The set up
We use Winter’s Formula to get the predicted pCO2 based on the bicarbonate.
1.5 x bicarbonate + 8 =
1.5 x 8 +8 = 20
His actual pCO2 is 22 which is close enough, so a pure metabolic acidosis with appropriate respiratory compensation.
138 – (114 +8) = 16
Yes, this is an anion gap metabolic acidosis.
2 x Na + Glucose / 18 + BUN / 2.8 + Ethanol / 4.6 = calculated osmolality
2 x 138 + 96 / 18 + 14 / 2.8 + 0 / 4.6 = 286
Osmolar gap = measured osm – calculated osm
Osmolar gap = 292 – 286 = 6
This is a normal osmolar gap. Poor foreshadowing by the question writer.
Bicarbonate before the anion gap = Bicarbonate + (Anion gap -12)
Bicarbonate before the anion gap = 8 + (16 –12)
Bicarbonate before the anion gap = 12
So the bicarbonate before the anion gap was 12 indicating a large non-anion gap metabolic acidosis and a relatively mild anion gap metabolic acidosis.
- chloride intoxication
- GI losses
The patient doesn’t seem to be suffering from chlorine gas intoxication or have an isotonic saline drip running so number one is not likely.
The low potassium could indicate GI losses as well as type 1 or 2 RTA. The urine anion gap in the face of severe metabolic acidosis will help here. In GI losses and chloride intoxication the urine amnion gap will be negative, in RTA it should be positive.
Urine anion gap = (Na + K) – Cl
Urine anion gap = (56 + 32) – 78
The positive anion gap indicates a lack of NH4+ in the urine. In diarrhea, the kidney will up ammonium excretion to get rid of the acid load. The increase cation load in the urine will be balanced by an increased in chloride in the urine. The increase Cl– will make the urine anion gap negative (in reality it is an unmeasured cation, or a positive cation gap, but by convention we use an anion gap). The positive urinary anion gap is the face of a severe acid load indicates a renal tubular acidosis.