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.

  1. The A-a gradient cannot be calculated
  2. The pH is not supplied
  3. The PaCO2 is not reported
  4. The SBE is not offered, but the bicarbonate is 15, suggetsing a metabolic acidosis.
  5. The respiratory compensation cannot be assessed.
  6. 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.
  7. 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.