Internal Medicine
Internal Medicine carries the largest share of the colloquium: it tests whether you can move from a symptom (dyspnea, chest pain, fever, confusion) to a prioritized differential, order the right tests, and interpret the numbers correctly.
Core rules
- Acid–base first. Identify the primary disorder from pH and PaCO2/HCO3, then test the compensation: Winter's formula gives expected PaCO2 = (1.5 × HCO3) + 8 ± 2 for metabolic acidosis.
- Anion gap = Na − (Cl + HCO3), normal 8–12 mmol/L; add 2.5 to the gap for every 1 g/dL albumin below 4.
- Community-acquired pneumonia: CURB-65 (Confusion, Urea >7 mmol/L, RR ≥30, low Blood pressure, age ≥65) guides admission.
- Type 2 diabetes: metformin first-line unless eGFR <30, decompensated heart failure, or acidosis; A1c ≥6.5% is diagnostic.
Worked example
A 54-year-old with vomiting and glucose 320 mg/dL: pH 7.22, PaCO2 22, HCO3 9, Na 132, Cl 92. Anion gap = 132 − (92 + 9) = 31 → a high-gap acidosis. Delta ratio = (31 − 12)/(24 − 9) = 1.27 → a single disorder, with no superimposed metabolic alkalosis. Expected PaCO2 = 1.5(9) + 8 = 21.5 ± 2, so the measured 22 is appropriate compensation. Conclusion: high-anion-gap metabolic acidosis with ketones positive - DKA.
Traps
- Calling a picture "compensated" without checking whether the compensation is adequate.
- Ignoring hypoalbuminemia, which hides a raised anion gap.
- Accepting a normal PaCO2 as reassuring in metabolic acidosis - it is not.
- Missing a second disorder by skipping the delta ratio.