Urinalysis and Body Fluids
Reagent strip chemistry
Protein is detected by the protein error of indicators, so a highly buffered alkaline urine can falsely raise it and dilute urine can falsely lower it. Glucose uses glucose oxidase/peroxidase. Ketone pads (nitroprusside) detect acetoacetate but not beta-hydroxybutyrate, so diabetic ketoacidosis can read falsely low. Blood and leukocyte esterase rely on peroxidase-like activity; nitrite detects Gram-negative nitrate reducers and can be negative with frequent voiding or Gram-positive infection. Specific gravity by refractometry is more reliable than the reagent pad.
Microscopy
Identify RBCs, WBCs, epithelial cells, casts, and crystals. Hyaline casts are nonspecific; RBC casts indicate glomerulonephritis; WBC casts indicate pyelonephritis; fatty casts suggest nephrotic syndrome; broad waxy casts suggest chronic renal failure. Calcium oxalate (envelope), uric acid, triple phosphate/struvite (coffin lid), and cystine (hexagonal) crystals are the classic set.
Body fluids
Synovial fluid: gout crystals are needle-shaped and negatively birefringent; pseudogout crystals are rhomboid and positively birefringent. CSF: xanthochromia distinguishes subarachnoid hemorrhage from a traumatic tap. Serous fluids are classified as transudate or exudate using protein, LDH, and cell count.
Traps
- A traumatic tap clears between tubes; true hemorrhage does not.
- Squamous epithelial cells in a urine culture suggest contamination.
- Urine colony counts above 100,000 CFU/mL are classic but lower counts can be significant in symptomatic patients.
Rule of thumb
RBC casts plus dysmorphic RBCs plus proteinuria equals a nephritic picture, not a lower urinary tract bleed.