From the Guidelines
Specific gravity on a urinalysis is a measure of the concentration of particles in urine, ranging from 1.002 to 1.030, and is used to assess hydration status, kidney function, and diagnose conditions like diabetes insipidus or nephrogenic diabetes insipidus. This value indicates how well the kidneys are concentrating or diluting urine. Normal specific gravity is generally between 1.005 and 1.030, with a reading of 1.000 representing pure water.
Key Points to Consider
- Low specific gravity (below 1.005) suggests dilute urine, which may occur with excessive fluid intake, diabetes insipidus, or kidney function problems.
- High specific gravity (above 1.030) indicates concentrated urine, potentially due to dehydration, heart failure, or conditions like syndrome of inappropriate antidiuretic hormone (SIADH).
- The measurement works because dissolved substances like electrolytes, glucose, and proteins increase urine density.
- Clinicians use specific gravity alongside other urinalysis parameters to assess hydration status, kidney function, and to help diagnose conditions.
- The test is performed using either a refractometer, urinometer, or reagent strips, with the refractometer method being most accurate. Some studies have found significantly lower morning urine specific gravity in children with enuresis compared with controls 1, whereas others have failed to find such a difference.
Clinical Application
- Specific gravity should be interpreted in context with a patient's clinical presentation, fluid intake, and other laboratory values for proper diagnosis.
- In the context of enuresis, first-morning urine specific gravity has been proposed as a specific test and as a way to predict which children might respond to DDAVP treatment 1.
- However, the relationship between specific gravity and enuresis is not fully understood and requires further research.
- A screening urinalysis, including specific gravity, is a crucial part of the diagnostic evaluation for enuretic children 1.
From the Research
Definition and Measurement of Specific Gravity
- Specific gravity is the ratio of the density of urine to that of water, and it is dependent on the number and weight of solute particles and on the temperature of the sample 2.
- The weight of solute particles in urine is constituted mainly of urea, chloride, sodium, potassium, phosphate, uric acid, and sulfate 2.
- Specific gravity can be measured by various methods, including weighing, hydrometer, refractometry, surface tension, osmolality, a reagent strip, or oscillations of a capillary tube 2.
Clinical Significance of Specific Gravity
- Urinary specific gravity is useful to assess the disorders of water balance and to discriminate between prerenal azotemia and acute tubular necrosis 2.
- The concentration of solids (or water) in urine can be used to evaluate the renal production of maximally concentrated urine and formation of dilute urine 2.
- Specific gravity provides a reliable assessment of the patient's hydration status 3.
- Criterion values for urine-specific gravity (USG) and urine color (UCol) can be used to represent adequate water intake in healthy adults, with a USG ≥1.013 detecting UOsm>500 mOsm/kg with high accuracy 4.
Relationship with Kidney Function and Health
- Dehydration, which can be assessed by specific gravity, may lead to chronic kidney disease through various mechanisms, including the effects of vasopressin on the kidney, the activation of the aldose reductase-fructokinase pathway, and the effects of chronic hyperuricemia 5.
- Increasing hydration, particularly with water, may actually prevent chronic kidney disease 5.
- Specific gravity measurement of urine can be useful in interpreting other findings of urinalysis, both chemical and microscopical, and in evaluating the disorders of water balance 2.