Pathology and Laboratory Medicine
Pathology and Laboratory Medicine tests interpretation of common tests in primary care: blood counts, liver and renal panels, urinalysis, coagulation, and tumor markers. The goal is to use tests to confirm a clinical hypothesis, not to screen indiscriminately.
Core rules
- Microcytic anemia: low ferritin confirms iron deficiency. Ferritin is an acute-phase reactant and can be normal in deficiency with inflammation.
- Macrocytic anemia: check B12, folate, thyroid, alcohol, and medications.
- Liver enzymes: ALT/AST reflect hepatocellular injury; ALP and GGT reflect cholestasis.
- Urinalysis: nitrites and leukocyte esterase suggest urinary tract infection; red cell casts suggest glomerulonephritis.
- Tumor markers are for monitoring, not diagnosis.
Worked example
A 35-year-old woman has fatigue, Hb 9.8 g/dL, MCV 72 fL, and ferritin 8 ng/mL. Reasoning: this is iron deficiency anemia. Do not stop at the blood test; ask about menorrhagia and gastrointestinal blood loss. Start oral iron and recheck the count in 4–8 weeks. If ferritin were normal or high with inflammation, interpret cautiously.
Traps
- Diagnosing iron deficiency on a normal ferritin during infection.
- Ignoring hemolysis when the reticulocyte count is high.
- Treating a spurious high potassium without repeating the sample.
- Using CA-125 to screen asymptomatic women.