A 69-year-old man with back pain and anemia has a newly found IgG kappa monoclonal protein of 4.8 g/dL [48 g/L]; total protein is 10.4 g/dL [104 g/L] and albumin 3.2 g/dL [32 g/L]. Results: sodium 137, potassium 4.2, chloride 110 and HCO3- 24 mmol/L; calcium 9.6 mg/dL [2.40 mmol/L]; creatinine 1.1 mg/dL [97 µmol/L]. He takes no lithium and no bromide-containing drugs. The anion gap, Na - (Cl + HCO3-), has a reference interval of 8-16 mmol/L and is corrected by adding 2.5 mmol/L for each 1 g/dL of albumin below 4.0 g/dL. The best explanation of his anion gap is:

Unmeasured cation from the IgG paraprotein

Chloride falsely raised by bromide

Low albumin reducing unmeasured anions

Hypercalcemia adding unmeasured cations

A laboratory measures osmolality with a vapor pressure osmometer. A patient known to have drunk methanol shows no osmolal gap. The reason is:

Methanol is volatile and is not detected

Methanol is in the calculated value

Methanol is bound to plasma proteins

Methanol's molecular weight is too high

pH 7.29, sodium 138, chloride 114 and bicarbonate 14 mmol/L (anion gap reference 8-16 mmol/L). Which anion gap and cause fit these results?

24 mmol/L, from lactic acidosis

Electrolyte calculations (osmolality, anion gap)

10 ASCP MLS practice questions on Electrolyte calculations (osmolality, anion gap), part of Chemistry. The first 10 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 69-year-old man with back pain and anemia has a newly found IgG kappa monoclonal protein of 4.8 g/dL [48 g/L]; total protein is 10.4 g/dL [104 g/L] and albumin 3.2 g/dL [32 g/L]. Results: sodium 137, potassium 4.2, chloride 110 and HCO3- 24 mmol/L; calcium 9.6 mg/dL [2.40 mmol/L]; creatinine 1.1 mg/dL [97 µmol/L]. He takes no lithium and no bromide-containing drugs. The anion gap, Na - (Cl + HCO3-), has a reference interval of 8-16 mmol/L and is corrected by adding 2.5 mmol/L for each 1 g/dL of albumin below 4.0 g/dL. The best explanation of his anion gap is:
1. AChloride falsely raised by bromide
2. BLow albumin reducing unmeasured anions
3. CHypercalcemia adding unmeasured cations
4. DUnmeasured cation from the IgG paraprotein
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Answer: D · Unmeasured cation from the IgG paraprotein
The anion gap is 137 - (110 + 24) = 3 mmol/L. Correcting for albumin adds 2.5 x (4.0 - 3.2) = 2 mmol/L, giving 5 mmol/L, which is still below the reference interval, so low albumin explains only part of it. Many IgG paraproteins are positively charged at blood pH; they act as unmeasured cations and are balanced by extra chloride and bicarbonate, which lowers the gap. Calcium is normal, and he takes no bromide or lithium, the other usual causes of a gap this low. An unexplained low anion gap in an adult is a reason to look for a paraprotein.
- A: He takes no bromide-containing drugs, so nothing is raising the chloride result.
- B: After correction for albumin the gap is still only 5 mmol/L, so albumin is not the whole cause.
- C: Calcium is normal at 9.6 mg/dL, so it adds no extra cation.
Question 2 · Hard
Sodium 138, chloride 104 and bicarbonate 20 mmol/L; albumin 2.4 g/dL (24 g/L). The gap is corrected by adding 2.5 mmol/L for each 1 g/dL of albumin below 4.0 g/dL (reference gap 8-16 mmol/L). What is the corrected anion gap?
1. A14 mmol/L, a normal gap
2. B20 mmol/L, a raised gap
3. C18 mmol/L, a raised gap
4. D10 mmol/L, a normal gap
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Answer: C · 18 mmol/L, a raised gap
The uncorrected gap is 138 - (104 + 20) = 14 mmol/L, which looks normal. Albumin is 1.6 g/dL below 4.0, so the gap is raised by 2.5 × 1.6 = 4, giving 18 mmol/L. The corrected value is above the reference range, which points to unmeasured acids such as lactate. Without this correction, a high anion gap acidosis can be missed in patients with low albumin.
- A: 14 is the uncorrected gap, which hides the added acid.
- B: 20 multiplies by the albumin itself rather than by the shortfall below 4.0.
- D: 10 subtracts the correction instead of adding it.
Question 3 · Medium
A laboratory measures osmolality with a vapor pressure osmometer. A patient known to have drunk methanol shows no osmolal gap. The reason is:
1. AMethanol is in the calculated value
2. BMethanol is volatile and is not detected
3. CMethanol is bound to plasma proteins
4. DMethanol's molecular weight is too high
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Answer: B · Methanol is volatile and is not detected
A vapor pressure osmometer measures how much the solutes lower the sample's vapor pressure. Volatile solutes such as methanol, ethanol and isopropanol evaporate and add to the vapor, so they are missed and no osmolal gap appears. Freezing point depression osmometers detect these alcohols and should be used when toxic alcohol ingestion is suspected. A missing gap with the wrong method can delay treatment.
- A: The formula uses only sodium, glucose and BUN.
- C: Methanol is not protein-bound; it stays dissolved in plasma water.
- D: Methanol is a small molecule (32 g/mol) that adds many osmoles per gram.
Question 4 · Medium
pH 7.29, sodium 138, chloride 114 and bicarbonate 14 mmol/L (anion gap reference 8-16 mmol/L). Which anion gap and cause fit these results?
1. A24 mmol/L, from ketoacidosis
2. B10 mmol/L, from ketoacidosis
3. C10 mmol/L, from diarrhea
4. D24 mmol/L, from lactic acidosis
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Answer: C · 10 mmol/L, from diarrhea
Anion gap = Na - (Cl + HCO3) = 138 - (114 + 14) = 10 mmol/L, which is normal. A metabolic acidosis with a normal gap and a high chloride means bicarbonate has been lost and replaced by chloride, as in diarrhea or renal tubular acidosis. Ketoacidosis and lactic acidosis add unmeasured anions and raise the gap. A value of 24 comes from leaving bicarbonate out of the formula.
- A: 24 is sodium minus chloride only; a normal gap excludes ketoacidosis.
- B: Ketoacids are unmeasured anions and would raise the gap.
- D: 24 leaves bicarbonate out of the formula; the true gap of 10 is normal.
Question 5 · Easy
Which formula is commonly used to estimate serum osmolality in conventional units?
1. A2 × Na + glucose/18 + BUN/2.8
2. BNa + glucose/18 + BUN/2.8
3. C2 × Na + glucose/2.8 + BUN/18
4. DNa + K - (Cl + HCO3)
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Answer: A · 2 × Na + glucose/18 + BUN/2.8
Sodium and its paired anions make up most of serum osmolality, so sodium is doubled. Glucose in mg/dL is divided by 18 and BUN by 2.8 to convert them to mmol/L. In SI units the formula is simply 2 × Na + glucose + urea, all in mmol/L. The calculated value is compared with the measured osmolality to find the osmolal gap.
- B: Sodium must be doubled to account for its paired anions.
- C: The divisors are swapped; glucose is divided by 18 and BUN by 2.8.
- D: This is the anion gap formula, not osmolality.
Question 6 · Medium
A patient receiving IV mannitol for raised intracranial pressure has a measured osmolality of 318 mOsm/kg and a calculated osmolality of 292 mOsm/kg. The gap of 26 is best explained by:
1. AMannitol producing organic acids
2. BMannitol falsely raising the sodium
3. CMannitol interfering with the method
4. DMannitol, an osmole not in the formula
Answer and explanation in LearnFree account
Question 7 · Hard
A patient with suspected bromide intoxication has sodium 140, chloride 129 and bicarbonate 24 mmol/L, giving an anion gap of -13 mmol/L. The most likely explanation is:
1. ALithium adding unmeasured cations
2. BIgG paraprotein adding unmeasured cations
3. CAlbumin loss lowering the unmeasured anions
4. DBromide falsely raising the measured chloride
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Question 8 · Hard
Sodium 138 mmol/L, glucose 108 mg/dL (6.0 mmol/L), BUN 28 mg/dL (10.0 mmol/L), ethanol 92 mg/dL (20 mmol/L), measured osmolality 330 mOsm/kg. Counting ethanol as mg/dL ÷ 4.6, how large is the osmolal gap that ethanol does not explain?
1. A34 mOsm/kg
2. B58 mOsm/kg
3. C38 mOsm/kg
4. D18 mOsm/kg
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Question 9 · Medium
Sodium 140, chloride 100 and bicarbonate 14 mmol/L. What is the anion gap and what does it suggest?
1. A26 mmol/L: a high anion gap acidosis
2. B26 mmol/L: metabolic alkalosis from vomiting
3. C12 mmol/L: a normal anion gap
4. D54 mmol/L: a high anion gap acidosis
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Question 10 · Hard
Sodium 140 mmol/L, glucose 90 mg/dL, urea nitrogen (BUN) 14 mg/dL; measured osmolality 320 mOsm/kg. What are the calculated osmolality and osmolal gap?
1. A290 mOsm/kg; gap 30, from simple dehydration
2. B280 mOsm/kg; gap 40, which is normal
3. C290 mOsm/kg; gap 30, suggesting a toxic alcohol
4. D300 mOsm/kg; gap 20, which is normal
Answer and explanation in LearnFree account

Electrolyte calculations (osmolality, anion gap)

Sodium 138, chloride 104 and bicarbonate 20 mmol/L; albumin 2.4 g/dL (24 g/L). The gap is corrected by adding 2.5 mmol/L for each 1 g/dL of albumin below 4.0 g/dL (reference gap 8-16 mmol/L). What is the corrected anion gap?

A laboratory measures osmolality with a vapor pressure osmometer. A patient known to have drunk methanol shows no osmolal gap. The reason is:

pH 7.29, sodium 138, chloride 114 and bicarbonate 14 mmol/L (anion gap reference 8-16 mmol/L). Which anion gap and cause fit these results?

Which formula is commonly used to estimate serum osmolality in conventional units?

A patient receiving IV mannitol for raised intracranial pressure has a measured osmolality of 318 mOsm/kg and a calculated osmolality of 292 mOsm/kg. The gap of 26 is best explained by:

A patient with suspected bromide intoxication has sodium 140, chloride 129 and bicarbonate 24 mmol/L, giving an anion gap of -13 mmol/L. The most likely explanation is:

Sodium 138 mmol/L, glucose 108 mg/dL (6.0 mmol/L), BUN 28 mg/dL (10.0 mmol/L), ethanol 92 mg/dL (20 mmol/L), measured osmolality 330 mOsm/kg. Counting ethanol as mg/dL ÷ 4.6, how large is the osmolal gap that ethanol does not explain?

Sodium 140, chloride 100 and bicarbonate 14 mmol/L. What is the anion gap and what does it suggest?

Sodium 140 mmol/L, glucose 90 mg/dL, urea nitrogen (BUN) 14 mg/dL; measured osmolality 320 mOsm/kg. What are the calculated osmolality and osmolal gap?