SIP

Subject Title: Clinical Chemistry

Name of Test: Urine specific gravity

The specific gravity of urine indicates the relative proportions of dissolved solid components to the total volume of the specimen. It measures the concentrating and diluting abilities of the kidney. Water has a specific gravity of 1.000. Since urine is a solution of minerals, salts and organic compounds in water, the specific gravity is greater than 1.000. The more concentrated the urine, the higher the urine specific gravity. Knowledge of the specific gravity is needed in interpreting the results of most tests performed in routine urinalysis. Specific gravity is measured using the refractometer and is determined by the principle of refractometry.

A refractometer

Principle of Test
An indirect method of measuring specific gravity is refractometry. The refractometer does this by measuring the refractive index of the solution. The refractive index of a solution is related to the content of dissolved solids present. It is the ratio of the velocity of light in air to the velocity of light in solution. This ratio varies directly with the number of dissolved particles in solution. The refractometer utilizes a light source, which pass through a chamber containing the specimen and a prism with scale readings. There is a direct relationship between the reading of the degree of refraction of light and the concentration of the specimen. This reading can be converted to specific gravity. Scale readings are in terms of specific gravity, refractive index and serum or plasma total protein. In my lab, we do not use refractometer to measure the total protein of patient sample but use Roche Modular analyzer to measure it. The refractometer is compensated to give correct readings on aqueous solutions at all temperatures ranging from 15°C-37°C. Readings need not be adjusted for either the temperature of the sample or the temperature of the room in which the refractometer is used.

Procedure

Before running any patient samples, we must first do quality control. In my lab, we use KOVA liqua-Trol level 1 (normal) and KOVA liqua-Trol level 2 (abnormal).

1) Label the test request form, urine container and a test-tube

2) Pipette a few ml of random urine into the labeled test-tube

3) Add a few drops of distilled water into the coverslip of the refractometer and clean it using gauze, ensuring the coverslip and prism are clean and dry

4) Add few drops of urine into the coverslip, making sure that no air bubbles are trapped within.

5) Read the specific gravity using the urine specific gravity scale. Avoid lifting the coverslip before the reading is made

6) Record the result on the test request form, which will be later keyed into the LIS

7) Add few drops of distilled water into coverslip again for washing off the urine and dry it using gauze. It is important that the refractometer is always maintained in a clean state

Reference Range

Normal values range from 1.003-1.030 but average is between 1.010 and 1.025. Specific gravity is highest in the first morning specimen, which is usually greater than 1.020 but not exceeding 1.030, due to the highly concentrated urine.

Clinical Interpretation
Low Specific Gravity
Diabetes insipidus, a rare disease caused by impaired functioning of the antidiuretic hormone (ADH), is the most obvious and severe example of the loss of effective concentrating ability of kidneys. This disease is characterized by large volume of urine with low specific gravity. Specific gravity in such case usually ranges between 1.001 and 1.003. Low specific gravity can also occur in patients with glomerulonephristis and pyelonephritis. Pyelonephritis is an ascending urinary tract infection that has reached the pelvis of the kidney. In these diseases, the kidney has lost its ability to reabsorb water due to tubular damage. Thus urine remains dilute. Urine with a fixed low specific gravity is indicative of severe renal damage with disturbance of both the concentrating and diluting abilities of the kidney.

High Specific Gravity
Specific gravity is high in patients when there has been excessive loss of water, as with
sweating, fever, vomiting and diarrhea. Increased secretion of ADH will cause increased tubular water reabsorption and decrease in urine volume. Hence it will have high specific gravity. Trauma, stress, infections, surgery and many drugs cause increased in ADH secretion. Moreover, abnormally high amounts of some of the urinary constituents, in particular glucose and protein, can increase specific gravity.

Urine specific gravity is used for monitoring of diseases and we can use urine osmolality test to further confirm the results. Osmolality is a more exact measurement of urine concentration as specific gravity depends on precise nature of the molecules in the urine and requires correction for the presence of glucose and protein. Urine specific gravity is easy and convenient to test.

Soong Ci Liang
TG01
0503333G