Urinalysis and Body Fluids
Collection and physical examination
First-morning midstream specimens concentrate solutes. Assess color, clarity, and specific gravity (roughly 1.003–1.030), which reflects hydration and concentrating ability.
Chemical and microscopic examination
Dipstick protein detects mainly albumin; it misses light chains, so use sulfosalicylic acid or immunofixation. Nitrite is specific for nitrate-reducing Enterobacterales but insensitive; leukocyte esterase detects neutrophils. Ascorbic acid causes false-negative blood and glucose. Microscopy identifies RBCs (dysmorphic in glomerular bleeding), WBCs, epithelial cells, casts (hyaline, granular, RBC, WBC), and crystals (calcium oxalate, uric acid, triple phosphate, cystine).
Body fluids
CSF: normal glucose is about 60% of serum and protein 15–45 mg/dL. Bacterial meningitis shows high neutrophils, low glucose, and high protein. Serous fluids: Light's criteria (pleural fluid/serum protein >0.5, LDH >0.6, or LDH above two-thirds the serum upper limit) identify exudates.
Traps
- A negative nitrite does not exclude urinary tract infection.
- Hyaline casts are nonspecific; RBC casts indicate glomerulonephritis.
- Amorphous urates can be mistaken for cells.
- Ascorbic acid and high specific gravity interfere with dipstick readings.
Rule of thumb: correlate dipstick, microscopy, and chemistry - a discrepancy usually signals an interfering substance or a nonrenal source.