A Multi-state Reproducibility Study of an Amplification Assay

Background:
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.

Objective:
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.

Method:
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.

Result:
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.

Conclusion:
When the initial RLU is greater than 600, the Gen-Probe APTIMA assay reliably repeats as positive, regardless of specimen source or gender.

Implications:
Reducing the number of tests that are repeated to improve PPV can result in cost savings to programs, allowing more screenings to be performed.