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Confirmatory
testing of specimens positive for N. gonorrhoeae by COBAS
AMPLICOR using a more specific method such as 16S rRNA PCR should be
considered.
Confirmation by 16S
rRNA PCR of the COBAS AMPLICOR CT/NG test for diagnosis of Neisseria
gonorrhoeae infection in a low-prevalence population.
Diemert DJ, Libman MD, Lebel P.
J Clin Microbiol. 2002;40:4056-4059.
Summary:
Question
What is the specificity and positive predictive value of the COBAS
AMPLICOR NG assay compared to a 16S rRNA PCR assay for the detection of N.
gonorrhoeae in samples from a low prevalence population?
Design
The sensitivity, specificity, and positive predictive value (PPV) of the
NG portion of the COBAS AMPLICOR test for the detection of N.
gonorrhoeae infection was assessed using genital swabs and urine
specimens collected from a low prevalence population (0.5%) by comparing
results to those of culture and an investigational 16S rRNA PCR assay.
Participants
Specimens (n = 9,772) from consecutive patients undergoing
investigation for N. gonorrhoeae infection (both symptomatic and
asymptomatic screening) at outpatient clinics affiliated with a large
Montreal, Canada, tertiary care center, as well as two health centers in
northern Quebec serving a primarily Native Canadian population were
analyzed. In addition, 62 specimens were sent for confirmation of a
positive test performed elsewhere.
Description of Tests and Diagnostic
Standard
The COBAS AMPLICOR N. gonorrhoeae PCR test (Roche Molecular Systems,
Branchburg, NJ) was performed on the automated COBAS instrument according
to the manufacturer's instructions. Specimens with an OD reading of >0.2
and <3.5 were considered equivocal and retested. Specimens were
considered positive if the OD reading of the first test was >3.5
or if two tests yielded OD readings of >0.2. All specimens
positive by AMPLICOR underwent testing by 16S rRNA PCR (Roche Molecular
Systems). This assay uses primers that target the N. gonorrhoeae 16S
rRNA gene, which is different from the gene targeted by the AMPLICOR
assay, and is thought to be highly conserved within the species. Specimens
with an OD reading of >0.2 and <0.8 were repeated in
duplicate. Specimens were interpreted as positive if the OD reading was >0.8
or if at least two tests yielded signals of >0.2
Main Outcome Measures
The specificity and PPV of the COBAS AMPLICOR NG PCR compared to the 16S
rRNA PCR were calculated for each type of specimen and according to the
gender of the patient.
Main Results
One hundred sixty-eight (1.7%) of 9,772 specimens were positive by
AMPLICOR PCR and underwent confirmatory testing using the 16S rRNA assay.
In addition, 62 specimens were referred from other centers for
confirmation of a positive AMPLICOR test yielding a total of 230 specimens
for confirmatory testing. Of these specimens, 158 were from females, and
72 were from males, including 149 urethral or cervical swab specimens and
81 urine specimens. Seventy-two specimens were confirmed positive by the
16S rRNA assay. The specificity and PPV by gender and specimen type of the
AMPLICOR result compared to the 16S rRNA assay result are shown in the
table. The PPV of the AMPLICOR test was significantly higher when the OD
was >3.5 on initial testing (65.1%) (86 samples) than when both
initial and repeat OD readings were within the equivocal zone (10.1%) (144
samples).
| Specificity and PPV
of COBAS AMPLICOR NG test compared to that of 16S rRNA PCR |
| Specimen |
Specificity,
% (95% CI) |
PPV, %
(95% CI) |
| Female
endocervical |
98.3
(97.9-98.6) |
7.1
(3.1-13.6) |
| Male
urethral |
98.8
(98.0-99.3) |
59.5
(42.1-75.2) |
| Female
urine |
99.2
(98.8-99.5) |
28.3
(16.0-43.5) |
| Male
urine |
99.2
(97.8-99.8) |
82.9
(66.4-93.4) |
| Total |
98.7
(98.4-98.9) |
31.3
(25.4-37.7) |
Authors' Conclusions
The COBAS AMPLICOR NG test is highly
sensitive and specific for the diagnosis of infection with N.
gonorrhoeae. However, in low-prevalence populations, the PPV of the
test is low. No alternate OD cutoff or testing algorithm would improve PPV
while maintaining sensitivity; therefore; confirmatory testing with
another, more specific assay such as the 16S rRNA PCR assay should be
performed on all specimens with an OD of more than 0.2 in populations with
a low prevalence of infection. Additionally, even in areas of higher
prevalence, specimens yielding an OD reading between 0.2 and 3.5 should be
considered for confirmatory testing.
Source of funding: None given.
For correspondence: David J. Diemert,
Malaria Vaccine Development Unit, National Institute of Allergy and
Infectious Diseases, Twinbrook I, Room 1123, 5640 Fishers Ln., Rockville,
MD 20852. E-mail address: ddiemert@niaid.nih.gov.
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