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) |
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.