Why the Doctor Asks for a Urine Sample

It's probably safe to say that no one really likes giving a urine sample. But the fact is that few tests can match the routine urine analysis for telling your doctor about what's going on inside your body.

The bladder can hold almost 2 cups of urine for two to five hours comfortably, and excretes about 2 quarts of waste products and additional water daily. And just as you use a dipstick to check your car's engine oil, doctors rely on a specially treated "dipstick," a plastic stick that has segments imbedded with different chemical indicators on it, to test urine. Unusual amounts of certain substances in your urine can indicate conditions ranging from minor urinary tract infections to diabetes and other more serious conditions.

One portion of the dipstick, for instance, measures the specific gravity (a measure of the amount of substances dissolved in the urine). The higher the specific gravity, the more concentrated the urine is. Using a numerical scale, the dipstick shows whether you're drinking enough fluids and how well your kidneys are filtering wastes from your body. Very concentrated urine suggests you are probably not drinking enough fluids, making it harder for your body to flush wastes. Urine is normally darker--more concentrated--in the morning, because your body's been storing wastes all night.

The dipstick registers the presence of a variety of chemicals that the kidneys filter, such as glucose (sugar) or minerals. Healthy people generally don't excrete sugar in their urine. Its presence may mean that you have diabetes or another condition that prevents your body from efficiently converting blood sugar into energy.

The urine test also can indicate infections by measuring nitrites and leukocytes. Bacteria produce an enzyme that converts nitrates, which are normally present in urine, to nitrites. Leukocytes are the white blood cells that fight infection. The presence of either indicates that your body may be fighting an infection in the bladder or somewhere else along the urinary tract. Red blood cells in the urine can be a sign of kidney stones, tumors, or other abnormalities, but further tests are needed to confirm these findings.

The dipstick gauges protein and pH levels. Urine shouldn't contain much, if any, protein, although some perfectly normal people do excrete tiny amounts. Low levels of protein in the urine can occur in normal patients for a variety of reasons, but can also be a sign of early or chronic kidney disease. Significantly high urine protein levels are always a sign of kidney disease. Urine pH, or acidity, normally ranges between 4.5 and 8, with most people's urine falling between 5.5 and 6.5. Urine pH can be affected by diet, drugs, and certain kidney problems. 

Ketones can be detected by dipsticks. They are substances produced when fat is used by the body for energy. They are eliminated from the body in the urine. Large amounts of urinary ketones indicate a diet low in carbohydrates, starvation, or, in a person with diabetes, a dangerous condition known as diabetic ketoacidosis.

Often urine tests can help monitor these and other illnesses for improvement or worsening.

Several other tests can be done on a urine sample without the dipstick. Urine pregnancy tests at home or in the office are common. Illicit (illegal) drug tests on urine are done in the workplace or in the health care provider's office. At times your health care provider will send a urine sample to a lab for a culture or microscopic evaluation. The urine culture will determine the specific bacteria and sensitivities to antibiotics for a bladder infection. This may be done if your doctor thinks you may have a more complicated bladder or kidney infection. Microscope evaluation can help determine types or causes of kidney dysfunction. Other tests can be very specific, such as a 24-hour urine collection for total protein, or for specific hormones.

Contact your doctor if you have any new urinary symptoms or changes in your urine.