The Xpert® CT/NG (Cepheid Inc., Sunnyvale, CA) is a rapid, fully automated real-time PCR test that simultaneously detects Chlamydia trachomatis (CT) and Neisseria gonorrhoeae (NG). Xpert ® CT/NG assay has high sensitivity and specificity, but also includes a Specimen Adequacy Control (SAC) that targets hydroxymethylbilane synthase (HMBS), a human cellular house-keeping gene. SAC controls for false negative results by confirming adequate patient sample and appropriate testing conditions. SAC is quantified by its cycle threshold, the number of cycles required to detect the presence of 1 HMBS gene target. A lower SAC indicates an earlier cycle threshold (Ct) and more cellular material detected. Our objective was to determine if SAC values could be used as a marker for inflammation and assist in potentially predicting clinical disease.
As part of NICHD HPTN 040, a multi-center clinical trial evaluating new treatment regimens for the prevention of HIV mother-to-child transmission, urine samples from 1167 HIV-infected pregnant women, collected at the time of labor/delivery underwent Xpert® CT/NG testing. Invalid results were excluded from analysis. Mean SAC Ct values and standard deviation (SD) were calculated. Student’s t-test was used to compare SAC Ct mean values compared to a reference of urine samples negative for CT and NG.
The urine CT positivity was 18.3% (214/1167) and NG, 4.6% (54/1167). The mean SAC Ct value in urine from women without CT or NG was 28.19 (SD: 4.20) and higher than the mean SAC Ct for CT positive specimens (27.43, SD: 3.83(P=.0153)), NG positive specimens (26.19 (SD: 3.01(, P<.0001), and in those CT/NG co-infected was 26.45 (SD: 3.01), P=.0049).
Lower SAC Ct values were significantly associated with chlamydial and gonococcal infections. While further studies are needed, SAC Ct values may show promise identifying the presence of inflammation and infection.