Urine Routine Examination
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Direct answer
Within an hour of voiding, a urine sample begins to change — bacteria multiply, cells lyse, casts dissolve — so a complete examination wants the first morning midstream specimen in a clean container, examined within an hour at room temperature or held a few hours at 2-8 degrees. The routine runs three layers: physical (colour, transparency, specific gravity 1.003-1.035), chemical (the dipstick panel of pH, protein, glucose, ketones, blood, bilirubin, urobilinogen, nitrite and leukocyte esterase) and microscopic (cells, casts, crystals and organisms in spun sediment). Each dipstick pad is a mini-reaction with known blind spots — the protein pad reads albumin and misses light chains, the ketone pad reads acetoacetate and ignores beta-hydroxybutyrate — and knowing those blind spots is the examination's real content.
What you must remember
- Collection: clean-catch midstream, first morning preferred (concentrated, acidic, best for nitrite and morphology); 10 mL suffices; 24-hour collections (with preservative per test) are for quantitative protein and creatinine clearance.
- Specific gravity 1.003-1.035 by refractometer or dipstick; fixed at about 1.010 (isosthenuria) indicates lost concentrating ability in chronic tubular damage.
- Protein pad: bromcresol-type indicator detects albumin preferentially — misses Bence Jones proteins and microalbuminuria; the sulfosalicylic acid test precipitates all proteins and is the confirmatory.
- Glucose pad: glucose oxidase — specific for glucose (negative in galactosaemia) but falsely lowered by ascorbic acid; ketones: sodium nitroprusside detects acetoacetate, not beta-hydroxybutyrate.
- Blood pad reacts to intact red cells, free haemoglobin and myoglobin — microscopy separates haematuria (cells present) from haemoglobinuria or myoglobinuria (pad positive, no cells).
- Nitrite (formed by nitrate-reducing gram-negative rods) and leukocyte esterase together screen urinary infection; nitrite needs bladder dwell time — the reason the first morning sample suits it.
- Sediment: more than 4-5 pus cells per high-power field is pyuria; red cell casts mean glomerulonephritis, white cell casts pyelonephritis, broad waxy casts chronic kidney disease.
- Crystals: calcium oxalate and uric acid in acidic urine, triple phosphate (staghorn calculi, Proteus) in alkaline urine, hexagonal cystine crystals in cystinuria.
A dipstick-microscopy mismatch
The most instructive urine reports are the ones where chemistry and microscopy disagree. A young man with cola-coloured urine after a sore throat has dipstick blood 3+ yet a sediment with only occasional red cells: free haemoglobin or myoglobin is crossing the pad's peroxidase chemistry. The next observation resolves which — the plasma. Pink plasma means intravascular haemolysis (haemoglobin escaped the red cells, spilled through glomeruli); clear plasma with dark urine points to myoglobinuria from rhabdomyolysis, and a creatine kinase level settles it. Post-streptococcal glomerulonephritis enters the differential with dysmorphic red cells and red cell casts on a repeat, properly collected sample.
The second mismatch: a 60-year-old with back pain has dipstick protein trace but sulfosalicylic acid testing showing heavy turbidity 3+. The dipstick under-reports because the excess protein is not albumin — it is monoclonal light chains, Bence Jones protein. Classically these proteins precipitate on heating around 40-60 degrees, dissolve at boiling, and reappear on cooling; the modern route is serum protein electrophoresis and free light chain assay, but the heat behaviour remains an examination favourite. Both mismatches teach the same lesson: the dipstick is a screening instrument whose negatives and weak positives must be read against the sediment, the plasma and the patient.
Where students slip
Reporting "protein: negative" on a myeloma patient's urine is the single classic trap — the dipstick is albumin-biased and light chains pass invisible; suspicion of paraproteinaemia demands the sulfosalicylic acid test or electrophoresis. Second, a diabetic taking vitamin C whose glucose pad reads negative while sugar appears in the reducing-substance test — ascorbate suppresses the peroxidase-coupled colour reaction; the candidate must know the interference rather than blame the patient. Third, calling heavy squamous epithelial cells and mixed flora a "urinary infection" — that is a contaminated perineal collection; the correct action is instructions and recollection, not a culture.
Frequently asked questions
Why is the first morning midstream specimen preferred?
It is concentrated, slightly acidic, richest in formed elements, and provides the bladder dwell time nitrite testing needs; midstream flushing reduces perineal contamination.
Which proteins does the dipstick miss and how are they caught?
Bence Jones light chains and low-level albumin; sulfosalicylic acid turbidity, urine protein electrophoresis and the albumin-to-creatinine ratio detect them.
What is the heat characteristic of Bence Jones protein?
Precipitation at 40-60 degrees Celsius, dissolution at 100 degrees, and reappearance on cooling — the classical bedside confirmation.
What do red cell and white cell casts indicate?
Red cell casts signal glomerulonephritis; white cell casts signal pyelonephritis or interstitial nephritis — localisation of urinary infection or inflammation.
Which crystals favour alkaline rather than acidic urine?
Triple phosphate (struvite) and amorphous phosphate crystals; calcium oxalate and uric acid favour acidic urine, and hexagonal cystine crystals appear in cystinuria.
What does a fixed specific gravity of 1.010 mean?
Isosthenuria — the tubules have lost the ability to concentrate or dilute, typical of advanced chronic kidney disease.