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AOBPa Pathology study materials
AOBPa Pathology study resources

AOBPa Pathology: Clinical Pathology / Laboratory Medicine: Interpreting the Numbers

AOBPa Pathology · Work through your exam, one topic at a time.

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Clinical Pathology / Laboratory Medicine: Interpreting the Numbers

Clinical pathology rewards disciplined interpretation of laboratory data: know the reference interval logic, the preanalytical pitfalls, and the pattern recognition that separates competing diagnoses.

Concepts to master

  • Preanalytical error: hemolysis (falsely raises potassium, LDH, AST; lowers glucose), prolonged tourniquet (raises potassium and protein), and EDTA contamination (raises potassium, lowers calcium). Most "impossible" lab values are preanalytical.
  • Anemias by indices: microcytic (iron deficiency, thalassemia, anemia of chronic disease, sideroblastic), normocytic (hemolysis, blood loss, renal), macrocytic (B12/folate, alcohol, hypothyroidism, drugs). Use the Mentzer index (MCV/RBC) to separate iron deficiency (>13) from thalassemia trait (<13).
  • Acid-base: calculate the anion gap, then the delta ratio to detect mixed disorders; winter's formula gives expected PaCO2 compensation.
  • Liver patterns: hepatocellular (AST/ALT predominant) vs. cholestatic (ALP/GGT predominant); the R factor quantifies the mix.
  • Coagulation: PT reflects the extrinsic and common pathway; aPTT the intrinsic and common. Mixing studies distinguish factor deficiency (corrects) from inhibitor (does not correct).

Traps

Do not interpret a single value in isolation - always ask whether the pattern is internally consistent. A "high potassium" with a normal ECG and no hemolysis on the smear should prompt a repeat draw. Beware the corrected calcium: calculate it when albumin is abnormal. And remember that a normal anion gap does not exclude a metabolic acidosis (hyperchloremic causes).

Rule of thumb

For any abnormal lab, run the three-question check: (1) Is it real (preanalytical)? (2) Is it isolated or part of a pattern? (3) Does the clinical picture fit? Only then commit to a diagnosis.

Worked example

pH 7.28, PaCO2 28, HCO3 13, Na 140, Cl 104. The anion gap is 140 − (104 + 13) = 23, so this is a high-anion-gap metabolic acidosis. Delta ratio = (23 − 12)/(24 − 13) ≈ 1.0, indicating pure high-AG acidosis with appropriate respiratory compensation, not a mixed disorder.