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Department: Diagnostic Update

What is urine specific gravity?

Flasar, Cathy APRN-BC, FNP, MSN

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doi: 10.1097/01.CCN.0000340988.20292.ba
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In Brief

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 kidney function.2 It provides an estimate of urine osmolality if the urine doesn't contain appreciable amounts of protein, glucose, or other large molecules. 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? USG measurements are indicated in many patients including those with alterations in fluid volume status.

Preparing the patient. When 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, may need to be discontinued.3

Collecting the specimen. Obtain a “clean-catch” or midstream specimen, or follow your facility's policies and procedures.

If you're testing USG at the bedside, do it soon after collecting the urine, following facility pol icy. 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. This method gives 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 USG value.
  • the hydrometer or urinometer, the least reliable method. A urine sample is placed in the small cylinder and the urinometer is floated in the urine. The USG is read at the meniscus.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
  • glomerulonephritis
  • pyelonephritis
  • aldosteronism.

Causes of increased USG include:

  • excessive water loss
  • diabetes mellitus
  • syndrome of inappropriate antidiuretic hormone secretion
  • 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
  • proteinuria
  • 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.

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.