Specific gravity measures the kidney's ability to concentrate or dilute urine in relation to plasma. Because urine is a solution of minerals, salts, and compounds dissolved in water, the specific gravity is greater than 1.000. The more concentrated the urine, the higher the urine specific gravity. An adult's kidneys have a remarkable ability to concentrate or dilute urine. In infants, the range for specific gravity is less because immature kidneys are not able to concentrate urine as effectively as mature kidneys.
Glucose, protein, or dyes used in diagnostic tests excreted into the urine increase the specific gravity. If none of these abnormal substances are present in the urine, there are two primary reasons why the kidney is producing concentrated urine with a high specific gravity. The first and most common reason for an increase in urine specific gravity is dehydration. The second reason for a high specific gravity is an increased secretion of anti-diuretic hormone (ADH). ADH causes increased tubular water re-absorption and decreased urine volume. Trauma, stress reactions, surgery, and many drugs cause an increase in ADH secretion.
A low specific gravity occurs in three situations.
It is important for the nurse to be aware of the physiological dynamics of surgical diuresis. After a major surgical procedure that produces high physiologic and psychological stress, increased secretion of antidiuretic hormone causes fluid retention within the vascular space. As stress after surgery decreases, ADH and other hormones, such as glucocorticosteroids, begin to drop to normal values, and the fluid that was held in reserve is excreted. This increase in urine volume a few days after surgery is sometimes referred to as a surgical diuresis. It is important for nurses to consider this type of fluid retention and related increase in urine specific gravity in the immediate post-operative patient to avoid excessive fluid replacement.