Evaluation of Repeat Testing of Positive Chlamydia trachomatis Using Combo and Stand Alone Targets

Background:
Georgia Public Health Laboratory (GPHL) implemented the Gen Probe Aptima Combo 2 (AC 2) Assay in 2006. Since testing positive specimens was recommended to be performed by two separate targets by CDC (MMWR 51, 2002), we compared the results of repeat positives using a stand alone target (16 S rRNA) and the AC 2 target (23 S r RNA) which is used in the AC 2 assay for Chlamydia trachomatis (CT).

Objective:
The objective of this comparison was to evaluate the repeat testing of positive results using combo and stand alone targets and to determine the positive predictive value for each target.

Method:
GPHL used TIGRIS DTS system by Gen Probe and set up a cut off value of 300 (x1000) RLU, instead of 100 (x1000) RLU recommended by the manufacturer. All positive results below 300 (x1000) RLU were repeated with the stand alone target for the first six months and with combo assay, thereafter. All invalid and equivocal results were also tested and the results were included in the overall positivity rates.

Result:
The results indicate that repeating positive results below 300 (x1000) RLU by the stand alone target reduces the false negative results. This difference is also reflected in the overall positivity rate increase from 10.3% to 10.7% with stand alone target as compared to AC2 target. In addition, we determined that choosing 300 (x1000) RLU for cut off value was a reasonable value since 25% of CT results did not repeat as positive for specimens that were between 100 (x1000) to 300(x1000) RLU.

Conclusion:
These results indicate that using an alternate target for repeating positives produces more reliable results for detection of Chlamydia infection.

Implications:
Using an alternate target for testing positive Chlamydia specimens below 300 (x1000) increases the positive predictive value.