Enhancing the specificity of the COBAS AMPLICOR CT/NG test for Neisseria gonorrhoeae by retesting specimens with equivocal results.
The COBAS AMPLICOR CT/NG test for Neisseria gonorrhoeae cross-reacts with certain strains of nonpathogenic Neisseria species. In some strains, the target sequence is identical to that of N. gonorrhoeae, whereas other strains have a small number of mismatches within the regions recognized by the primers or probe used in the COBAS AMPLICOR NG test. These cross-reactive strains are occasionally present in urogenital specimens, causing false-positive results in the COBAS AMPLICOR NG test. Analysis of the data generated in a large multicenter clinical trial showed that 2.9% of the specimens gave signals between A(660)s of 0.2 and 3.5 but that one-half of these equivocal specimens did not contain N. gonorrhoeae. Most of these equivocal specimens were correctly classified as true positive or true negative by retesting in duplicate and defining a PCR-positive result as two of three results with an A(660) of > or =2.0. If specimens had been classified as positive or negative based on a single test result using a cutoff of an A(660) of 0.2, specificity would have ranged from 96.2 to 98.9% depending on specimen type, sex, and presence of symptoms. By employing the equivocal zone-retesting algorithm, specificity increased to 98.6 to 99.9% with little effect (0.1 to 4.9% decrease) on sensitivity in most specimen types, enabling the test to achieve a positive predictive value of at least 90% in populations with a prevalence of 4% or higher. In lower-prevalence populations, the test could be used to screen for presumptive infections that would have to be confirmed by an independent test.
Van Der Pol
B
B
Indiana University School of Medicine, Indianapolis, Indiana, USA.
Martin
D H
DH
Schachter
J
J
Quinn
T C
TC
Gaydos
C A
CA
Jones
R B
RB
Crotchfelt
K
K
Moncada
J
J
Jungkind
D
D
Turner
B
B
Peyton
C
C
Kelly
J F
JF
Weiss
J B
JB
Rosenstraus
M
M
D000328:Adult; D000465:Algorithms; D002690:Chlamydia Infections / Q000382:microbiology; D002692:Chlamydia trachomatis / Q000235:genetics; D005189:False Positive Reactions; D005260:Female; D006069:Gonorrhea / Q000175:diagnosis* / Q000382:microbiology*; D006801:Humans; D008297:Male; D009343:Neisseria / Q000235:genetics / Q000302:isolation & purification; D009344:Neisseria gonorrhoeae / Q000235:genetics / Q000302:isolation & purification*; D016133:Polymerase Chain Reaction / Q000379:methods* / Q000592:standards; D012680:Sensitivity and Specificity