Question 3.3

For each set of the following biochemical and arterial blood gas parameters:

Any reasonable scenario accepted that was both biochemically correct AND clinically likely.

College Answer

Increased anion gap metabolic acidosis and metabolic alkalosis.

Clinical scenario – acute renal failure with vomiting .

Let us dissect these results systematically.

The A-a gradient cannot be calculated - the information is missing
There is a normal pH
The PaCO₂ is normal
The SBE is not supplied, but the bicarbonate is 25, which is slightly higher than expected, suggestive of a metabolic alkalosis
The respiratory compensation is inadequate - the expected PaCO₂(0.7 × 25) + 20 = 37.5mmHg, and thus there is also a very mild respiratory acidosis
The anion gap is raised:
(145) - (96 + 25) = 24, or 28 when calculated with potassium
The delta ratio is of no use here.
The urinary electrolytes and pH would not be helpful.

Thus, this is a high anion gap metabolic acidosis with a coexisting metabolic alkalosis. Scenarios in which this might arise include the following:

Any cause of metabolic acidosis with a high anion gap (eg. lactic acidosis, uremic acidosis, etc etc), as well as
- vomiting / high NG aspirates
- diuretic use
- recovery from hypercapnea

Goodkin, David A., Gollapudi G. Krishna, and Robert G. Narins. "The role of the anion gap in detecting and managing mixed metabolic acid-base disorders."Clinics in endocrinology and metabolism 13.2 (1984): 333-349.