sample drawn at the same time gives sodium 125 mmol/L on the blood gas analyzer (direct ISE). Measured serum osmolality by freezing point is 259 mOsm/kg (reference 275-295). How should the sodium result be interpreted?
1. ATranslocational hyponatremia from glucose
2. BPseudohyponatremia from excess protein
3. CTrue hypotonic hyponatremia
4. DPseudohyponatremia from lipemia
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Answer: C · True hypotonic hyponatremia
Direct ISE measures sodium activity in plasma water and is not affected by lipid, so its result of 125 mmol/L shows that sodium in plasma water really is low. The indirect result is about 4 mmol/L lower because the lipid displaces some water from the diluted sample, but that small effect does not explain the hyponatremia. A low measured osmolality of 259 mOsm/kg confirms a hypotonic state, whereas in pseudohyponatremia measured osmolality is normal. Glucose is near normal, so there is no translocational shift, and total protein is normal. Lipemia does not rule out true hyponatremia, so this patient needs the usual workup and careful correction of a hypotonic hyponatremia.
- A: A glucose of 112 mg/dL is far too low to draw water out of cells.
- B: Total protein is normal, and measured osmolality is low rather than normal.
- D: In pseudohyponatremia direct ISE sodium and measured osmolality are normal; both are low here.
Question 2 · Medium
Which cause of a low sodium gives the same low result by both direct and indirect ISE?
1. ALipoprotein X in cholestasis
2. BSevere hyperglycemia
3. CSevere hypertriglyceridemia
4. DMultiple myeloma paraprotein
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Answer: B · Severe hyperglycemia
High glucose raises plasma osmolality and draws water out of cells, which truly dilutes the sodium in plasma water. Both methods therefore read low, and the measured osmolality is high rather than normal. Lipids, paraproteins and lipoprotein X lower sodium only by indirect methods, because they displace plasma water. Direct ISE gives a normal sodium in those samples.
- A: Lipoprotein X displaces plasma water, giving pseudohyponatremia by indirect ISE only.
- C: Triglycerides displace plasma water, so only indirect ISE reads low.
- D: Paraprotein displaces plasma water, so direct ISE reads normally.
Question 3 · Easy
The sensing membrane of a sodium ion-selective electrode is usually made of:
1. AGas-permeable silicone
2. BValinomycin in plastic
3. CSilver-silver chloride
4. DSodium-selective glass
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Answer: D · Sodium-selective glass
Sodium electrodes usually use a glass membrane made with aluminum silicate, which is far more selective for sodium than for potassium. The potential across the membrane depends on the sodium activity in the sample. Some electrodes use a neutral sodium carrier in a polymer membrane instead. Protein build-up on the membrane slows the response, so electrodes are cleaned regularly.
- A: A gas-permeable membrane is part of the pCO2 electrode.
- B: Valinomycin is the ionophore in potassium electrodes.
- C: Silver-silver chloride is used in reference electrodes.
Question 4 · Easy
In an ISE analyzer, valinomycin is the ionophore in the electrode for which ion?
1. AIonized calcium
2. BChloride
3. CPotassium
4. DSodium
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Answer: C · Potassium
Valinomycin is a ring-shaped antibiotic whose cavity fits potassium ions far better than sodium ions. Held in a PVC membrane, it lets the electrode respond to potassium even though sodium is about 30 times more concentrated. Sodium electrodes use special glass, and chloride electrodes use anion exchangers. Ionized calcium electrodes use a different neutral carrier.
- A: Ionized calcium electrodes use a calcium-selective carrier, not valinomycin.
- B: Chloride electrodes use an anion-exchange membrane.
- D: Sodium is measured with sodium-selective glass.
Question 5 · Easy
Which form makes up most of the total CO2 measured on a chemistry panel?
1. ABicarbonate ion
2. BCarbonic acid
3. CDissolved CO2 gas
4. DCarbamino compounds
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Answer: A · Bicarbonate ion
Total CO2 includes bicarbonate, dissolved CO2, carbonic acid and carbamino compounds, but bicarbonate makes up most of it (roughly 90 to 95%). Total CO2 is therefore used as an estimate of bicarbonate and runs slightly higher than it, by about the dissolved CO2. Methods convert all forms to bicarbonate or to CO2 before measuring. The ASCP reference range for total CO2 is 22 to 33 mmol/L.
- B: Carbonic acid is present only in tiny amounts.
- C: Dissolved CO2 is only about 0.03 x pCO2, near 1.2 mmol/L.
- D: Carbamino compounds carry CO2 mainly inside red cells, not in serum.
Question 6 · Hard
A lipemic sample has a plasma water fraction of 0.80 instead of the usual 0.93. Direct ISE sodium is 140 mmol/L. The indirect ISE sodium will be about:
1. A163 mmol/L
2. B120 mmol/L
3. C112 mmol/L
4. D140 mmol/L
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Question 7 · Medium
Indirect ISE gives a falsely low sodium in severe hypertriglyceridemia because the lipid:
1. ACoats and poisons the sodium electrode membrane
2. BDraws water out of cells to dilute plasma
3. CBinds sodium so that the electrode cannot sense it
4. DTakes up volume that would normally be plasma water
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Question 8 · Medium
A centrifuged serum tube for electrolytes is left uncapped on the bench for 2 hours. The result most affected is:
1. AChloride, which rises
2. BTotal CO2, which falls
3. CSodium, which falls
4. DPotassium, which rises
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Question 9 · Medium
For a sodium electrode at 37 °C, a tenfold rise in sodium activity changes the measured potential by about:
1. A30.8 mV
2. B61.5 mV
3. C6.15 mV
4. D123 mV
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Question 10 · Easy
Indirect ISE methods differ from direct ISE methods in that the sample is:
1. ARead by flame emission instead
2. BDiluted before it reaches the electrode
3. CMeasured undiluted as whole blood
4. DDeproteinized with acid first
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Question 11 · Easy
In the enzymatic total CO2 method, the indicator reaction that oxidizes NADH is catalyzed by:
1. ACarbonic anhydrase
2. BMalate dehydrogenase
3. CLactate dehydrogenase
4. DPhosphoenolpyruvate carboxylase
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Question 12 · Medium
A patient taking a bromide-containing medicine has chloride 125 mmol/L by ISE and an anion gap near zero. What is the most likely explanation?
1. ABromide makes the kidneys retain true chloride
2. BBromide falsely lowers the sodium result
3. CBromide falsely raises the bicarbonate
4. DBromide is read as chloride by the electrode
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Question 13 · Hard
A sample with triglycerides 4,000 mg/dL (45.2 mmol/L) has sodium 128 mmol/L by indirect ISE. Which further results confirm pseudohyponatremia?
1. ADirect ISE sodium also low; osmolality low
2. BDirect ISE sodium normal; osmolality normal
3. CDirect ISE sodium also low; osmolality high
4. DDirect ISE sodium also low; osmolality normal
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Question 14 · Hard
A 70-year-old with multiple myeloma has total protein 13.0 g/dL (130 g/L), sodium 127 mmol/L by indirect ISE and measured osmolality 289 mOsm/kg (reference 275-295). He has no symptoms of hyponatremia. What should the laboratory do next?
1. AMeasure sodium by direct ISE
2. BUltracentrifuge, then rerun the sodium
3. CReport it as true hyponatremia
4. DDilute further and rerun by indirect ISE
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Question 15 · Medium
In a hemolyzed sample, which analyte is falsely increased the most?
1. APotassium
2. BBicarbonate
3. CChloride
4. DSodium
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Electrolytes: sodium, potassium, chloride, bicarbonate

Which cause of a low sodium gives the same low result by both direct and indirect ISE?

The sensing membrane of a sodium ion-selective electrode is usually made of:

In an ISE analyzer, valinomycin is the ionophore in the electrode for which ion?

Which form makes up most of the total CO2 measured on a chemistry panel?

A lipemic sample has a plasma water fraction of 0.80 instead of the usual 0.93. Direct ISE sodium is 140 mmol/L. The indirect ISE sodium will be about:

Indirect ISE gives a falsely low sodium in severe hypertriglyceridemia because the lipid:

A centrifuged serum tube for electrolytes is left uncapped on the bench for 2 hours. The result most affected is:

For a sodium electrode at 37 °C, a tenfold rise in sodium activity changes the measured potential by about:

Indirect ISE methods differ from direct ISE methods in that the sample is:

In the enzymatic total CO2 method, the indicator reaction that oxidizes NADH is catalyzed by:

A patient taking a bromide-containing medicine has chloride 125 mmol/L by ISE and an anion gap near zero. What is the most likely explanation?

A sample with triglycerides 4,000 mg/dL (45.2 mmol/L) has sodium 128 mmol/L by indirect ISE. Which further results confirm pseudohyponatremia?

A 70-year-old with multiple myeloma has total protein 13.0 g/dL (130 g/L), sodium 127 mmol/L by indirect ISE and measured osmolality 289 mOsm/kg (reference 275-295). He has no symptoms of hyponatremia. What should the laboratory do next?

In a hemolyzed sample, which analyte is falsely increased the most?