Pathology and Laboratory Medicine
Laboratory medicine tests pre-analytical discipline, interpretation in clinical context, and safe transfusion practice.
Core rules
- Pre-analytical errors cause most laboratory mistakes: hemolysis, delayed processing, wrong tube, prolonged tourniquet time. Hemolysis raises potassium and lowers glucose on delay.
- Anemia by MCV: iron deficiency shows low ferritin, high TIBC, low saturation; anemia of chronic disease shows low TIBC with normal or high ferritin; thalassemia shows normal ferritin and TIBC with a Mentzer index <13.
- Coagulation: PT/INR monitors warfarin, aPTT monitors heparin; a mixing study separates factor deficiency from an inhibitor.
- Transfusion: ABO/Rh compatibility, crossmatch, and bedside identity check; report any reaction immediately.
Worked example
A 40-year-old with fatigue: Hb 9.8 g/dL, MCV 72 fL, ferritin 8 ng/mL, TIBC high. This is iron deficiency anemia. Do not simply prescribe iron - find the source: gastroscopy and colonoscopy in men and postmenopausal women to exclude gastrointestinal blood loss, plus celiac serology.
Traps
- Reading a hemolyzed potassium as true hyperkalemia.
- Relying on ferritin alone during inflammation, when it rises as an acute-phase reactant.
- Assuming a normal aPTT excludes lupus anticoagulant.
- Transfusing on a number rather than on clinical need.