Question 25.2

List 3 causes for the following combination of findings observed on a serum sample.

Test	Value	Normal Range
Measured osmolality*	340 mOsm/kg	(280 – 290)
Sodium	138 mmol/L	(135 – 145)
Potassium	4 mmol/L	(3.5 – 5.0)
Chloride	98 mmol/L	(95 – 105)
Bicarbonate*	15 mmol/L	(22 – 32)
Glucose	6 mmol/L	(4 – 6)
Urea	8 mmol/L	(6 – 8)

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College Answer

•    Raised osmolar gap with raised AG
•     Methanol
•    Ethylene glycol
•     Ethanol
•    Lactic acidosis can lead to a raised OG and AG, however, the osmolar gap does not reach the levels seen here.

Discussion

These biochemistry results are provided without any history.

Let us dissect them systematically.

The A-a gradient cannot be calculated
The pH is not supplied
The PaCO₂ is not reported
The SBE is not offered, but the bicarbonate is 15, suggetsing a metabolic acidosis.
The respiratory compensation cannot be assessed.
The anion gap is raised:
(138) - (98 + 15) = 25, or 29 when calculated with potassium
The delta ratio, assuming a normal anion gap is 12 and a normal bicarbonate is 24, would therefore be (25 - 12) / (24 - 15) = 1.44
This delta ratio suggests that there is a pure high anion gap metabolic acidosis here.
The osmolar gap is raised: 340 - (2 × 138 + 6 + 8) = 50

Thus, this high anion gap metabolic acidosis is also associated with a raised osmolar gap.

Most likely differentials:

Toxic alcohol intoxication, which includes ethanol intoxication. This is probably the only commonly seen cause of such a massively raised osmolar gap associated with a high anion gap metabolic acidosis.

Less likely differentials (osmolar gap is probably a bit too high for these to be realistic differentials):

References

Jacobsen, Dag, et al. "Anion and osmolal gaps in the diagnosis of methanol and ethylene glycol poisoning." Acta Medica Scandinavica 212.1‐2 (1982): 17-20.

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