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PCR is a sensitive
and specific test for detection of T. pallidum.
Use of PCR in the diagnosis of early
syphilis in the United Kingdom.
Palmer HM, Higgins SP, Herring AJ, Kingston
MA.
Sexually
Transmitted Infections 2003;79:479-483.
Summary:
Question
How well does a T. pallidum PCR test perform for the laboratory
diagnosis of early syphilis in the United Kingdom?
Design
This study describes the results obtained with a T. pallidum PCR
test performed on ulcer swabs from men and women with suspected syphilis
and compares them with the diagnoses given by the clinicians, which were
made on the basis of their clinical information and serological test
results.
Participants
Ulcer samples from 98 patients with suspected syphilis, who were also
tested by serological methods, were submitted for PCR testing. Seventy of
the patients had attended GUM clinics in the Greater Manchester area.
Eighty-six patients were male and, of these, 58 were MSM. Of the MSM, 24
were HIV positive. Two patients presented with two separate infections
during the 19 month study period so that 100 samples were included in the
analyses.
Description of Tests and Diagnostic
Standard
Swabs of 42 penile, 23 oral, 19 anal, 12 vulvar, 3 rectal, and 1
unspecified ulcer were collected, placed into a dry tube or a tube with
transport medium, and sent to the PCR testing laboratory. One mL of saline
was added to the samples in the dry tubes. Specimens were vortexed,
centrifuged, and the DNA was extracted from the cell pellets. The DNA was
amplified using primers KO3A and KO4, which amplify a 260 bp region of the
T. pallidum 47 kDa integral membrane lipoprotein gene. Amplicons
were detected by electrophoresis. A dilution series of a known amount of
purified T. pallidum DNA was used to determine the detection limit
of the PCR.
Clinicians diagnosed the patients as
having primary or secondary syphilis, or as not having syphilis, according
to the clinical details and their available laboratory test results. The
laboratory tests varied according to local diagnostic practices and
included the serological tests of RPR, VDRL, TPHA, TPPA, an EIA detecting
IGM and IgG, and an EIA detecting IgM only. Dark ground microscopy was
performed on 34 cases.
Main Outcome Measures
The sensitivity, specificity, positive predictive value (PPV), and
negative predictive value (NPV) of the PCR for the diagnosis of primary
and secondary syphilis, as determined by clinicians using serological
and/or microscopic test results, were calculated.
Main Results
The detection limit of the PCR was approximately 800 genome copies of T.
pallidum. The results of the PCR, RPR, and TPHA or TPPA tests on
specimens from clinician-diagnosed cases of primary, secondary, or no
syphilis are shown in the table. Two HIV positive patients tested PCR
positive 12 and 21 days, respectively, before syphilis seroconversion. The
sensitivity, specificity, PPV, and NPV of the PCR were 94.7%, 98.6%,
94.7%, and 98.6%, respectively, for primary syphilis and 80%, 98.6%,
88.9%, and 97.2%, respectively, for secondary syphilis.
Authors' Conclusions
PCR successfully detected T. pallidum
from oral, genital, and anal ulcers present during primary or secondary
syphilis. The PCR results correlated well with the serology. PCR provided
an earlier diagnosis than serology in some cases, and offered a
confirmatory diagnosis or a differential diagnosis between T. pallidum
and HSV in other cases.
Source of funding:
Not given
For correspondence:
Helen M. Palmer, Scottish Neisseria gonorrhoeae Reference Laboratory,
Department of Medical Microbiology, Royal Infirmary of Edinburgh, Little
France, Edinburgh, EH16 4SA, UK. E-mail address: Helen.palmer@luht.scot.nhs.uk
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