Urine microscopy (sediment examination)

Principle

• Centrifugation concentrates formed elements (cells, casts, crystals, organisms) for microscopic identification and semi-quantitative reporting.

Specimen

• Fresh first morning or random urine examined within 2 hours, or refrigerated if delayed.

Reagents and equipment

• Conical tubes, centrifuge, slides and coverslips or standardized slide systems; optional Sternheimer–Malbin supravital stain.
• Brightfield microscope with reduced light; phase contrast and polarized light are helpful.

Procedure

• Mix well and centrifuge a standard volume (commonly 10–12 mL) at about 400–450 × g for 5 minutes.
• Decant to leave a fixed volume (e.g. 0.5–1 mL), resuspend gently and place a set volume on the slide.
• Scan with 10× objective for casts (especially near coverslip edges) and report per low-power field (LPF).
• Use 40× objective for cells and report RBCs and WBCs per high-power field (HPF), averaging about 10 fields.

Results

• Typical findings (local ranges apply): RBC 0–2/HPF, WBC 0–5/HPF, hyaline casts 0–2/LPF, few squamous epithelial cells.
• Casts: hyaline (normal, exercise), RBC (glomerulonephritis), WBC (pyelonephritis), granular (tubular damage), waxy (advanced renal failure, stasis), fatty (nephrotic syndrome), broad (chronic renal failure).
• Dysmorphic RBCs (e.g. acanthocytes) suggest glomerular bleeding.
• Normal acid urine crystals: uric acid, calcium oxalate, amorphous urates; normal alkaline: triple phosphate (coffin-lid), amorphous phosphates, calcium carbonate, ammonium biurate.
• Abnormal crystals: cystine (hexagonal plates), tyrosine, leucine, cholesterol, bilirubin, and drug crystals (e.g. sulfonamides).

Quality control and pitfalls

• Standardize volumes, speed and time so results are comparable.
• Casts and cells lyse in dilute, alkaline or old urine.
• Mucus threads, fibers, starch and oil droplets mimic casts or cells; yeast can mimic RBCs (RBCs lyse with dilute acetic acid, yeasts do not).
• Correlate with dipstick (e.g. blood positive but no RBCs suggests hemoglobinuria, myoglobinuria or lysed cells).

Clinical use

• Evaluation of kidney disease, UTI, hematuria, stones and metabolic disorders.