Urine Specific Gravity
USG· also: SG, Specific Gravity, Urine SG
Clinical Overview
Urine specific gravity (USG) measures the concentration of solutes in urine relative to pure water (specific gravity 1.000). A higher specific gravity indicates more concentrated urine (more solutes); a lower value indicates more dilute urine. USG reflects the kidney's ability to concentrate and dilute urine — one of the most fundamental tests of tubular and kidney function. It is measured as part of routine urinalysis by refractometer or dipstick.
Why This Test Matters
USG is a simple proxy for urine osmolality and hydration status. Critically ill patients with polyuria (excessive urine output) can be rapidly assessed: USG <1.005 with polyuria suggests central diabetes insipidus (DI), nephrogenic DI, or excessive fluid intake. USG fixed at 1.010 (isosthenuria — urine always the same specific gravity as plasma) indicates severe renal tubular dysfunction unable to concentrate or dilute urine — typically seen in advanced CKD. USG also affects interpretation of other urinalysis parameters: a highly concentrated urine (USG >1.025) may produce false-positive dipstick results for protein and leukocyte esterase, while a dilute urine may cause false-negative results. USG is a poor substitute for formal osmolality measurement in precise clinical decisions — urine osmolality should be measured when DI or SIADH is suspected.
Reference RangesWHO/IFCC standards
| Reference Range | Unit | Notes |
|---|---|---|
| 1.005 – 1.03 | g/mL | Random specimen; normal range varies with hydration status |
What Causes Abnormal Results?
High USG Causes
- Dehydration (most common cause of high USG)
- Syndrome of Inappropriate Antidiuretic Hormone (SIADH — inappropriately concentrated urine)
- Heart failure, cirrhosis, nephrotic syndrome (decreased effective circulating volume stimulates ADH)
- Glycosuria (glucose increases osmolality — USG rises more than expected from other solutes)
- Proteinuria (heavy protein content raises USG)
- Contrast media (radiocontrast causes marked but transient elevation in USG)
Low USG Causes
- Overhydration or excessive fluid intake (diluted urine)
- Diabetes insipidus (central or nephrogenic — inability to concentrate urine)
- Early CKD with impaired tubular concentrating ability
- Psychogenic polydipsia (compulsive water drinking)
- Medications: diuretics, alcohol, lithium (causes nephrogenic DI)
Signs & Symptoms to Watch For
How to Prepare for This Test
USG is measured on a fresh random urine sample as part of routine urinalysis. For accurate assessment of concentrating ability, first morning urine (after overnight water restriction) should have USG ≥1.020 in a healthy individual. If evaluating DI, formal water deprivation test with vasopressin measurement and urine osmolality are needed.
Factors That Can Affect Results
- Glucose and protein significantly raise USG above what the kidney's water-handling would predict (glucose: +0.004 per 1 g/dL; protein: +0.003 per 1 g/dL)
- Radiocontrast media massively elevate USG for hours after IV contrast administration
- Dipstick method less accurate than refractometer — does not detect glucose or contrast effects accurately
- Neonates have limited concentrating ability — USG 1.001–1.020 is normal in newborns
Related Topics
Frequently Asked Questions
What does a very low specific gravity of 1.001 mean?
USG of 1.001–1.003 represents maximally dilute urine and indicates the kidney is excreting large amounts of free water. The most common cause in an outpatient is excessive fluid intake (overhydration) or recent diuretic use. However, in a patient with polyuria (producing large volumes of very dilute urine day and night), this suggests diabetes insipidus (DI) — either central DI (deficiency of ADH/vasopressin from pituitary damage) or nephrogenic DI (kidneys cannot respond to ADH, e.g., from lithium toxicity, hypercalcemia, or hereditary mutation). A formal water deprivation test distinguishes DI subtypes and confirms the diagnosis.
Can I use urine specific gravity to tell if I am dehydrated?
USG is a reasonable rough guide to hydration. USG <1.010: well-hydrated or overhydrated. USG 1.010–1.020: normal/adequate hydration. USG >1.020: concentrated urine — suggests you need more fluids, especially if you also notice dark yellow urine. USG >1.025–1.030: significantly concentrated — a sign of dehydration in most circumstances. However, high protein intake, glucose in urine, or recent radiocontrast can falsely elevate USG independent of hydration. For sports performance, military, and clinical hydration assessment, urine osmolality (>300 mOsm/kg indicates dehydration) is more precise than USG.