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What is urine specific gravity?

Flasar, Cathy APRN, BC, FNP, MSN

doi: 10.1097/01.NURSE.0000325315.41513.a0
Department: upFront: EYE ON DIAGNOSTICS
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Cathy Flasar is a family nurse practitioner and an assistant professor at the Goldfarb School of Nursing at Barnes-Jewish College in St. Louis, Mo.

URINE SPECIFIC GRAVITY (USG) measures the concentration of particles in urine and the density of urine compared with the density of water.1 Measuring USG is an easy and convenient way to gauge a patient's hydration status as well as the functional ability of his kidneys.2 It provides a fair estimate of urine osmolality if the urine doesn't contain appreciable amounts of protein, glucose, or other large molecules such as radiocontrast media. As ordered, you may perform specific gravity testing at the patient's bedside or send a urine specimen to the lab.

Who gets the test? Urine specific gravity measurements are indicated in many patients including those with alterations in fluid volume status, renal dysfunction, and certain conditions, such as diabetes insipidus.

Preparing the patient. When a special evaluation of specific gravity is ordered separately from the urinalysis, the patient should fast for 12 hours before specimen collection.3 Before you obtain a routine USG (which is part of a urinalysis), certain drugs, including those that increase USG, such as carbenicillin, I.V. albumin, and I.V. dextran, may need to be discontinued.3

Collecting the specimen. Collect the urine as a “clean-catch” or midstream specimen, or follow your facility's policies and procedures. For a regular urinalysis with USG, collect about 20 mL.3

If you're testing USG at the bedside, do it soon after collecting the urine, following facility policy. If testing won't be done right away, refrigerate the specimen; a delay of more than 2 hours can cause unreliable results.4

Performing the test. You can test USG with one of these methods:3

  • a refractometer or total solids meter, the most accurate method, can determine the density of the solution. Place a drop of urine on a clear glass plate of the urinometer; put another plate on top. The path of light deviates (refracts) when it enters the solution, giving you the refractive index, the ratio of the velocity of light in air to the velocity of light in the solution being tested. The degree of refraction is directly proportional to the density of the urine.
  • a multiple-test dipstick, which has a separate area with a reagent for USG. An indicator changes color based on the concentration of ions. Read the chart that accompanies the dipstick for the numerical value of the color you get to determine its USG value.
  • the hydrometer or urinometer, the least reliable method, which may become obsolete. Put a sample of urine (10 to 20 mL) in the small cylinder, then float the bulb-shaped instrument in the urine. Read the USG at the meniscus, using the

calibrated scale.3

Interpreting the results. Consistently using the same USG testing method for an individual patient is critical for evaluating trends. Results vary slightly by lab instruments and methods. Normal results in adults generally range from 1.010 to 1.020. Abnormal results are generally those below 1.010 or above 1.020. In patients with certain kidney diseases, USG doesn't vary with fluid intake and is called a fixed specific gravity.1

Causes of decreased USG include:

  • excessive fluid intake
  • diabetes insipidus and other causes of polydipsia, such as schizophrenia
  • glomerulonephritis
  • pyelonephritis
  • aldosteronism
  • edema from brain injury, including brain surgery.

Causes of increased USG include:

  • excessive water loss, especially in older or pediatric patients
  • diabetes mellitus
  • syndrome of inappropriate secretion of antidiuretic hormone
  • congestive heart failure.

What can affect the result? These factors can falsely decrease USG:

  • drugs such as lithium and diuretics
  • alkaline urine (with dipstick method).3

These factors can falsely increase USG:

  • drugs such as isotretinoin, I.V. albumin, and antibiotics including penicillin
  • moderate amounts of protein in the urine
  • residues of detergent on specimen containers
  • radiographic contrast media, minerals, and dextran (with refractometer method)
  • cold urine specimens (with hydrometer method).3

What care does the patient require? If a patient's USG is abnormal, he may need additional diagnostic testing.

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REFERENCES

1. Smeltzer SC, et al. Brunner & Suddarth's Textbook of Medical-Surgical Nursing, 11th edition. Lippincott Williams & Wilkins, 2007.
2. Porth CM. Essentials of Pathophysiology: Concepts of Altered Health States, 2nd edition. Lippincott Williams & Wilkins, 2006.
3. Fischbach FT. A Manual of Laboratory and Diagnostic Tests, 7th edition. Lippincott Williams & Wilkins, 2003.
4. Simerville JA, et al. Urinalysis: A comprehensive review. American Family Physician. 71(6):1153–1162, March 2005.
© 2008 Lippincott Williams & Wilkins, Inc.