| Use
of non-amplified C. trachomatis detection assays with confirmation
may be preferred over nucleic acid amplification tests when routinely
testing low-risk women in a high volume laboratory.
Evaluation
of sequential testing strategies using non-amplified and amplified methods
for detection of Chlamydia trachomatis in endocervical and urine
specimens from women.
Semeniuk H, Zentner A, Read R, Church D.
Diag Microbiol Infect Dis. 2002;42:43-51.
Summary:
Question
Are sequential test strategies that combine an initial non-amplified C.
trachomatis detection method with secondary testing of equivocal
samples by an amplified method feasible alternative strategies in a high
volume laboratory primarily testing low risk populations?
Design
This study describes a comparison of two unique sequential testing
strategies that employed two different commercial nucleic acid
amplification test (NAAT) methods to detect C. trachomatis in a
population of women with a widely varying risk of infection. Two
endocervical swabs were collected from each woman. One swab was tested
using Access enzyme immunoassay (EIA) and COBAS AMPLICOR CT assay. The
other swab was tested using the PACE 2 CT hybridization assay and the
AMP-CT TMA assay. A brief history documenting the participant's risk of
acquiring infection was obtained at the time of enrollment.
Participants
Five hundred four women, aged 15 to 75, being tested for cervical C.
trachomatis infection were enrolled from the following sites around
Calgary, Alberta: 1) physician's offices, 2) obstetric, gynecologic, and
family practice clinics at hospitals, 3) hospital emergency departments,
and 4) the regional sexually transmitted diseases clinic (n = 323).
Description of Tests and Diagnostic
Standard
Two endocervical swab specimens were collected from each woman so that the
order of collection for each testing strategy was alternated between
sequentially enrolled women. The swab to be tested for the PACE 2 CT assay
(Gen-Probe, San Diego, CA) and the AMP-CT TMA assay (Gen-Probe) was placed
into Gen-Probe transport medium. The swab to be tested for the Access EIA
(Beckman, Missisauga, Ontario, Canada) and the COBAS AMPLICOR CT PCR
method (Roche Diagnostic Systems, Branchburg, NJ) was placed into the EIA
transport tube. Aliquots for testing by each method were removed from each
sample prior to performing any assay. All tests were performed according
to the manufacturers' instructions. For the EIA assay, all specimens
falling within the equivocal zone had a confirmatory blocking assay
performed. A reduction of at least 50% in the optical density in the
blocked assay compared to the unblocked test indicated that the specimen
was a true positive. A direct fluorescent antibody assay was also
performed as a secondary confirmatory test. All specimens with initial
equivocal results by the PACE 2, AMP-CT, and COBAS assays were repeated.
COBAS assays were also repeated on samples that were negative for both C.
trachomatis and the internal control. A true positive was defined as
any patient who was positive on both NAAT. Repeat testing was performed to
resolve any discrepant results between the two amplified methods.
Main Outcome Measures
The sensitivity, specificity, positive predictive value (PPV), and
negative predictive value (NPV) of each test method compared to the gold
standard described above were determined. The number of repeats performed
and the time each assay required were also recorded.
Main Results
Using positivity by both amplification methods to define true positives,
28 of 504 (5.6%) women were positive for C. trachomatis infection,
and the overall performance characteristics of the four assays were
calculated (table). The Access EIA had the lowest sensitivity, and the
PACE 2 assay was less sensitive than either of the amplified assays. The
COBAS assay had the lowest PPV because of the assay's high false positive
rate and the highest repeat factor due to the presence of inhibitory
substances detected by the internal control.
| Performance
of assays for detection of C. trachomatis in endocervical
swabs. |
| Test |
Sensitivity% |
Specificity% |
PPV% |
NPV% |
Number of
Repeats |
| Access
EIA |
71.4 |
100 |
100 |
98.3 |
3 |
| PACE 2 |
85.7 |
98.9 |
82.8 |
99.2 |
5 |
| AMP-CT |
100 |
99.2 |
87.5 |
100 |
4 |
| COBAS CT |
100 |
98.5 |
80 |
100 |
24 |
Authors' Conclusions
Despite the high sensitivity and specificity
of the amplified assays, the PPVs were low, especially in low risk women.
Approximately two-thirds of all false positive results occurred in women
who did not have any risk factor for infection. Utilization of a
sequential testing strategy is not recommended for the clinical
laboratory. The use of non-amplified assays with non-amplified
confirmation may be preferred when routinely testing low-risk women in a
high volume laboratory.
Source of funding: Beckman, Gen-Probe,
and Roche Diagnostics
For correspondence: Deirdre Church,
Calgary Laboratory Services, Calgary, Alberta, Canada. E-mail address: deirdre.church@cls.ab.ca.
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