Lab case 359 interpretation

Question 1:

PH = 7.38, that is within the normal range, toward the acidotic side.

pCO2 is 62 mmHg, that is high. Usually this number is associated with respiratory acidosis.

Not clear if the rise in CO2 is acute or chronic. We will calculate the compensation to try to figure that.

For acute respiratory acidosis we expect HCO3 to increase by 1 for every 10 pCO2 above 10. Accordingly, the expected HCO3 should be 26.2 if the respiratory acidosis is acute.  HCO3 here is 36, that is much higher than the value we have for acute respiratory acidosis.

Next, we will calculate the compensation for chronic respiratory acidosis. For chronic respiratory acidosis, we expect the HCO3 to increase by 4 for every 10 pCO2 above 40, accordingly, the expected HCO3 should be 32.8 if the case was chronic respiratory acidosis. HCO3 is even higher than that 36, So we have additional metabolic acidosis. (We have more HCO3 than what is expected).

It is good to calculate the anion gap to exclude the presence of an associated HAGMA.  We are going to add K to the calculation as the level here is elevated.

Anion gap = (Na+ K)- (Cl + HCO3) = (116 + 7.7) – (88+36) = – 0.3 So, we have low anion gap here.

Other abnormal findings.

Na = 116 mmol/L, that is severe hyponatraemia, not clear if it is acute or chronic.

K = 7.7 mmol/L, that is severe hyperkalaemia.

Cl = 88 mmol/L, that is hypochloraemia

Creatinine = 146, that is high. for a 65 year old female, this will give her GFR of 32, that is stage 3B CKD.

Final conclusion: Respiratory acidosis (most probably chronic) with associated metabolic acidosis  with severe hyponatraemia and severe hyperkalaemia and stage 3 CKD.

 Question 2:

Anion GAP is the difference between unmeasured anion and unmeasured cations. It is usually calculated as the difference between the measured cations and measured anions.

Low or negative anion gap is caused by either reduced unmeasured anions or increase unmeasured cations. (Also can be caused by analytical error).

Albumin is the major unmeasured anion, it contributes to almost the whole value of the anion gap.

Unmeasured cations include Ca, Mg, IgG (Multiple myeloma) and Lithium (in case of overdose).

Other causes

  • Dilution – leads to low anions
  • Analytical errors (Increased viscosity, increased lipids, increased Na and Iodide ingestion.)
  • Bromide OD (leads to falsely elevated chloride measurements)


This patient had COPD and she was alcoholic, she was non compliant with restricted fluids intake. She had generalised oedema with albumin level of 20. Renal impairment caused by new UTI and she was on diuretics including spiranolactone.

We can correct Anion gap to albumin level. That is, for every 1g/L decrease in albumin below 35 anion gap will decrease by 0.25

Accordingly, corrected anion gap for this patient will be – 0.3 + (15 x 0.25) = 3.45.