Investigations and Applied Physiology/Pathology: Deriving the Diagnosis From the Data
Investigations and Applied Physiology/Pathology tests interpretation of laboratory and physiological data: acid–base disorders, ECG, renal and liver function, haematology and diagnostic test performance. The candidate must derive the diagnosis from the numbers rather than pattern-match a remembered label.
Core rules
- Interpret an arterial blood gas in sequence: pH, PaCO2, HCO3, then calculate the anion gap and compare with the expected compensation.
- Correct the anion gap for albumin: add 2.5 mmol/L to the measured gap for every 1 g/dL that albumin is below 4 g/dL.
- Use Winter’s formula for metabolic acidosis: expected PaCO2 = 1.5 × HCO3 + 8 ± 2.
- Use the delta ratio to detect mixed metabolic disorders: delta anion gap divided by delta bicarbonate.
- For any diagnostic test, remember that positive predictive value rises with prevalence and falls in low-prevalence screening populations.
Worked example
A blood gas shows pH 7.28, PaCO2 30 mmHg, HCO3 14 mmol/L, Na 140 mmol/L and Cl 100 mmol/L, with albumin 4.0 g/dL. Anion gap = 140 − (100 + 14) = 26 mmol/L, so this is a high-anion-gap metabolic acidosis. Winter’s expected PaCO2 = 1.5 × 14 + 8 = 29 ± 2 mmHg; the measured 30 mmHg is appropriate respiratory compensation. Delta ratio = (26 − 12)/(24 − 14) = 1.4, consistent with a pure high-anion-gap acidosis. Check ketones, lactate, creatinine and a toxicology screen to identify the cause.
Traps
- Ignoring hypoalbuminaemia, which falsely lowers the anion gap.
- Assuming full respiratory compensation when the measured PaCO2 lies outside Winter’s range.
- Missing a mixed disorder because the pH happens to be near normal.