Evaluating the Predictive Value of Urine Tests for Urinary Tract Infections
Results from a new study could help refine approaches to diagnosing and treating uncomplicated urinary tract infections (UTIs) in healthy, premenopausal women. UTIs are very common and occur most often in women. The majority of UTIs are caused by the bacterium Escherichia coli (E. coli), but a number of other microbes can infect the bladder, as well. To help diagnose a woman who has UTI symptoms, a healthcare provider may ask her to provide a urine sample, which she collects into a sterile container while in the midst of voiding her bladder. Once collected, the urine can be tested for the presence of certain microbes—often by seeing what types and amounts of microbes will grow in a laboratory setting. However, these so-called “clean catch” midstream urine samples can be contaminated by microbes living near the opening through which urine leaves the body, potentially confusing the results. Also, there is some uncertainty about how well the values for microbial growth used for diagnosing UTIs from these samples actually reflect the presence or burden of UTI-causing microbes in the bladder itself.
To determine how well midstream void samples predict causative agents in UTIs, researchers recruited 202 healthy, premenopausal women with symptoms of acute UTI. They obtained two urine samples from each woman, one from the bladder (by using a catheter inserted into the bladder via the urethra, the tube that carries urine from the bladder to outside the body), the other from a midstream void, and then compared the presence and burden of microbes in these paired samples. The researchers found that UTI-causing microbes grew in 142 specimens of bladder urine and 157 specimens of voided urine. The majority (65 percent) of women with UTI symptoms had E. coli in their urine, and there was a strong correlation between voided and bladder urine, even at E. coli growth values below those currently used to diagnose UTI. In contrast, they found that presence in midstream urine samples did not accurately predict bladder infection by two other, commonly detected microbes thought to cause UTIs. These and other results suggest that testing of midstream void samples is best used for detecting and diagnosing the most common cause of UTIs, E. coli. This new information can help guide health care providers who treat UTIs. In addition, the fact that bladder urine samples from over a quarter of women with UTI symptoms did not yield any microbes under standard laboratory growth conditions serves as a reminder that there may be other microbes not identified by the tests, and/or non-infectious conditions with the same symptoms as UTIs.