There are a number of reasons why your tests can come back negative.
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1. Dipstick tests and mid-stream urine cultures are the standard tools GPs rely on to diagnose UTIs. Dipstick tests measure chemical signs of bacterial infection and the number of white blood cells in urine, while culturing aims to identify which bacteria are present and in what numbers by growing them in a laboratory.
However recent research suggests that these tests are failing UTI sufferers, missing 50 per cent of infections. Current standards for dipstick tests are based on research from the 1950s on a small study of pregnant women with severe kidney infections.
Experts increasingly believe that the threshold for diagnosing a bacterial infection has been set too high and infection exists even when there are relatively low numbers of bacteria in the urine.
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2. Women who have recently taken antibiotics will have reduced numbers of bacteria in their urine. If they have been drinking lots of water to try to flush out the infection this will dilute their urine. This can give a negative test result.
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3. Scientists’ understanding of bacteria has also changed. For urine cultures, strong growth of a single strain of bacteria must be found. A test result of “weak mixed growth” used to be thought to mean the sample had been contaminated, but it is now proven that infections are commonly caused by a mixture of strains of bacteria and also by types of bacteria that either produce very slowly (and therefore are missed) or will not reproduce in the presence of oxygen and so cannot be grown in a laboratory.
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4. The way that urine samples are collected and stored at the GP surgery before being sent for analysis is another reason why a test could come back negative. During this time the urine degrades and the bacteria die.
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5. Standard tests do not take into account cells from the bladder wall. These are found in greater numbers in the urine of chronic UTI sufferers as the bacterial infection weakens the skin or epithelium of the bladder.
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New research suggests that microscopy – analysing fresh urine under a microscope – combined with listening to a patient’s symptoms and story is the best way to diagnose UTI.
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