Clinical Laboratory and Diagnostic Aids
This domain tests sample collection, common investigations and the meaning of normal values, plus safe handling of specimens.
Collection and tubes
Collect in the correct vacutainer: plain red or serum separator for biochemistry and serology, lavender EDTA for haematology such as complete blood count, light blue sodium citrate for coagulation tests (fill to the marked line, blood-to-anticoagulant ratio 1:9), green heparin for plasma biochemistry, and grey sodium fluoride or potassium oxalate for blood glucose, because fluoride stops glycolysis.
Common tests and reference values
Haemoglobin 13–17 g/dL in men and 12–15 g/dL in women; total leucocyte count 4,000–11,000/µL; platelets 1.5–4.5 lakh/µL. Fasting plasma glucose 70–110 mg/dL; HbA1c below 5.7% is normal, 5.7–6.4% prediabetes and 6.5% or above diabetes. Lipid profile: total cholesterol under 200 mg/dL, LDL under 100, HDL above 40 in men and 50 in women, triglycerides under 150. Serum creatinine 0.6–1.2 mg/dL. Urine routine examines colour, albumin, sugar, pus cells and casts.
Diagnostic aids
ECG for cardiac rhythm and ischaemia; X-ray, ultrasound, CT and MRI for imaging; endoscopy for direct visualisation. For tuberculosis, sputum CBNAAT (GeneXpert MTB/RIF) is the preferred first-line test. Malaria is diagnosed by rapid antigen test and thick and thin smears; HIV follows the three-test national algorithm.
Quality and safety
Calibrate analysers, run controls, observe cold chain, apply universal precautions and segregate waste at the point of collection.
Worked example
A fasting glucose is requested. Reasoning: glucose falls in the grey fluoride tube because fluoride inhibits glycolysis; a plain tube left standing gives a falsely low result. Collect, label at the bedside and transport promptly.
Traps that cost marks
Sending an EDTA sample for biochemistry; underfilling the citrate tube, which falsely prolongs clotting times; leaving a tourniquet on too long, which raises potassium.