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Identifying false-positive chlamydia and gonorrhea results using nonmanufacturer relative light unit cutoffs for the
Savannah N Rios1, Derrick J Chen2
1University of Wisconsin School of Medicine and Public Health, Madison, WI, United States.
Objective:
Chlamydia trachomatis and Neisseria gonorrhoeae present substantial public health challenges. Accurate diagnostic testing is essential to prevent misdiagnosis and unnecessary treatment. Although nucleic acid amplification tests offer excellent performance, they are not infallible. This study sought to evaluate the semiquantitative utility of relative light unit (RLU) values from the Hologic Aptima Combo 2 Assay to improve the diagnostic accuracy of testing for C trachomatis and N gonorrhoeae.
Methods:
Data were analyzed from January 2021 to December 2021. Manufacturer guidelines define results as positive if the RLU value is above 100 for C trachomatis only, above 150 for N gonorrhoeae only, and above 250 for dual C trachomatis and N gonorrhoeae detection; equivocal if the RLU value is 25 to 99 for C trachomatis, 60 to 149 for N gonorrhoeae, and 85 to 249 for both; and negative if the RLU value is below 25 for C trachomatis, below 60 for N gonorrhoeae, and below 85 for both. Manufacturer guidance recommends repeat testing only for equivocal results. In contrast, the University of Wisconsin University Hospital adopted a modified criterion, classifying all results with an RLU value at or below 900 as equivocal and requiring repeat testing.
Results:
In this retrospective review of 20 875 Aptima Combo 2 assays performed from January to December 2021, 7 patients had initial positive results, with RLU values at or below 900. Of these, 5 were ultimately determined to be false positives.
Conclusions:
These findings demonstrate that expanding the definition of equivocal results to include low positive RLU values (≤900) increases identification of false positives with minimal additional repeat testing. This modified approach may improve diagnostic specificity and reduce unnecessary treatment and patient anxiety.
