Pathology and Laboratory Medicine: Interpreting Data Critically
Laboratory questions test interpretation, not memorization. Start with the reference range and the clinical context; a value outside the range matters only if it changes management. Understand the complete blood count: anemia classification by mean corpuscular volume (microcytic, normocytic, macrocytic) guides the differential. Microcytic anemia suggests iron deficiency, thalassemia, or chronic disease; check ferritin, which is the best single test for iron deficiency when inflammation is absent.
Body fluids and chemistry
Interpret arterial blood gases systematically: pH, then carbon dioxide, then bicarbonate, then compensation and anion gap. In metabolic acidosis, calculate the anion gap; a normal gap suggests bicarbonate loss or renal tubular acidosis. Elevated creatinine with a normal urea suggests early renal injury; a low urea-to-creatinine ratio can suggest liver disease or low protein intake. Liver enzymes: hepatocellular pattern (high transaminases) versus cholestatic pattern (high alkaline phosphatase and bilirubin).
Microbiology and hematology
Know specimen collection principles - culture before antibiotics - and the meaning of Gram stain. In hematology, recognize the peripheral smear findings of iron deficiency (hypochromia, microcytosis) versus megaloblastic anemia (hypersegmented neutrophils).
Traps
Examiners present a lab value in isolation to tempt you to treat a number rather than a patient. Also beware of tests with high sensitivity but poor specificity, and of interpreting a single value without trend.
Rule of thumb
For any lab question, ask: what is the pretest probability, and does this result change the plan? If not, the answer is usually to repeat or observe.