“Unnecessary” gonorrhoea tests

AUSTRALIAN women are being falsely diagnosed with gonorrhoea on the basis of unnecessary tests, often performed without their consent or their GP’s request, research suggests.

A study published online today by the MJA raises concern that pathology laboratories are now mostly using a dual nucleic acid amplification test (NAAT) for chlamydia and gonorrhoea rather than a single chlamydia detection test, even when GPs have only requested chlamydia testing. (1)

This is despite warnings from the manufacturers that NAAT testing has a low positive predictive value for gonorrhoea in low-prevalence settings such as the general community.

The study authors said there was likely to have been a “substantial number of false-positive results and unnecessary treatment” for gonorrhoea as a result.

Their study looked at gonorrhoea notifications for Victoria between 2008 and 2013 and gonorrhoea culture test results at the Melbourne Sexual Health Centre (MSHC) over the same period.

According to Medicare data testing with dual NAAT in Victoria rose 2.3-fold over the 6 years, with the proportion of positive tests remaining stable. The number of cases identified through NAAT (without culture) climbed from 98 in 2008 to 343 in 2013.

In the same period there was no observable increase in the proportion of women diagnosed by culture at the MSHC “Taken collectively, these data suggest that the prevalence of gonorrhoea among women remains very low and stable in Victoria (0.2%‒0.3%) and at the MSHC (0.4%‒0.6%), and that the rise in notifications is likely due to false-positive results”, the authors wrote.

The study also considered the cases of 25 women referred to the centre for culture testing after receiving a positive NAAT result for gonorrhoea. Only 10 of these women had a positive culture result (and of these nine presented with symptoms), with the authors suggesting that the remaining 60% could be false-positive results.

They concluded that positive NAAT results among low-risk women should be “regarded as doubtful” and followed up with confirmatory cultures.

“Currently, we are likely testing too many low-risk women, who are presumably attending a GP for chlamydia screening”, the authors wrote. They suggested that laboratories should suppress gonorrhoea results for women who were not at increased risk of the disease.

However, Dr Raymond Chan, speaking for the Royal College of Pathologists of Australasia, said suppressing positive results would be problematic.

“Some of these positive results are going to be true and we don’t know which ones”, he said.

The issue had generated much discussion among sexual health experts and microbiologists, Dr Chan told MJA InSight. He said many laboratories now routinely tested two targets before reporting a positive gonorrhoea result.

“If I was a GP and got a positive result back, I would phone the laboratory and ask if they had confirmed the result using a second target”, he said. “That doesn’t absolutely rule out the risk of a false-positive in a really low prevalence population, but it makes it less likely.”

Dr Chan also suggested it would be wrong to attribute most of the rise in gonorrhoea notifications among women over the past decade to false-positive results.

He said the 38 cases in the study identified by NAAT in 2008, rising to 86 cases in 2013, that were confirmed by culture, showed “clearly there has been a rise, although not nearly to the extent that NAAT testing alone suggests”.

Professor Basil Donovan, head of the sexual health program at the Kirby Institute at the University of NSW, told MJA InSight laboratories should suppress the NAAT gonorrhoea test result until the result of a confirmatory test on a second target was known but agreed that even then occasional false-positive results would still occur.

“Fortunately, for any individual GP this will be a rare event, and a third NAAT test or culture may help to sort out discrepant results.”

Professor Donovan said some increase in false-positive results was inevitable, with the increase in gonorrhoea testing.

However, he downplayed the authors’ suggestion that the finding that only 10 of 25 women attending MSHC with a positive NAAT had a positive culture result represented a false-positive rate of 60%.

“Some of those women might have spontaneously cleared their infection or may have taken antibiotics in the meantime that resulted in a negative culture”, he said, noting it was only a small number of cases.

The authors of an accompanying editorial in the MJA drew attention to the psychological impacts of false-positive test results, including anxiety and depression, feelings of guilt and self-blame, loss of self-esteem and self-confidence, feelings of social isolation and existential concerns. (2)

“Diagnosis of a sexually transmitted infection can also affect long-term sexual relationships, leading to concerns about trust and fidelity, and fear about disclosing results to a partner”, they wrote.

1. MJA 2015; Online 30 March
2. MJA 2015; Online 30 March

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