Urinalysis and Body Fluids
Urinalysis and Body Fluids
This domain tests chemical dipstick interpretation, microscopic sediment analysis, and body fluid cell counts, including the corrections that separate true disease from artifact.
Core rules
- Dipstick protein reacts mainly with albumin; a positive protein with a benign sediment warrants a sulfosalicylic acid test for Bence Jones protein.
- Leukocyte esterase detects esterases from neutrophils; nitrite reflects nitrate-reducing Gram-negative rods. Both positive strongly suggests urinary tract infection but culture remains required.
- Specific gravity by refractometry is affected by protein and glucose; the reagent pad is not.
- RBC casts localize bleeding to the glomerulus; white cell casts suggest pyelonephritis or interstitial nephritis; broad waxy casts suggest chronic renal failure.
- In a traumatic tap, subtract the WBCs expected from blood contamination: expected WBC = CSF RBC × (blood WBC/blood RBC).
Worked example
CSF: RBC 6,000/µL, WBC 30/µL. Peripheral blood: WBC 8.0 × 10^3/µL, RBC 5.0 × 10^6/µL. Expected WBC from blood = 6,000 × (8,000/5,000,000) = 9.6/µL. Corrected CSF WBC = 30 − 9.6 ≈ 20/µL, still elevated, so a traumatic tap does not explain the pleocytosis. A decreasing RBC count across tubes 1 to 3 supports trauma; a constant count suggests hemorrhage.
Traps that cost marks
- Calling any positive nitrite a contamination rather than a probable infection.
- Interpreting squamous epithelial cells as inflammation.
- Failing to correct CSF WBC for blood contamination.
- Reporting crystals without noting urine pH, where the crystal type depends on it.