Medical Laboratory Science (Clinical Pathology, Hematology, Biochemistry, Microbiology)
Medical Laboratory Science is the largest technical domain: clinical pathology, haematology, biochemistry and microbiology. Expect specimen collection and anticoagulants, the principle of common tests, reference ranges, quality control and interpretation of abnormal results.
Core content
- Anticoagulants: EDTA (lavender) for haematology counts; 3.2% sodium citrate (blue) for PT and aPTT at a 9:1 blood-to-anticoagulant ratio; heparin (green) for biochemistry; fluoride-oxalate (grey) for glucose.
- Haematology: haemoglobin by the cyanmethaemoglobin method; PCV, MCV, MCH and MCHC derived from the CBC; ESR by Westergren; PT/INR reflects the extrinsic pathway, aPTT the intrinsic pathway.
- Biochemistry: glucose by glucose oxidase or hexokinase; creatinine by the Jaffé reaction; urea by urease; ALT and AST for liver, ALP for cholestasis; quality control through Levey–Jennings charts and Westgard rules.
- Microbiology: Gram stain uses crystal violet, iodine, decolouriser and safranin, leaving Gram-negative organisms pink and Gram-positive purple; MacConkey agar detects lactose fermenters; Kirby–Bauer disc diffusion tests susceptibility; Ziehl–Neelsen staining detects acid-fast bacilli.
Worked example
A WBC count reads 12,000/µL and the film shows 20 nucleated RBCs per 100 WBCs. Analysers count nucleated RBCs as white cells, so apply the correction: 12,000 × 100/(100 + 20) = 10,000/µL. The corrected count is always lower than the raw figure.
Traps
- Under-filling or over-filling citrate tubes, which distorts PT and aPTT.
- Reporting a WBC count uncorrected for nucleated RBCs in anaemic patients.
- Ignoring haemolysis, which falsely raises potassium, LDH and AST.
- Treating an abnormal ESR as a diagnosis; it is a non-specific marker.