Introduction False-positive results due to contamination of NAATs have been described. Apart from the laboratory, also the area where samples from patients are collected can be the source of the contamination.

Methods and results: In a 46 days period, 62 (7.3%) of male patients visiting the STI outpatient clinic with a low risk for gonorrhoea showed a positive NAAT (AC 2, Hologic-GenProbe) for Neisseria gonorrhoeae (NG) in urine. This was only 0.8% in the previous 6 months. The prevalence of positive NAAT results for Chlamydia trachomatis (CT) remained unchanged. Culture was positive in only 2/24 NG-NAAT-positive patients whose cultures were available. The prevalence of NG among high-risk patients as determined by culturing, and the positive NG-NAAT results from vaginal, rectal and pharyngeal swabs from the STI clinic and from urines received from other practises remained unchanged.

All 5 environmental swabs from the male bathroom and all 4 swabs from transport trays were positive in NG-NAAT, but only 1 of these 9 was positive for CT. Swabs from trays from the laboratory, routinely cleaned with chlorine, were negative. An audit showed that some clients do not deliver their urine samples in a hygienic way and employees who transferred urine into Aptima tubes might have touched the seal of these tubes.

The pseudo-outbreak ended after daily cleaning of bathrooms and trays with chlorine and strictly following anti-contamination guidelines. Afterwards only 0.2% of low-risk male patients had a positive NG-NAAT in urine. Thirty-seven patients who had been treated for gonorrhoea were informed about the possible incorrect diagnosis.

Conclusion This pseudo-outbreak was most likely a consequence of external contamination of trays and test tubes with nucleic acids from the sampling site, in combination with inadequate handling of tubes during pipetting.