NEW YORK – A recent study of Abbott's RealTime CT/NG assay has supported the potential of the test for use with rectal and pharyngeal samples to detect Chlamydia trachomatis and Neisseria gonorrhoeae infections.
The study, published in the Journal of Molecular Diagnostics, reported the analytical performance of Abbott's test with such samples. Rectal infections are associated with increased risk of HIV, and oropharyngeal infections can be a source of gonococcal antimicrobial resistance, the researchers noted. The US Centers for Disease Control and Prevention estimated 1.7 million cases of chlamydia and more than 500,000 cases of gonorrhea in the US in 2017, with the number of cases on the rise.
The in vitro PCR-based RealTime CT/NG assay is currently only cleared by the US Food and Drug Administration for use with urogenital specimens: endocervical swabs, vaginal swabs, urethral swabs, and urine specimens.
Researchers analyzed the test's limit of detection, inclusivity, and analytic specificity, and also performed assay verification. For chlamydia, the test detects DNA located within plasmids, and for gonorrhea it detects genomic DNA. Limits of detection for both infections were 320 copies of target DNA, corresponding to around 30 to 40 organisms per test, according to the study results. These limits of detection are "similar or slightly lower than the reported claims for the Abbott RealTime CT/NG assay in urogenital specimens," the study noted. They were also lower than those for Cepheid's Xpert CT/NG assay, a CT/NG assay cleared by the FDA for use with extragenital specimens.
"It's a little hard to compare between assays, because other assays all have different units they compare to," said lead author Paul Adamson, an infectious diseases fellow at the University of California, Los Angeles. "But this is certainly on par with the other tests."
Inclusivity and analytic specificity were both 100 percent for each sample type, meaning the test detected all the different strains of chlamydia and gonorrhea that researchers tested for and there was no cross-reactivity against the 28 other bacteria researchers tested. It also had 100 percent accuracy and precision among specimens tested, according to the verification study.
Testing from extragenital samples isn't necessarily better than using urogenital samples, Adamson noted, but more samples can provide more information about performance of the test. There could be different bacteria present in urine or cervical swabs, he said. "Environments have varying pH levels, and that might be something that affects the test," Adamson said.
Although he was careful to mention this study wasn't a clinical study, Adamson added there could also be differences in concentration depending on the specimen. "If you're someone who has had an asymptomatic oropharyngeal infection, the swab that you take, if you didn’t have any symptoms or signs of infection, the amount of bacteria there might be very low," he said. "Whereas if you're someone who has a symptomatic urogenital infection and you have pus and you're swabbing the pus, the concentrations of bacteria at those different sites will be different."
Adamson said testing with these new specimen types could be used for people with symptomatic infections but could also be used to screen high-risk patients – men who have sex with men, for example.
Although Abbott's test, which runs on the firm's m2000 RealTime instrument, is already cleared for urogenital specimens, Adamson said he thinks there's a definite need for testing that works with rectal and pharyngeal specimens, especially for high-risk patients. Providing more testing options could also help improve access to testing for patients, Adamson said.
Hologic's Aptima Combo 2 assay and Cepheid's Xpert CT/NG test are two CT/NG tests cleared by the FDA for use with extragenital specimens and urogenital specimens. The 510(k) clearances for these tests were supported by an earlier clinical trial evaluating the performance of Hologic and Cepheid's tests, along with the Abbott assay. This multi-site study collected data from more than 2,500 patients and was coordinated by the Antibacterial Resistance Leadership group.
Both Hologic and Cepheid received clearance for their assays last May.
Urogenital testing remains the dominant sample used in the market, but a Hologic spokesperson said the company is seeing increasing customer demand for extragenital testing, particular for at-risk populations.
Before the Hologic and Cepheid assays were cleared by the FDA, Adamson noted, individual labs each had to perform their own verification processes to be able to do the testing. "There's definitely a need for additional testing platforms, and it gives various labs different options for platforms that can be used," Adamson added.
"At the end of the day, we found it to be a very accurate and reliable test," Adamson said.