Urinalysis and Body Fluids
Urinalysis and body fluids tests chemical strip interpretation, microscopic sediment findings, and the analysis of cerebrospinal, synovial, pleural, and peritoneal fluids. The exam stresses the reasons a strip result can be misleading and the sediment findings that localize disease.
Collection and chemical testing
The first-morning midstream clean-catch specimen is preferred; random specimens are acceptable for screening. On the reagent strip, the protein pad reacts mainly with albumin and can miss light chains, so suspected Bence Jones proteinuria requires sulfosalicylic acid testing or urine protein electrophoresis. The glucose pad uses glucose oxidase and peroxidase and can be falsely negative with high ascorbate. The blood pad detects peroxidase activity of heme. Leukocyte esterase and nitrite are surrogate markers for urinary tract infection; nitrite can be falsely negative with Enterococcus, Staphylococcus, or very frequent voiding. Specific gravity by strip is affected by protein and ketones.
Microscopic sediment
Dysmorphic red cells and red cell casts indicate glomerular bleeding. White cell casts point to pyelonephritis. Hyaline casts are nonspecific and dissolve in alkaline or very dilute urine; granular casts suggest tubular injury. Crystals include calcium oxalate (envelope or dumbbell), uric acid, triple phosphate (coffin lid, alkaline urine), and cystine (hexagonal). Starch, hair, and air bubbles are common artifacts mistaken for red cells.
Other body fluids
CSF: cell count and differential, protein, glucose with a simultaneous serum glucose, and lactate. Bacterial meningitis shows neutrophilic pleocytosis, low glucose (CSF-to-serum ratio below 0.4), and high protein; viral meningitis shows lymphocytosis with normal glucose. Synovial fluid: monosodium urate crystals are needle-shaped and negatively birefringent in gout; calcium pyrophosphate crystals are rhomboid and positively birefringent in pseudogout; a septic joint typically exceeds 50,000 WBCs/µL.
Worked example
CSF shows 800 WBCs/µL with 90% neutrophils, protein 180 mg/dL, glucose 25 mg/dL against a serum glucose of 95 mg/dL (ratio 0.26), and lactate 5.0 mmol/L. The pattern is bacterial meningitis, and empiric therapy should not wait for culture.
Traps that cost marks
- Relying on the strip protein pad to detect light chains.
- Reading strips outside the stated time window.
- Ignoring a traumatic tap when interpreting CSF cell counts and protein.
- Confusing urate and pyrophosphate crystals under polarized light.