NephrologyTier 2Topic

Normal urine composition

Composition

Normal urine is ~95% water, with dissolved urea (the major nitrogenous waste), creatinine, uric acid, electrolytes (Na, K, Cl), and minimal protein/glucose/blood
Composition varies substantially with hydration, diet, and time of day - a single spot sample reflects recent intake as much as underlying physiology

Normal values

Normal daily output: ~800-2000mL; urine osmolality 50-1200 mOsm/kg (wide range reflecting the kidney's concentrating/diluting capacity); specific gravity 1.003-1.030

Filtration and reabsorption

The glomerular filtration barrier (size- and charge-selective) normally excludes albumin and cells - their presence indicates a breach (glomerular disease) or overflow (e.g. light chains in myeloma, haemoglobin in haemolysis, myoglobin in rhabdomyolysis)
Minimal physiological proteinuria (<150mg/day, mostly Tamm-Horsfall protein/uromodulin) is normal and not detected by standard dipstick

Deviations

Any protein/blood/glucose/nitrite on dipstick is a deviation from this normal composition and prompts further characterisation (quantification, culture, or imaging as appropriate)
Glucosuria - normally absent (filtered glucose fully reabsorbed up to the renal threshold, ~10mmol/L plasma glucose) - its presence suggests hyperglycaemia exceeding the threshold, or a primary tubular defect (renal glycosuria, SGLT2 inhibitor effect)

Clinical use

Understanding normal composition is the baseline against which every urinalysis and 24-hour urine collection is interpreted

Physiology links

Glomerular filtration barrier physiology, tubular reabsorption physiology, SGLT2 inhibitor mechanism (therapeutic glucosuria)

Variation through the day

Urine composition is dynamic within the day (the first-morning sample is most concentrated) - relevant to how samples are collected and interpreted (e.g. first-morning albumin:creatinine ratio preferred for reproducibility)

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