Patient Assessment, Physical Examination, and Clinical Laboratory Interpretation
What the exam tests
This domain asks you to interpret physical findings and laboratory data and link them to drug therapy. Expect cases with electrolytes, blood gases, renal and liver panels, CBC and ECGs.
Core rules
- Read vital signs first: hypotension, tachycardia, hypoxia and fever narrow the differential.
- Anion gap = Na+ − (Cl− + HCO3−); normal is 8–12 mmol/L; correct for hypoalbuminemia by adding 2.5 per 1 g/dL fall in albumin.
- Corrected calcium = measured Ca + 0.8 × (4.0 − albumin g/dL).
- Assess renal function with eGFR or Cockcroft–Gault for drug dosing; do not use eGFR alone for narrow-therapeutic-index drugs in extremes of body weight.
- Liver panels: ALT/AST reflect hepatocellular injury; ALP and bilirubin reflect cholestasis; Child–Pugh and MELD guide prognosis and dosing.
- ECG: QT prolongation risk with macrolides, fluoroquinolones, antipsychotics and ondansetron; correct potassium and magnesium.
Worked example
A patient with vomiting has Na+ 132, Cl− 92, HCO3− 14, K+ 3.2 and albumin 2.5 g/dL. The anion gap is 132 − (92 + 14) = 26, and the albumin-corrected gap is higher still, suggesting a primary metabolic acidosis with an additional process. Treat the cause, replace potassium, and review renally cleared drugs.
Traps
Interpreting total calcium without albumin correction; ignoring a normal anion gap in hypoalbuminemia; measuring drug levels at the wrong time; attributing ECG changes to drugs without checking electrolytes; using creatinine alone in low muscle mass.