Urinalysis: looking into the void - includes related article, last part

FDA Consumer, Oct, 1989 by Dixie Farley

URINALYSIS Looking into the Void

In the past two issues, FDA Consumer reported on blood chemistry tests and the complete blood count as part of a series on in vitro diagnostics, tests performed on samples of tissue or fluid taken from the body. This is the concluding article.

One of the most familiar (if undignified) experiences during a visit to the doctor is that trip to the bathroom, cup in hand. Indeed, more than 36.7 million urinalyses were carried out in U.S. hospitals in 1988, making the urinalysis second only to the complete blood count as the most frequently performed group of tests.

The idea of examining urine for signs of disease is centuries old. When diabetes was suspected in a patient in ancient China, for instance, the physician would simply spill the urine sample on the ground, wait a while, and then check to see whether ants had gathered. Father of medicine Hippocrates also saw diagnostic value in watching bodily waters. He correctly observed that frothy urine is associated with kidney disease.

Today, urinalysis is more precise and systematic, thanks to increased knowledge of the body and such advances as the high-power microscope and chemical dipsticks. A single dipstick can be used to check for pH, sugar, protein, bilirubin (bile), ketones (waste products of the body's breakdown of fatty acids), and blood. The results are rapidly expressed as a color change. These tests and a visual inspection of the urine sample are easily done in the physician's office. A complete urinalysis, though, includes a weight comparison called urine specific gravity and a microscopic examination of small particles called urinary sediment. If the physician's office isn't set up to do these tests, the sample can be sent to an outside laboratory. Ranges of normal vary with such factors as patient age and sex and the equipment used, so it should be kept in mind that the following "normals" are only examples.

Visual Inspection

Measurement: Evaluation of the appearance and odor of urine. Normal range: Pale to dark yellow color, clear to slightly cloudy transparency, and a mild, faintly pungent odor. Significance of results: Urine appearance and odor can vary for reasons as serious as kidney failure or as harmless as last night's dinner. Numerous medications, such as sulfa drugs, can affect urine color. Examples of other changes and their possible causes are: * colorless--increased fluid intake or inability of the kidneys to concentrate minerals, salts, and other substances in urine (as occurs in diabetes insipidus); * cloudy--alkalinity, pus or bacteria; * opalescent--bacteria; fat due to kidney disease or a crushing injury, especially to the long bones; * yellow-orange--excess urine concentration due to fever or to bilirubin buildup caused by liver or gallbladder disease; * red--from eating foods such as beets or from blood in the urine due, for instance, to urinary tract injury or bladder cancer; * pungent, grass-like odor--asparagus consumption; * asparagus odor--kidney failure; * sharp, medicine-like odor--multivitamins in the diet; * pleasant, fruity odor--infection with Pseudomonas bacteria (also turns the urine blue-green), responsible for such diseases as meningitis, pneumonia, and a type of ear malady known as hot-weather ear.

Urine Specific Gravity

Measurement: Concentration of urine. Normal range: 1.003 to 1.030 specific gravity. Comments: Urine specific gravity is a comparison of urine weight with the weight of an equal volume of distilled water. Specific gravity increases in proportion to increases in the concentration of minerals, salts, and other substances in urine. The resulting measurement reflects how well the kidneys concentrate and dilute urine. Method: In one method, the operator sets afloat in the sample a small tubular device called a urinometer. Urine density determines the depth to which the device sinks, and a graduated measurement on the tube is read. The operator makes corrections for variables such as temperature and the presence of sugar or excess protein and then records the results. The devices can be inaccurate, however, so before their first use they must be checked using a solution with a known specific gravity.

The refractometer, another hand instrument, gives a more dependable test. Instead of measuring specific gravity, it measures what is known as the urine refractive index, a comparison of the velocity of light in urine to that in a vacuum. Like specific gravity, the refractive index increases in proportion to increases in the amount of dissolved substances in urine. For easy use by people unfamiliar with this measurement, the device is calibrated in terms of specific gravity. It requires only a few drops of urine, has a built-in temperature control, and provides a convenient, accurate and rapid test.

There also are automated specific gravity tests, including a dipstick test. These and the other dipstick tests can be performed on equipment connected to a computer to get a printout of the findings. Significance of results: High values can accompany reduced fluid intake, dehydrating conditions (such as fever and profuse sweating), ingestion of preservatives or X-ray contrast media, diabetes mellitus, shock, congestive heart failure, and certain tumors. Low values may be seen following use of diuretics and in exaggerated fluid intake, abnormally low body temperature, kidney disease, diabetes insipidus, and sickle cell anemia. Severe kidney damage causes urine specific gravity to remain fixed at 1.0010 to 1.0012.