Wednesday, March 12, 2008
All state public health laboratories in the CDC Region VIII Infertility Prevention Project (CO, MT, ND, SD, UT and WY) perform chlamydia and gonorrhea screening with the Gen-Probe APTIMA Combo2 assay. To improve the positive predictive value (PPV) of the assay, each state uses state-specific criteria for repeating an initial equivocal or positive test on the same specimen.
Although the APTIMA specimen is not considered a homologous suspension, a retrospective study was conducted to determine the reproducibility of a repeat test performed on the same specimen.
Initially equivocal (RLU ≥25) or positive (RLU >100) Chlamydia specimens and initially equivocal (RLU ≥60) or positive (RLU >150) gonorrhea specimens were stratified by state, gender and specimen source.
For gonorrhea, regardless of gender, specimen source, or state, all (100%) initial RLU values >150 tested positive on repeat testing (28/28). For Chlamydia, regardless of gender or specimen source, 134 of 135 (99.3%) of initial RLU values >600 repeated as positive. The one specimen that failed to repeat, a female urine specimen, repeated in the equivocal range (RLU between 25 and 100). When initial Chlamydia RLUs were >300 and <600, only 86.8% (46/53) repeated as positive. When stratified by gender in this range, 13/14 (92.9%) of males repeated as positive, but only 33/39 (84.6%) of females repeated as positive. 100% (11/11) of these female urine specimens repeated as positive, but only 78.8% (21/27) of cervical specimens repeated. This lower repeatability was not state dependent.
When the initial RLU is greater than 600, the Gen-Probe APTIMA assay reliably repeats as positive, regardless of specimen source or gender.
Reducing the number of tests that are repeated to improve PPV can result in cost savings to programs, allowing more screenings to be performed.