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Vaginal swabs were
acceptable specimens for the detection of C. trachomatis and N.
gonorrhoeae by the BDProbeTec ET system, but use of an
amplification control with this specimen had minimal impact on test
performance.
Detection of Chlamydia
trachomatis and Neisseria gonorrhoeae by strand
displacement amplification and relevance of the amplification
control for use with vaginal swab specimens.
Cosentino LA, Landers DV, Hillier SL.
Journal of Clinical
Microbiology 2003;41:3592-3596.
Summary:
Question
How well do vaginal swabs perform compared to cervical swabs as specimens
for the detection of C. trachomatis and N. gonorrhoeae by
the BDProbeTec ET system, and how does use of an amplification control
affect performance?
Design
The detection of C. trachomatis and N. gonorrhoeae by the
BDProbeTec ET system using vaginal swabs was compared to detection by the
BDProbeTec ET system, by PCR (for C. trachomatis), and by culture
(for N. gonorrhoeae) using endocervical swabs. The use of an
amplification control for identification of indeterminate results of
vaginal swabs by the BDProbeTec ET system was evaluated.
Participants
Four hundred fifty-five symptomatic women, aged 18 to 40, attending
primary health care and STD clinics in Pittsburgh, PA, were tested for the
study comparing the results of vaginal and cervical swab specimens. For
the study of only vaginal swab specimens tested by BDProbeTec ET using the
amplification control, 2973 women, enrolled at five locations, were
tested.
Description of Tests and Diagnostic
Standard
Cervical and vaginal swabs were tested using a strand displacement
amplification assay (SDA) on the BDProbeTec ET instrument (Becton
Dicksinson, Sparks, MD) for the detection of C. trachomatis and N.
gonorrhoeae, including an amplification control to detect inhibitors
of amplification, according to the manufacturer's instructions. Separate
cervical swab specimens were tested for C. trachomatis by Amplicor
PCR (Roche Diagnostic Systems, Branchburg, NJ) according to the
instructions in the manufacturer's package insert, and for N.
gonorrhoeae by standard culture on Thayer-Martin and chocolate medium
and identification by Gram staining, oxidase test, and the Gonocheck II
system (EY Laboratories, Inc., San Mateo, CA). In cases of disagreement
between the C. trachomatis results of SDA and PCR or the N.
gonorrhoeae results of SDA and culture, a fourth cervical swab was
tested by LCR (Abbott Laboratories, Chicago, IL) according to the
manufacturer's instruction. The collection order for the swabs was rotated
to reduce potential sampling bias. Samples were considered positive for C.
trachomatis if positive by two nucleic acid amplification tests (NAAT),
and for N. gonorrhoeae if positive by culture or by two NAAT.
Main Outcome Measures
The sensitivity, specificity, positive predictive value (PPV), and
negative predictive value (NPV) of C. trachomatis detection by PCR
and SDA in cervical and vaginal swabs, and of N. gonorrhoeae
detection by culture and SDA in cervical and vaginal swabs, as determined
by the definition described above, were calculated. The prevalence of
indeterminate results for the BDProbeTec ET assay when tested on vaginal
swab specimens was determined.
Main Results
Of the 455 women tested by SDA, PCR, and N. gonorrhoeae culture, 37
(8.1%) and 39 (8.6%) were positive for C. trachomatis and N.
gonorrhoeae, respectively. The performances of PCR and SDA on vaginal
and cervical swab specimens for the detection of C. trachomatis,
and of culture and SDA on vaginal and cervical swabs for the detection of N.
gonorrhoeae are shown in the table. According to the BDProbeTec ET
amplification control, indeterminate results were obtained for 0.7% of
cervical swab and 6.4% of vaginal swab specimens. All indeterminate
results were resolved as negative by repeat testing.
Of 1326 vaginal swabs tested by
BDProbeTec ET using the amplification control, 375 (28%) were
indeterminate. When 134 of the indeterminate specimens were retested
undiluted, only 43 (32%) were resolved. When 484 specimens were diluted
1:10 and retested, all the originally indeterminate results resolved as
negative; however, seven samples that were originally positive for C.
trachomatis or N. gonorrhoeae became negative. Of 1647
additional vaginal swabs tested by BDProbeTec ET using the amplification
control, 413 (25%) yielded indeterminate test results. Following 1:1
dilution and retesting, 6 (1.5%) of the initially indeterminate results
resolved as positive for C. trachomatis or N. gonorrhoeae.
Authors' Conclusions
SDA of vaginal swab specimens has a
sensitivity and specificity equivalent to those of cervical swab specimens
for the detection of C. trachomatis and N. gonorrhoeae and
are an acceptable alternative to cervical swab specimens in the BDProbeTec
ET system. Although one in four vaginal swab specimens yielded an
indeterminate result when the amplification control was used, retesting of
indeterminate samples by repeating SDA or by PCR usually yielded negative
results.
Source of funding: In part by the U.
S. Department of Defense and by Becton-Dickinson.
For correspondence: Sharon L. Hillier,
Department of Obstetrics, Gynecology, and Reproductive Sciences,
University of Pittsburgh, Magee-Womens Hospital, 300 Halket St.,
Pittsburgh, PA 15213-3180. E-mail address: slh6+@pitt.edu.
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