Urinalysis is a frequent laboratory test that offers a non-invasive look at the body’s metabolic and excretory processes. Urine specific gravity (USG) is a rapid indicator of the kidneys’ ability to manage water balance. This measurement shows how concentrated or dilute a urine sample is, reflecting overall fluid status and renal function. Understanding USG helps interpret a wide range of health conditions.
Defining Urine Specific Gravity
Urine specific gravity (USG) compares the density of a urine sample to the density of pure water. Pure water has a specific gravity of 1.000; therefore, any higher number indicates the presence of dissolved substances, or solutes, in the urine. These solutes include electrolytes (sodium and chloride), nitrogenous waste (urea), and organic molecules like protein or glucose.
The USG value acts as a proxy for the total concentration of particles excreted by the kidneys. A higher value indicates concentrated urine, meaning more particles are packed into the fluid. Conversely, a lower value suggests fewer dissolved particles, resulting in more dilute urine. The expected range for USG in adults typically falls between 1.005 and 1.030, though this varies based on hydration.
How the Measurement is Perform
Determining USG often begins with a simple screening method using a urine dipstick. This thin plastic strip has a color-sensitive pad that changes color when immersed in the sample. The color change corresponds to a specific gravity value, providing a quick, though less precise, estimate of concentration.
For a more accurate determination, laboratories frequently use a refractometer. This instrument measures how much a beam of light is bent, or refracted, as it passes through the urine sample. The degree of light refraction is directly proportional to the concentration of dissolved solids. Automated systems in larger medical laboratories can also perform this measurement efficiently alongside a full chemical analysis.
What High and Low USG Results Indicate
A USG reading outside the typical range suggests the kidneys are either aggressively conserving water or inappropriately excreting it. A high specific gravity, generally above 1.030, points to concentrated urine, meaning the body is trying to retain water.
Causes of High Specific Gravity
The most common reason for an elevated result is simple volume loss, such as dehydration caused by insufficient fluid intake, excessive sweating, vomiting, or diarrhea. Concentrated urine can also result from large molecules that increase density. High amounts of glucose in poorly managed diabetes artificially elevate the USG reading. Similarly, significant protein in the urine (proteinuria) can lead to a high specific gravity. Conditions causing the release of too much antidiuretic hormone, which prompts water retention, also result in a highly concentrated sample.
A low specific gravity, especially a reading of 1.003 or less, suggests the urine is maximally diluted. This is often a consequence of overhydration, where the kidneys are working to excrete surplus water after a person consumes excessive fluids. However, if the kidneys consistently produce a very dilute sample even when fluid intake is restricted, it may signal an issue with the organ’s concentrating mechanism.
Low USG is characteristic of conditions like diabetes insipidus, a disorder where the body either lacks antidiuretic hormone or the kidneys fail to respond to it. Certain kidney diseases, such as acute tubular necrosis or pyelonephritis, can also impair the kidney tubules’ ability to reabsorb water. In these cases, the low specific gravity indicates the kidneys have lost the capacity to adjust urine concentration based on the body’s needs.
Common Factors That Affect Accuracy
Several external or temporary physiological events can influence the USG reading, sometimes leading to results that do not accurately reflect the patient’s underlying fluid status or kidney function.
Factors Affecting Accuracy
Falsely High Readings: The recent injection of intravenous radiographic contrast dyes (used in CT scans or MRIs) temporarily increases urine density. Extremely high levels of glucose or protein also physically increase density, which can be misleading when using the dipstick method.
Falsely Low Readings: Certain medications, such as diuretics, promote water excretion. Their recent use results in a lower-than-expected specific gravity, independent of kidney disease.
For the most accurate assessment, medical providers often consider these temporary influences and may instruct a patient to avoid certain substances or restrict fluid intake before a USG test.