Sodium 143  mmol/L (137-145)
Potassium 2.6   mmol/L (3.1-4.2)
Chloride 117  mmol/L (101-109)
Bicarbonate   18 mmol/L (22-32)
Urea       7.0  mmol/L   (3.0-8.0)
Creatinine  0.08 mmol/L (0.05-0.12)

List 3 likely causes for the above plasma biochemistry

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College Answer

Drugs:

1) RTA 1 or 2
2) Ampho B,
3) Acetazolamide
4) GI losses

Discussion

Let us dissect this set of results.

There is some sort of acidosis. There is no SBE, but the bicarbonate is 18. The anion gap is (143) - (117 + 18) = 8, or 10.6 when calculated with potassium

Thus, it is a normal anion gap metabolic acidosis.

What could possibly cause a normal anion gap metabolic acidosis?

In the absence of history, one falls back on broad differentials:

  • Pancreatic secretion loss
  • Acetazolamide, or renal tubular acidosis Type 2
  • Normal saline intoxication
  • Diarrhoea
  • Aldosterone antagonists
  • Renal tubular acidosis Type 1 (distal)
  • Ureteric diversion
  • Small bowel fistula
  • Hyperalimentation (TPN)

Those offered by the college are frankly bizarre. I cannot imagine how a candidate could have scored full marks with that. Acetazolamide use is a pharmacological form of Type 2 RTA, and "Ampho B" probably refers to amphotericin-induced distal (Type 2) renal tubular acidosis (and I cannot imagine how even the most time-constrained examiner might have been so rushed that they actually failed to finish the word "amphotericin" in their model answer).

Thus, the three non-overlapping causes I would have given would be the following:

  • Normal saline intoxication
  • Renal tubular acidosis (Type 1 or 2) - not 4, as the K+ is low
  • Diarrhoea or other gastrointestinal losses

References

References

Story DA. Hyperchloraemic acidosis: another misnomer? Crit Care Resusc. 2004 Sep;6(3):188-92.