Urinalysis and Body Fluids
What to master
Work through physical, chemical, and microscopic phases. Specimen: first-morning midstream, timely analysis, appropriate preservative. Physical: color, clarity, and specific gravity by refractometry (correct for glucose and protein) versus reagent strip. Chemical pads and interferences: glucose oxidase (false negative with high vitamin C), protein dye-binding (false positive in very alkaline urine or with quaternary ammonium compounds), peroxidase blood pad (reactive with hemoglobin and myoglobin, false positive with bleach), leukocyte esterase, and nitrite, which requires about four hours of bladder dwell and a nitrate-reducing gram-negative organism.
Microscopic: dysmorphic red cells with red cell casts indicate glomerular bleeding; white cell casts indicate pyelonephritis; fatty casts indicate nephrotic syndrome. Know cast types (hyaline, granular, waxy) and crystals (calcium oxalate envelope, triple phosphate coffin lid, uric acid, hexagonal cystine). Body fluids: cell counts, crystal identification (monosodium urate needles with negative birefringence versus calcium pyrophosphate rhomboids with positive birefringence), CSF xanthochromia, pleural fluid by Light's criteria, synovial analysis, semen analysis, and amniotic fluid maturity tests.
Traps
Delayed analysis lyses cells and casts and alters pH; normal crystals are not disease; a negative nitrite does not exclude infection.
Worked example
Pleural protein 4.2 g/dL with serum protein 7.0 (ratio 0.6), pleural LDH 220 with serum LDH 180 (ratio 1.22). Both ratios exceed the Light thresholds of 0.5 and 0.6, so the fluid is an exudate.