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Urine Specific Gravity : Test, Results, Clinical meaning and Treatment

A urine specific gravity test is one of the urinalysis parameters, commonly used for the evaluation of kidney function and to aid in the diagnosis of various kidney diseases. This article explains the test, the different analysis methods, and what the results may indicate.



◉ Normal value of Urine specific gravity

Normal value ranges may vary slightly from lab to lab. Some labs use different measurements or test different samples. However, in general, the normal specific gravity value for urine is 1.003 to 1.030.

◉ Causes

◉ High urine specific gravity (Hypersthenuria)

Increased urine specific gravity may be due to conditions such as:

◉ Low urine specific gravity (Hyposthenuria)

Decreased urine specific gravity may be due to:

◉ Fixed specific gravity

In patients with certain chronic kidney diseases, the kidney cannot produce urine with a higher or lower concentration than protein-free plasma (1.008 to 1.012). This situation is called fixed urinary specific gravity or isosthenuria, the specific gravity does not vary with fluid intake

Note : The specific gravity value should always be interpreted in conjunction with other clinical information and individual patient circumstances.

◉ What is the test used for?

A healthcare professional may order the measurement of specific gravity of urine for several reasons. Here are some common prescribing goals:

◉ How to measure the specific gravity of urine

Different laboratories or health care providers may use different methods or equipment to measure the specific gravity of urine. It is important to use the same method consistently to maintain consistency of results and facilitate correct interpretation.

Before the test, a healthcare professional may advise the person to temporarily discontinue or avoid certain medications and foods that may affect the results.

There are several methods for measuring the urine specific gravity. Here are some commonly used methods :

◉ Measurement of urinary density by refractometry

It is considered the most accurate method. This method uses a refractometer, which measures the bending of light (refraction) as it passes through the urine sample.

Measurement of specific gravity of urine by refractometry

Refractometer

◉ Urinary specific gravity measurement by dipstick

An indicator changes color depending on the concentration of ions in the urine. However, dipstick urine specific gravity measurements can have limitations in terms of accuracy and reliability.

Procedure

Results

A Urine Specific Gravity test is read by comparing the color of the test strip to a color chart provided by the manufacturer.

urine specific gravity color chart

Urine specific gravity color chart

◉ Measurement of urine specific gravity by Urinometry

Urinometers are cylindrical glass or plastic tubes with a weighted float that measures the specific gravity of the urine sample. The higher the specific gravity, the deeper the float will sink into the urine.

◉ Measurement of urinary specific gravity by automated analyzers

Some modern laboratory instruments can automatically measure the urine specific gravity. These analyzers use different principles, such as refractometry or conductivity, to determine specific gravity.


◉ What can affect the result?

Other factors that may cause an inaccurate result include:

◉ Additional analysis

Additional testing may be required to identify the underlying cause of the abnormality.


Reference

  1. Urine specific gravity test
  2. Verena Gounden; Harshil Bhatt; Ishwarlal Jialal - Renal Function Tests
  3. Rugheed Ghadban, MD - Specific Gravity
  4. labpedia - Urine Specific Gravity and Its Significance
  5. Cathy Flasar - What is urine specific gravity?
  6. healthline - Urine Specific Gravity Test
  7. medicalnewstoday - What is a urine specific gravity test?
  8. Urine concentration test
  9. A S de Buys Roessingh, A Drukker, J-P Guignard - Dipstick measurements of urine specific gravity are unreliable