Basic Laboratory Investigations: Haematology, Biochemistry, Microbiology and Urinalysis
Domain overview
This domain tests the common haematology, biochemistry, microbiology and urinalysis investigations done at PHC level, with emphasis on normal ranges and safe interpretation.
Core rules and concepts
Haematology: haemoglobin is about 13–17 g/dL in men and 12–15 g/dL in women; MCV 80–100 fL; WBC 4,000–11,000/µL; platelets 1.5–4.5 lakh/µL. ESR by Westergren is normally below 15 mm/h in men and 20 mm/h in women. A peripheral smear shows cell shape and helps distinguish anaemia types.
Biochemistry: fasting plasma glucose 70–99 mg/dL; creatinine about 0.7–1.3 mg/dL; total bilirubin below 1.2 mg/dL; ALT and AST vary by laboratory. Always use the reporting laboratory's reference range, because methods differ.
Microbiology: Gram stain separates bacteria by cell-wall type; culture identifies the organism and guides antibiotics; Ziehl–Neelsen stain detects acid-fast bacilli; KOH mount shows fungal hyphae.
Urinalysis: dipstick screens for protein, glucose, ketones, blood, nitrite and leukocyte esterase; microscopy examines pus cells, red cells, casts and crystals. A midstream sample reduces contamination.
Worked example
A 30-year-old woman has Hb 9 g/dL, MCV 72 fL, WBC 7,000/µL and platelets 2.5 lakh/µL. The picture is a microcytic anaemia. Iron deficiency is most likely, so check ferritin; a low ferritin confirms it. Thalassaemia trait also causes microcytosis, but ferritin is usually normal and the Mentzer index (MCV ÷ RBC count) is often below 13. Do not start iron blindly without looking for the cause, especially in adults.
Traps that cost marks
Ignoring the laboratory's own reference range; diagnosing from a single abnormal value; forgetting that haemolysis falsely raises potassium, AST and LDH; and allowing delay before glucose estimation, which lowers the result.