🔬 Urine Examination: A Practical Guide

📑 Contents

👁️ Appearance

Examination of the urine is a routine extension of the physical examination in all patients.

• Depending on concentration, normal urine is clear.
• Cloudy urine may result from high concentrations of leucocytes, epithelial cells, or bacteria.
• The appearance of urine may assist diagnosis at an early stage.
• Blood causes a pink to black discoloration, depending on the number of RBCs.
• Jaundice may cause dark yellow or brown urine.
• Haemoglobinuria and myoglobinuria cause dark urine that tests +ve for blood on dipstick.
• Normal urine tends to darken on standing.

Causes of coloured urine

👃 Odour

• Offensive urine usually denotes infection.
• Sweet urine suggests ketones.
• Certain rare metabolic diseases confer characteristic smells.

⚖️ Specific gravity

Refers to the weight of a solution with respect to an equal weight of distilled water. Normal range: 1.003 – 1.035 in urine.

💧 Osmolality

• Refers to the solute concentration of a solution.
• It cannot be measured with a dipstick.
• Low osmolality results from polyuria.

🧪 Urinary pH

• Urinary pH ranges from 4.5 to 8.0, usually 5.0 – 6.0.
• Most people pass acidic urine the majority of the time.

🍬 Glucose (glycosuria)

• Glycosuria results when tubular reabsorptive capacity for glucose is exceeded (>10 mmol/L).
• A valuable screening tool, but less useful for diagnosis and monitoring of diabetes mellitus.
• Renal glycosuria occurs when proximal tubular injury leads to failure to reabsorb filtered glucose.

🥚 Protein & proteinuria

• Urinary protein excretion should not exceed 150 mg/day, of which less than 20 mg is albumin.
• The remainder consists mainly of non‑serum‑derived tubular mucoprotein (e.g., Tamm‑Horsfall/uromodulin).
• Increased excretion of albumin is a sensitive marker of renal, particularly glomerular, disease.
• Protein excretion can be measured in untimed (“spot”) or timed (usually 24h) samples.

Proteinuria classification:

• Normal: <30 mg/g
• Microalbuminuria: 30–300 mg/g
• Overt proteinuria: >300 mg/g

🩸 Red blood cells (haematuria)

• Haematuria is defined as the presence of two RBCs per high‑powered field in urine.
• The amount determines whether it is visible to the naked eye (macroscopic) or requires dipstick/microscopy.
• Dipsticks detect haemoglobin – colour change (usually green). Detect as few as two RBCs per field.
• If dipstick is +ve, confirmatory microscopy is still desirable.
• Dipsticks remain +ve after RBC lysis, and also detect haemoglobinuria (intravascular haemolysis) and myoglobinuria (muscle breakdown).
• Glomerular bleeding: RBCs passing through inflamed/damaged glomeruli may show budding, spiculation, or surface irregularities (dysmorphic).
• Neutrophils (pyuria) – prominent in urinary infection but also present in inflammatory renal conditions (GN, TIN).
• Sterile pyuria – leucocytes seen consistently but culture sterile. Causes: partially treated UTI, calculi, prostatitis, bladder tumour, papillary necrosis, appendicitis.
• Lymphocytes – may be seen in prostatitis, cystitis.

🦠 Microorganisms & urine culture

• Bacteriuria: normal urine is sterile. Simultaneous presence of leucocytes suggests true infection.
• Fungi: Candida species most frequent – may result from genital contamination.
• Trichomonas: oval and flagellate (motile if alive) – usually a genital contaminant.
• Schistosoma haematobium: ova detection in endemic areas.

Urine culture: Culture and sensitivity differentiates contamination from true infection and guides treatment. A pure growth of >10 colony‑forming units (CFU)/mL is the conventional diagnostic criterion for UTI.

🧵 Casts

Casts are plugs of Tamm‑Horsfall mucoprotein within the renal tubules, with characteristic cylindrical shape. They are valuable clues to renal disease.

Non‑cellular casts

• Hyaline casts: mucoprotein alone, virtually transparent. Non‑specific, occurs in concentrated urine.
• Granular casts: granular material (protein or cellular remnants). Often pathological but non‑specific.
• Broad or waxy casts: waxy appearance – form in dilated, poorly functioning tubules of advanced CKD.

Cellular casts

• Red cell casts: virtually diagnostic of glomerulonephritis (GN).
• White cell casts: characteristic of acute pyelonephritis; also occur in tubulointerstitial nephritis (TIN).
• Epithelial cell casts: sloughed epithelial cells – non‑specific feature of acute tubular necrosis (ATN) or GN.
• Fatty casts: contain lipid‑filled tubular epithelial cells or free lipid globules. Maltese cross appearance under polarised light – occur in nephrotic syndrome. Clumped lipids = oval fat bodies.
• Other casts: any constituent (microorganisms, crystals, bilirubin, myoglobin) may become entrapped.

💎 Crystals

Detected by examining urine under polarised light. Most crystals are clinically irrelevant.

• Uric acid: lozenges with yellow‑brown hue, precipitate at acid pH. A few may be normal (high meat intake). Quantities may indicate hyperuricosuria; present in acute urate nephropathy.
• Calcium oxalate: monohydrated (ovoid) or bihydrated (pyramidal, like envelope). Prefer acid pH. A few normal (spinach, chocolate). Can denote hypercalciuria/hyperoxaluria. Diagnostic clue in ethylene glycol poisoning.
• Calcium phosphate: heterogeneous appearance (needles, prisms, stars). Favoured by alkaline pH. Risk factor for calcium stone formation.
• Triple phosphate (magnesium ammonium phosphate): coffin lid appearance – associated with urease‑producing bacteria (e.g., Proteus).
• Cystine: hexagonal crystals. Always significant (not normal). Marker of cystinuria.
• Cholesterol: thin plates with sharp edges – occur with heavy proteinuria.

Drug‑induced crystalluria

Many drugs precipitate in renal tubules and may cause AKI:

• Antibiotics: sulfadiazine, amoxicillin
• Antivirals: aciclovir, valaciclovir, famciclovir, ganciclovir, valganciclovir, indinavir
• Methotrexate
• Primidone (barbiturate)
• Triamterene
• Vitamin C (calcium oxalate deposition)