Urinalysis (UA) is a standard, non-invasive diagnostic test that analyzes a urine sample to provide a snapshot of a patient’s health. This routine examination involves physical, chemical, and microscopic assessments. Specific Gravity (SG) is a fundamental measurement within the chemical analysis, offering immediate insight into the body’s fluid balance and kidney function. Measuring SG evaluates how well the kidneys manage water and dissolved waste. The result helps healthcare providers determine if the urine is appropriately concentrated or overly diluted based on the body’s needs.
What Specific Gravity Measures
Specific Gravity (SG) measures the concentration of all dissolved substances (solutes) in the urine compared to the density of pure water. Since pure water has an SG of 1.000, any reading above this indicates the presence of particles like electrolytes, urea, and creatinine. SG reflects the kidneys’ ability to balance filtering waste and conserving or excreting water. Normally functioning kidneys adjust this concentration throughout the day based on fluid intake and loss.
SG is a direct indicator of the body’s hydration status when the sample was collected. A higher SG means the urine is concentrated, while a lower SG indicates a dilute sample. For a healthy adult, a random urine sample typically registers an SG between 1.005 and 1.030. This range shows the flexibility kidneys have in concentrating urine to conserve water or diluting it to eliminate excess fluid.
Methods Used to Determine Specific Gravity
Two primary methods are used in the clinical laboratory to determine the Specific Gravity value. The most precise technique uses a refractometer, a device that measures the refractive index of the urine sample. This method passes light through the urine and measures how much the beam bends as it interacts with dissolved particles. The degree of light refraction is directly proportional to the total concentration of solutes, providing an accurate, density-based SG reading.
The second, more common method uses a chemical reagent strip, or dipstick. This technique relies on a chemical reaction sensitive to the concentration of ions, such as sodium and chloride, rather than overall density. The dipstick contains a pad impregnated with a polyelectrolyte that changes its acid strength (pKa) based on the urine’s ionic concentration. This change triggers an indicator dye, like Bromothymol blue, to change color, which is then compared to a standardized chart to yield the SG result.
What Specific Gravity Results Mean
Interpreting the SG result is crucial for assessing fluid status and kidney health, particularly when the reading falls outside the 1.005 to 1.030 range.
High Specific Gravity (Hypersthenuria)
A high SG, generally above 1.030, indicates hypersthenuria, meaning the urine is highly concentrated. The most frequent cause is simple dehydration, where the body is actively conserving water, resulting in low urine volume and a high concentration of waste products.
Other factors can also elevate the SG by increasing the urine’s density. These include high levels of glucose, common in uncontrolled diabetes mellitus, or the recent injection of radiographic contrast dye. Heart conditions that reduce blood flow to the kidneys, such as congestive heart failure, may also cause excessive urine concentration as the body compensates for perceived low blood volume.
Low Specific Gravity (Hyposthenuria)
Conversely, a low SG, often below 1.005, is known as hyposthenuria and suggests very dilute urine. This finding can be benign, simply indicating overhydration due to excessive fluid intake, which prompts the kidneys to excrete the surplus water. Persistently low readings, however, may signal an impaired ability to concentrate urine, suggesting an underlying medical condition. Diabetes insipidus, caused by a problem with the hormone controlling water balance, is a classic cause of very low SG. Chronic kidney diseases, pyelonephritis, or acute tubular necrosis can also impair the concentrating ability of the kidney tubules, leading to a low SG even if the patient is dehydrated.
Fixed Specific Gravity (Isosthenuria)
A highly concerning and fixed SG result is known as isosthenuria, where the reading remains constant at approximately 1.010 to 1.012. This range mirrors the SG of the fluid initially filtered by the kidney. Isosthenuria indicates the kidneys have lost almost all ability to either concentrate or dilute the urine. This is a significant finding that suggests severe impairment of kidney function, commonly seen in advanced chronic kidney disease or acute kidney injury.