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

  • Describe the abnormalities.
  • Give one example of an associated clinical scenario.

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



Normal Adult Range


145 mmol/L

135 – 145


4.0 mmol/L

3.2 – 4.5


96 mmol/L

100 – 110


25 mmol/L

24 – 32



7.35 – 7.45


40 mmHg (5.2 kPa)

35 – 45 (4.6 – 5.9)

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College Answer

Increased anion gap metabolic acidosis and metabolic alkalosis.

Clinical scenario – acute renal failure with vomiting .


Let us dissect these results systematically.

  1. The A-a gradient cannot be calculated - the information is missing
  2. There is a normal pH
  3. The PaCO2 is normal
  4. The SBE is not supplied, but the bicarbonate is 25, which is slightly higher than expected, suggestive of a metabolic alkalosis
  5. The respiratory compensation is inadequate - the expected PaCO2(0.7 × 25) + 20 = 37.5mmHg, and thus there is also a very mild respiratory acidosis
  6. The anion gap is raised:
    (145+4.0) - (96+25) = 28
    The delta ratio is of no use here.
  7. 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.