How can I tell if it’s really a UTI?

Q: I tend to get repeat UTIs, so last month when I felt a burning sensation when I urinated, I took a home test to see if it was another one, but the test came up clear. Then my symptoms got worse, so I retested and still got a negative result. I went to my doctor, and it turned out I did have a UTI. Why didn’t my self-test catch it?

A: Home test kits for a urinary tract infection (UTI) are very helpful, but there are a variety of factors that can skew the results. First, many of these tests are designed to detect the nitrites produced by bacteria such as E. coli. And while E. coli is responsible for more than 90 percent of UTIs, other types of bacteria that can cause these infections don’t produce nitrites. What’s more, at the first sign of an infection, many women increase their fluid intake in an attempt to flush out the bad bacteria. That’s a smart strategy, but it can dilute the urine, lowering nitrites to undetectable levels.

Treating UTI symptoms with over-the-counter remedies that are formulated with phenazopyridine hydrochloride (an analgesic that reduces pain, burning and itching) can also interfere with screening tests because these products often contain dyes that can skew test results. Finally, high levels of vitamin C in the body can react with indicator substances in the test strips and result in false negative results. So using the test after taking a vitamin C supplement, for instance, can prevent a urinary tract infection from being detected.

If this happens again, I suggest seeing your health-care provider if symptoms worsen or persist for 24 hours. As you’ve found, she can make a more accurate diagnosis with the aid of lab tests that check urine for the presence of white blood cells and substances that signal infection.