The combined nucleic acid amplification test (NAAT) Aptima Combo 2 (AC2) was more sensitive at detecting Neisseria gonorrhoeae (GC), from genital and extragenital sites, than culture using swabs transported in Amies medium before plating, in a retrospective UK study.

New GUM attendees at Bath and Bristol clinics between April 2006 and March 2008 were studied.

The sensitivity of AC2, which detects both GC and Chlamydia trachomatis in a single sample, was compared with culture results. Inclusion criteria were: patients diagnosed as having GC from at least one specimen with GC identified by AC2 and/or culture.

The demographics recorded included: age, gender, sexuality, ethnicity, sexual history, day of attendance, symptoms, signs and specimens taken.

For most asymptomatic men, a urine sample was taken for AC2 testing; a urethral swab was taken for microscopy and culture for those with symptoms; a rectal sample was taken as well in men who have sex with men.

Asymptomatic women had a self-sampled vulvovaginal swab and symptomatic women had a speculum examination with endocervical swab taken for both AC2 and microscopy and culture.

Of 20,908 new male attendees 251 were found to be GC positive (1.2%); of the 17,074 new female attendees, 105 (0.6%) were GC positive.

AC2 was consistently more sensitive than routine culture in identifying GC from genital and extragenital sites in both sexes (see table 1).

‘If dual testing NAATs are available, routine culture should be limited to those individuals in whom treatment will be offered unless they are symptomatic and male,’ the authors conclude.

The findings from this study are in keeping with others that concluded that AC2 is more sensitive than culture for detecting GC at genital and extragenital sites.

The poor sensitivity of cultures in this particular study was put down to storage of Amies samples for six hours or longer which would explain particularly poor performance on a Friday when specimens might not have beeen plated out for 72 hours in the laboratory. Most GUM clinics directly plate the samples on agar dishes; it was not the case in this study which means the results are generalisable to everyday general practice where this facility does not exist.

If larger studies confirm that NAAT testing for GC is more sensitive than using Amies swabs, then this would improve detection of gonorrhoea in general practice. It would mean less invasive tests for men and women; first catch urine would suffice in men and self-taken vulvovaginal swabs in women.

It would also mean more people could be tested for common STIs such as gonorrhoea and chlamydia in the community.

Dr Richard Ma