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The ligase chain
reaction assay is a very sensitive method for detecting N.
gonorrhoeae in rectal and pharyngeal samples.
Evaluation of ligase chain reaction
for the non-cultural detection of rectal and pharyngeal gonorrhea in men
who have sex with men.
Young H, Manavi K, McMillan A.
Sexually
Transmitted Infections 2003;79:484-486.
Summary:
Question
How well does a nucleic acid amplification test (ligase chain reaction (LCR))
compare with culture for the detection of N. gonorrhoeae in rectal
and pharyngeal specimens from men who have sex with men (MSM)?
Design
This paper describes a prospective, double-blinded investigation to assess
the performance of the LCR test for detecting rectal and pharyngeal
gonorrhea, as determined by a gold standard based on both LCR and culture,
in MSM attending a genitourinary medicine clinic.
Participants
Two hundred fifty-one consecutive MSM attending a clinic of genitourinary
medicine for full screening for sexually transmitted infections were
tested.
Description of Tests and Diagnostic
Standard
Duplicate rectal swabs were collected from the distal 3 cm of the anal
canal. Duplicate throat swabs were collected from the tonsillar area and
posterior pharyngeal wall. One swab from each site was immediately rolled
over MNYC medium and incubated at 37oC in CO2
enriched atmosphere. Cultures were examined after 24 and 48 h.
Gram-negative diplococci from oxidase-positive colonies were confirmed as N.
gonorrhoeae by the Phadebact Monoclonal GC test and by rapid
carbohydrate utilization tests. The second swab from each site was placed
in LCR transport medium and stored at -20oC. The specimens were
processed and tested for N. gonorrhoeae DNA using the ligase chain
reaction assay (Abbott Laboratories) according to the manufacturer's
protocol. Specimens with LCR positive results not corroborated by culture
were tested by a second LCR reaction that used probes based on the N.
gonorrhoeae pilin gene. True positive specimens were defined as those
with positive culture results regardless of the LCR results or, if culture
negative, two positive LCR results.
Main Outcome Measures
The sensitivity, specificity, positive predictive value (PPV), and
negative predictive value (NPV) of culture and LCR for the diagnosis of
rectal and pharyngeal N. gonorrhoeae infections were determined.
Main Results
The prevalence of N. gonorrhoeae among 251 duplicate throat swabs from MSM
was 12.7% (32 of 251) and 6.0% (15 of 251) by LCR and culture,
respectively. LCR and culture were concordant in 230 (91.6%) specimens.
The prevalence of N. gonorrhoeae detected by LCR was significantly
higher than by culture (p = 0.005). The prevalence of N. gonorrhoeae
among 227 duplicate rectal swabs was 7.0% (16 of 227) and 4.0% (9 of 227)
by LCR and culture, respectively. LCR and culture were concordant in 219
(96.5%) specimens. The performances of LCR and culture for the diagnosis
of rectal and pharyngeal gonorrhea in MSM, as defined by the gold standard
described above, are shown in the table.
Authors' Conclusions
Routine screening for N. gonorrhoeae
infection of rectal and pharyngeal sites by a nucleic acid amplification
test (NAAT) method should be considered. However, because false positive
reactions can occur with NAATs, it is prudent to consider the initial
result as a "presumptive" positive and to confirm the findings
with a second NAAT based on a different target gen.
Source of funding:
Not given
For correspondence:
Hugh Young, Scottish Neisseria gonorrhoeae Reference Laboratory,
Directorate of Medical Microbiology, The Royal Infirmary of Edinburgh, 51
Little France Crescent, Edinburgh EH16 4SA, UK. E-mail address: hugh.young@luht.scot.nhs.uk
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