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Introduction ⬇

Specific gravity (SG) is a measurement of the kidneys’ ability to concentrate urine. The test compares the density of urine against the density of distilled water, which has an SG of 1.000. Because urine is a solution of minerals, salts, and compounds dissolved in water, the SG is a measure of the density of the dissolved chemicals in the specimen. As a measurement of specimen density, SG is influenced by both the number of particles present and the size of the particles. Osmolality is a more exact measurement and may be needed in certain circumstances.

The range of urine SG depends on the state of hydration and varies with urine volume and the load of solids to be excreted under standardized conditions; when fluid intake is restricted or increased, SG measures the concentrating and diluting functions of the kidney. Loss of these functions is an indication of kidney impairment.

Pretest Patient Care

  1. Explain purpose of test, procedure for urine collection, and interfering factors.

  2. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Posttest Patient Care

  1. Review test results; report and record findings. Modify the nursing care plan as needed. Counsel the patient regarding abnormal findings. Monitor appropriately for conditions associated with altered SG.

  2. Follow guidelines in Chapter 1 for safe, effective, informed posttest care.

Normal Findings ⬆ ⬇

Normal hydration and volume: 1.005–1.030 (usually between 1.010 and 1.025)

Concentrated urine: ≥1.025

Dilute urine: 1.001–1.010

Infant <2 years old: 1.001–1.006

Procedure ⬆ ⬇

There are three methods of determining SG:

  1. A multiple-test dipstick has a separate reagent area for SG. An indicator changes color in relation to ionic concentration, and this result is translated into a value for SG.

  2. A refractometer, or total solids meter, is used to determine the specimen’s refractive index, or the ratio of the velocity of light in air to the velocity of light in the test solution. A drop of urine is placed on a clear glass plate of the refractometer, and another plate is pressed on top of the urine sample. The path of light is deviated when it enters the solution, and the degree of deviation (refraction) is directly proportional to the density of the solution.

  3. A urinometer (hydrometer) is the least accurate method. It consists of a bulb-shaped instrument that contains a scale calibrated in SG readings. Urine (10–20 mL) is transferred into a small test tube–like cylinder, and the urinometer is floated in the urine. The SG is read off the urinometer at the meniscus level of the urine. This method is becoming obsolete owing to the ease of dipstick testing.

Specimen collection:

  1. For regular UA testing, about 20 mL of a random sample is needed (UA including SG).

  2. When a special evaluation of SG is ordered separately from the UA, the patient should fast for 12 hours before specimen collection.

Clinical Implications ⬆ ⬇

  1. Normal SG: SG values usually vary inversely with the amount of urine excreted (decreased urine volume = increased SG). However, this relationship is not valid in certain conditions, including:

    1. Diabetes—increased urine volume, increased SG

    2. Hypertension—normal volume, decreased SG

    3. Early chronic renal disease—increased volume, decreased SG

  2. Hyposthenuria (low SG, 1.001–1.010) occurs in the following conditions:

    1. Diabetes insipidus (low SG with large urine volume). It is caused by absence or decrease of ADH, a hormone that triggers kidney absorption of water. Without ADH, the kidneys produce excessive amounts of urine that are not reabsorbed (sometimes 15–20 L/d).

    2. Glomerulonephritis and pyelonephritis. SG can be low in glomerulonephritis, with decreased urine volume. Tubular damage affects the kidneys’ ability to concentrate urine.

    3. Severe kidney damage with disturbance of both concentrating and diluting abilities of urine. The SG is low (1.010) and fixed (varying little from specimen to specimen); this is termed isosthenuria.

  3. Hypersthenuria (increased SG, 1.025–1.035) occurs in the following conditions:

    1. DM

    2. Nephrosis

    3. Excessive water loss (due to dehydration, fever, vomiting, diarrhea)

    4. Increased secretion of ADH and diuretic effects related to the stress of a surgical procedure

    5. Heart failure

    6. Toxemia of pregnancy

Interfering Factors ⬆

  1. Radiopaque x-ray contrast media, minerals, and dextran may cause falsely high SG readings on the refractometer. The reagent dipstick method is not affected by high-molecular-weight substances. SG >1.040 suggests radiopaque contrast material is in the urine.

  2. Temperature of urine specimens affects SG; cold specimens produce falsely high values using the hydrometer.

  3. Highly buffered alkaline urine may also cause low readings (with dipsticks only).

  4. Elevated readings may occur in the presence of moderate amounts of protein (100–750 mg/dL) or with patients receiving intravenous albumin.

  5. Detergent residue (on specimen containers) can produce elevated SG results.

  6. Diuretics and antibiotics cause high readings.

  7. see Appendix E for drugs that affect test outcomes.