Evaluation of Repeat Testing for Positive Samples of Chlamydia trachomatis and Neisseria gonorrhoeae Using the Gen-Probe Aptima Combo 2 Assay

Background:
In the CDC Guidelines Screening Tests to Detect Chlamydia trachomatis and Neisseria gonorrhoeae Infections – 2002, it was suggested that laboratories consider additional testing on positive specimens in order to improve test specificity and increase the positive predictive value (PPV) of results.

Objective:
To assess the need for routine repeat testing of all positive samples in order to increase the positive predictive value and reliability of positive results.

Method:
Four states in the Region IV Infertility Prevention Project (IPP) routinely retested positive samples of Chlamydia trachomatis (CT) and Neisseria gonorrhoeae (GC) using the Gen-Probe Aptima Combo 2 Assay. Two platforms were used: the semi-automated platform using the TECAN DTS and the automated platform using the TIGRIS DTS. The RLUs were evaluated and the percentage of repeat positive, negative, or equivocal within each range determined.

Result:
Specimens were evaluated in 100 RLU increments between <100 to 1000 RLU and >1000 RLU.
Semi-automated: of 910 CT samples, 79.1% repeated Positive, 15.8% repeated Negative, and 5.1% repeated Equivocal. Among 96 GC samples, 65.6% repeated Positive, 13.5% repeated Negative, and 20.8% repeated Equivocal.
Automated platform: of 656 CT samples, 79.1% repeated Positive, 13.0% repeated Negative, and 7.9% repeated Equivocal. Among 50 GC samples, 86% repeated Positive, 2% repeated Negative, and 12% repeated Equivocal.
In both systems, samples >600 RLU repeated 98.8% for CT and 100% for GC.

Conclusion:
Repeat testing of positive Gen-Probe Aptima Combo 2 results for CT and GC specimens <600 RLU may be considered in order to reduce false positive results and increase the PPV.

Implications:
Reducing the number of repeat tests can reduce the cost of screening to programs, allowing more tests to be performed.