Urinalysis and Body Fluids
Core concepts
Urinalysis is physical (color, clarity, specific gravity), chemical (dipstick: pH, protein, glucose, ketones, blood, leukocyte esterase, nitrite, bilirubin, urobilinogen), and microscopic (RBCs, WBCs, epithelial cells, casts, crystals). Report formed elements per high-power field after centrifugation, and casts per low-power field. Body fluid analysis extends these principles: CSF cell counts and differential with glucose and protein compared to serum; synovial fluid crystal identification under polarized light (negatively birefringent monosodium urate versus positively birefringent calcium pyrophosphate); pleural fluid classification using Light's criteria.
Traps
- The protein pad is most sensitive to albumin and poorly detects globulins or Bence Jones protein; a sulfosalicylic acid test complements it.
- Ascorbic acid and high specific gravity can cause false-negative blood and glucose readings; delayed transport causes false-positive nitrite and bacterial overgrowth.
- Refrigeration precipitates amorphous urates that can be mistaken for cells; warm the specimen or examine it fresh.
- Report CSF glucose alongside a simultaneous serum glucose, since the normal ratio is about 0.6.
Worked example
Pleural fluid protein 3.2 g/dL with serum protein 6.0 g/dL gives a ratio of 0.53, above the 0.5 cutoff. Fluid LDH 200 U/L with serum LDH 150 U/L gives a ratio of 1.33, above the 0.6 cutoff. Both exudative criteria are met, so the fluid is classified as an exudate.