Clinical Pathology and Body Fluid Analysis
Clinical pathology and body fluid analysis covers urinalysis, cerebrospinal fluid, pleural and ascitic fluid, and synovial fluid. The exam tests systematic description, quantitative chemistry and the reasoning that separates a transudate from an exudate or a traumatic tap from subarachnoid haemorrhage.
Core rules
- Urine dipstick: blood and leucocyte esterase are read with care; high glucose or ascorbic acid can blunt leucocyte esterase, while myoglobin also gives a positive blood pad.
- Hyaline casts are non-specific; red cell casts indicate glomerular bleeding; leucocyte casts suggest pyelonephritis.
- CSF: normal protein 0.15–0.45 g/L and glucose about 60% of plasma. Xanthochromia and uniform blood in all tubes indicate haemorrhage, not a traumatic tap.
- Light's criteria: pleural fluid is an exudate if protein ratio >0.5, LDH ratio >0.6, or fluid LDH above two-thirds the upper normal serum limit.
- Serum–ascites albumin gradient ≥11 g/L indicates portal hypertension.
Worked example
Pleural fluid protein 45 g/L (serum 70) and LDH 320 U/L (serum 180). Ratios: 45/70 = 0.64 and 320/180 = 1.8, both above threshold, so the fluid is an exudate by Light's criteria. That directs the differential toward infection, malignancy or pulmonary embolism; a transudate would have pointed to cardiac failure or cirrhosis. Always send paired serum samples at the same time.
Traps
Reporting a CSF cell count without correcting for blood contamination; calling synovial fluid normal without polarised microscopy for urate or pyrophosphate crystals; and interpreting a dipstick protein that reacts mainly to albumin and misses globulins, such as Bence Jones protein.