🔬 Urinalysis: A Complete Guide
Appearance, dipstick tests, microscopy, casts, crystals, and interpretation of findings.
📑 Contents
👁️ Appearance
Depending on concentration, normal urine is clear or given a light yellow hue by urochrome and uroerythrin pigments.
- Cloudy urine – may result from high concentrations of leucocytes, epithelial cells, or bacteria. Precipitation of phosphates can also produce turbidity in urine refrigerated for storage.
- Specific circumstances result in characteristic changes in appearance that may assist diagnosis at an early stage.
- Blood causes a pink to black discoloration, depending on the number of RBCs and length of time present.
- Jaundice (conjugated hyperbilirubinaemia) may cause dark yellow or brown urine.
- Haemoglobinuria (intravascular haemolysis) and myoglobinuria (muscle breakdown) both cause dark urine that tests +ve for blood on dipstick. If centrifuged, supernatant remains coloured and tests +ve, but no RBCs on microscopy.
- Normal urine tends to darken on standing (urobilinogen oxidises to coloured urobilin).
- Chyluria – milky urine, settles into layers; results from lymphatic‑urinary fistula (malignancy, lymphatic obstruction).
- Beetroot can produce red urine (betalaine pigment) in genetically susceptible individuals.
Causes of coloured urine
Urine that darkens on standing
- Alkaptonuria (homogentisic acid), imipenem‑cilastatin, melanoma (melanogen), methyldopa, metronidazole, porphyria (porphobilinogen).
👃 Odour & Pneumaturia
Offensive urine usually denotes infection (bacterial ammonium production).
Sweet urine suggests ketones.
Certain rare metabolic diseases confer characteristic smells (e.g., maple‑syrup urine).
Pneumaturia (air bubbles in urine) suggests a vesicocolic fistula (diverticular disease, colonic cancer, inflammatory bowel disease).
⚖️ Specific Gravity & Osmolality
Specific gravity (normal 1.003–1.035) – estimated by dipstick, accurate with urinometer.
Osmolality (range 50–1350 mOsmol/kg) – central role in plasma osmolality regulation. In absence of significant glycosuria, Na⁺, Cl⁻, and urea are main determinants.
Low osmolality is useful in polyuria and hyper‑/hyponatraemic states. Recurrent stone formers can monitor specific gravity to maintain dilute urine.
🧪 Urinary pH
Range 4.5–8.0 (usually 5.0–6.0). Most people (except vegans) pass acidic urine. Main clinical use: investigation of systemic metabolic acidosis – a fall in pH to ~5 is expected; failure suggests renal tubular acidosis.
📊 Dipstick Tests
Common parameters: specific gravity, pH, leucocytes, nitrites, glucose, urobilinogen, bilirubin, ketones, protein/albumin, blood.
Leucocyte esterase & nitrites
Indicators of infection. Nitrite test exploits bacterial reduction of nitrate → nitrite (requires dietary nitrate and bladder dwell time >4h). Good specificity but modest sensitivity. Not a substitute for microscopy and culture.
Bilirubin and urobilinogen
Conjugated bilirubin (water‑soluble) → excreted in urine in hepatic/cholestatic disease. Absence of urobilinogen in a jaundiced patient suggests biliary obstruction.
Glucose
Glycosuria occurs when plasma glucose >10 mmol/L (tubular reabsorptive capacity exceeded). Renal glycosuria from proximal tubular injury.
Ketones
Dipsticks detect acetoacetate (not β‑hydroxybutyrate). Positive in DKA, prolonged fasting, alcoholic ketoacidosis, severe volume depletion, isopropyl alcohol poisoning.
🥚 Proteinuria
Normal urinary protein excretion <150 mg/day (<20 mg albumin). Increased albumin excretion is a sensitive marker of glomerular disease.
Dipsticks – convenient, highly specific, less sensitive. Detect albumin >300 mg/L. Microalbuminuria (30–300 mg/day) requires ELISA or radioimmunoassay.
| Result | Estimated quantity (g/L) |
|---|---|
| Trace | 0.15–0.3 |
| + | 0.3 |
| ++ | 1.0 |
| +++ | 2.5–5.0 |
| ++++ | >10 |
If dipstick ≥1+, repeat after 1–2 weeks. If persistent, verify with uPCR or uACR (spot urine). uACR and uPCR correct for concentration – first morning specimen preferable. uPCR ×10 approximates 24h excretion in g/day.
Nephrotic range (≥3.5 g/24h). For timed 24h collection: discard first void, collect all subsequent including overnight, add first void next day.
🩸 Haematuria
Defined as ≥2 RBCs per high‑powered field (hpf) in spun urine. Dipstick detects haemoglobin (also myoglobin). Confirm with microscopy.
Glomerular bleeding – dysmorphic RBCs (budding, spiculation).
Non‑glomerular bleeding – normal morphology.
🔬 Urine Sediment & Microscopy
Procedure: discard first few mL, collect 20 mL, centrifuge 10 mL at 400g for 10 min, remove 9.5 mL supernatant, resuspend pellet, examine unstained with phase contrast.
Cellular elements: neutrophils (infection, GN, TIN), sterile pyuria (partial treatment, calculi, prostatitis, TB, TIN), lymphocytes (chronic TIN), eosinophils (TIN, RPGN, atheroemboli), renal tubular cells (ATN, TIN), squamous epithelial cells (contaminant), transitional cells (cystitis), malignant cells.
Microorganisms: bacteria (gram stain), Candida, Trichomonas, Schistosoma haematobium ova.
🧵 Casts
Casts are Tamm–Horsfall mucoprotein plugs. Non‑cellular: hyaline (concentrated urine), granular (pathological but non‑specific), broad/waxy (advanced CKD). Cellular casts: RBC casts (diagnostic of GN), WBC casts (pyelonephritis, TIN), epithelial cell casts (ATN, GN), fatty casts (nephrotic syndrome – Maltese cross under polarized light).
💎 Crystals
Detected by polarised light. Common types: uric acid (lozenges, acid pH), calcium oxalate (monohydrate/bihydrate, ethylene glycol poisoning), calcium phosphate (alkaline pH), triple phosphate (magnesium ammonium phosphate), cystine (hexagonal, always significant), cholesterol (thin plates, heavy proteinuria).
Drug‑induced crystalluria – sulfadiazine, amoxicillin, aciclovir, methotrexate, triamterene, vitamin C (calcium oxalate).
📏 Determining GFR
GFR is the best overall measure of kidney function. Normal values ~120–125 mL/min. Useful for staging CKD, monitoring progression, drug dosing. Measured via clearance of markers (inulin = gold standard, but not routine). Clinically estimated by serum creatinine, eGFR formulae (CKD‑EPI, MDRD), creatinine clearance from 24h urine, or isotopic clearance (EDTA‑GFR, DTPA‑GFR).
Ideal clearance marker must be freely filtered, not protein‑bound, no extra‑renal elimination, not reabsorbed/secreted/metabolised by kidney.