A test that is routinely used by doctors to diagnose urinary tract infections wrongly gives a negative result in a fifth of cases, scientists have found.

The findings imply that a large proportion of women who seek medical help for UTIs such as cystitis are being misdiagnosed, with some being told their problem is psychological. Many women with severe symptoms are also likely to have been refused antibiotics.

Stefan Heytens, a practising GP and a researcher at the University of Ghent, said: “A substantial percentage of women visiting their GP with symptoms of a UTI, who test negative for a bacterial infection, are told they have no infection and sent home without treatment.”

Rather than using unreliable tests, he said, women should be diagnosed and treated on the basis of their symptoms.

Each year an estimated four million people, the vast majority women, will have a bout of cystitis. It typically involves bladder pain, an intense burning sensation when passing urine and the need to urinate urgently, sometimes several times an hour.

While unpleasant and inconvenient, cystitis normally clears without medicine within a couple of days. But if symptoms are severe or linger and antibiotics are required, GPs often carry out a “dipstick” test using a litmus-style indicator or send a urine sample to a microbiology lab, where it is cultured to see whether harmful bacteria are present.

In at least a fifth of cases, these tests come back negative, and doctors had been at a loss to explain what was wrong with this subset of patients. Some have been diagnosed with unexplained “urethral syndrome” while for others it has been suggested the root cause might be psychological.

The latest work proposes a third explanation: the tests are at fault.

The study, published in Clinical Microbiology and Infection, recruited 220 women who were visiting their GP for UTI symptoms and 86 healthy volunteers, all of whom gave urine samples.

The standard culture test detected bacteria in 81% of the samples. But a more advanced technique, designed to spot tiny quantities of bacterial DNA, found evidence of an infection in 98% of the women with symptoms.

Only about 10% of the healthy women tested positive for bacteria such as E. coli, suggesting that the results were not simply explained by trace levels of bacteria that are always present.

The authors are not sure why some infections failed to grow in culture. “The microbiologists in our institution do not have the slightest idea,” said Heygens. “They are just surprised that their test is not as infallible as they thought.”

However, he had this advice for doctors: “The woman that is visiting you with typical urinary complaints has an infection. There is nothing more to explore.”

The findings come after researchers concluded last year that cranberry juice, traditionally recommended by doctors as a natural cure, has no discernible effect on cystitis.

Prof James Malone Lee, who runs a specialist clinic for chronic UTIs at Whittington Hospital in London, highlighted the inadequacies of current testing in parliament last year. “The patients attending our centre describe frequent occurrences of them presenting with typical symptoms of urinary infection but being denied treatment because the tests are negative,” he said. “They are told emphatically that nothing is wrong.”

The consequences for the minority of women who suffer chronic infections could be devastating, he added. “It is appalling that patients coming to our centre have been told that their problems are psychological,” he said. “We have got to accept that our tests are discredited and we must start to consider what happens to those who go untreated because they tested negative.”

Prof Helen Stokes-Lampard, chair of the Royal College of GPs, said the vast majority of women who present at their GP surgery with a UTI are given a test aimed at identifying infection. “GPs rely on the results of these tests, so if [they are] ineffective in properly determining the type of infection – as this research suggests the urine test is – it is certainly concerning, and it needs to be addressed,” she said.