Diagnosing Urinary Tract Infections: Laboratory Tests

November 22, 2009

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Diagnosing Urinary Tract Infections: Laboratory Tests

by Lynn Cates, M.D., F.A.A.P.
reviewed by Laura Jana, M.D., F.A.A.P.

Whether or not your child has any signs or symptoms pointing to her urinary tract when she has a urinary tract infection (UTI), her healthcare provider will test her urine whenever he suspects one. Although the urine culture is the gold standard for diagnosing this condition, a routine urine analysis may suggest the presence of a urinary tract infection and, in some circumstances, other tests may be helpful in confirming the diagnosis, or gauging the severity of the infection.

Urine collection
Your child's urine cannot be evaluated accurately unless the specimen has been properly collected-that is, it has not been contaminated with the bacteria that normally reside on her perineum (that is, her genital and rectal area). A good culture should be obtained before antibiotics are started because once they have begun it will probably be impossible to tell whether she really had a urinary tract infection and what bacterium caused it. As you can imagine, it can be very difficult to collect a clean urine specimen from an infant or young child. Several techniques may be employed:

Bagging. One of the most common (and convenient) ways urine can be collected from an infant or young child is by bagging, or taping a bag over carefully cleansed genitalia (easier said than done in little girls and uncircumcised boys). Unfortunately, these specimens are easily contaminated, making it look like a child has a urinary tract infection when he does not have one. Also, as parents who have waited anxiously for their child to urinate know all too well, it may take a long time (even hours) before the child pees again. If a culture of a bagged urine specimen does not have any bacterial growth, a urinary tract infection can be ruled out. However, if bacteria do grow, it is impossible to know if it is because of an infection or just contamination, so the test must be repeated. For this reason, bagging is not recommended if antibiotics are to be started right away.

"Clean catch" midstream. Sometimes it is possible to get a good urine specimen from an older child by carefully cleaning the external genitalia and catching urine in a sterile container during midstream (that is, after the child has started urinating but before she is finished). Just as with a bagged specimen, if there is no bacterial growth, an infection can be ruled out. On the other hand, since the culture will have to be repeated if there is any bacterial growth, a "clean catch" urine is not recommended if antibiotics are going to be started right away.

Catheterization. A much better way to get a clean urine specimen is to insert a catheter (or narrow rubber tube) directly into the child's bladder from the outside through the urethra and collect the urine in a sterile container. There is a small chance that urine collected in this manner will be contaminated with bacteria from the skin, but the chance of contamination is much lower than in bagged or "clean catch" midstream specimens.

Suprapubic aspiration. The best way, by far, to obtain a non-contaminated urine specimen from infants is suprapubic aspiration (or suprapubic tap) of urine from the bladder. This procedure consists of inserting a needle directly into the bladder by sticking it carefully into the lower abdomen just above the pubic bone. Although this procedure sounds uncomfortable to parents, the needle stick lasts no longer than drawing blood or giving an immunization, and this procedure almost completely ensures a good specimen. It is particularly useful in boys with a very tight opening of the foreskin (or phimosis) which precludes inserting a catheter into their urethra. If any bacteria grow in a specimen obtained this way, the child has a urinary tract infection.

Urine tests
Once your child's urine has been collected, it is sent for a urine analysis and a urine culture:

Urine analysis. A urine analysis consists of looking for blood cells (both red blood cells and white blood cells, or pus cells), protein, sugar, and bacteria. The presence of bacteria, pus cells, or positive leukocyte esterase or nitrite tests suggest that a urinary tract infection may be present. However, these tests are not very accurate and cannot be used either to confirm or rule out a urinary tract infection.

Urine culture. A urinary tract infection can only be diagnosed for certain by culturing an appropriately collected urine specimen for bacteria. Because the culture depends on bacterial growth in the laboratory, the results usually are not available for at least a couple of days. This test reveals what kind of bacteria are causing the infection and what kinds of antibiotics are most likely to be effective.

Other laboratory tests
Other tests may be used in conjunction with urine tests to determine how sick your child is, whether it is more likely she has an upper or lower urinary tract problem, and whether the infection has spread to other parts of her body. In general, the more signs of inflammation, the more likely the infection involves the kidneys and/or has spread beyond the urinary tract. Some tests that are commonly employed to look for inflammation in general are a complete blood count (CBC), an erythrocyte sedimentation rate (ESR), and C-reactive protein (CRP). Her healthcare provider also may obtain a blood culture and even do a spinal tap to look for spread to the brain (meningitis). These tests are performed routinely in newborns with UTIs because they are less likely than older children to be able to confine the infection to the urinary tract--and they may have gotten the urinary tract infection from elsewhere in the body. A blood culture should also be performed if a child of any age has shaking chills with her urinary tract infection because this may be a sign of bloodstream infection (or sepsis).


RELATED INFORMATION

			Treatment Of Urinary Tract Infections
			Urinary Tract Infections: Overview
			Urinary Tract Problems

Created February 28, 2000
Reviewed March 12, 2001

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