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Rapid plasma reagin
and rapid syphilis screening tests do not perform well for the
diagnosis of syphilis in field conditions.
Performance of the rapid plasma reagin
and the rapid syphilis screening tests in the diagnosis of syphilis in
field conditions in rural Africa.
West B, Walraven G, Morison L, Brouwers J,
Bailey R.
Sex Transm Infect.
2002;78:282-285
Summary:
Question
How well do the rapid plasma reagin (RPR) test and a new rapid syphilis
test (RST) perform in the field as primary screening methods for syphilis?
Design
Serum samples were collected and tested for syphilis seropositivity in
temporary field laboratories using an RPR and an RST strip. The results of
the field tests were compared to those obtained using a repeat RPR and a
TPHA test carried out in a standard laboratory.
Participants
A total of 1325 women, aged 15-54, who participated in a large
community-based reproductive health survey in 20 villages in the Gambia
were tested.
Description of Tests and Diagnostic
Standard
Sera were tested on site, immediately following collection, by two
syphilis screening tests: 1) a standard RPR 18 mm circle card test (Quorum
Diagnostics, Vancouver, BC, Canada) using one drop of sample and mixing
for 8 minutes, and 2) a one step immunochromatographic strip test
utilizing a 47 kDa recombinant antigen of T. pallidum (RST, Quorum
Diagnostics) according to the manufacturer's guidelines. At the end of
each day, samples were transported to a permanent, well-equipped
laboratory, frozen, and tested within one week by RPR and TPHA (Fujirebio,
Mast Laboratories, UK) using standard techniques. The same laboratory
assistant performed the tests in the field and in the laboratory. A
diagnosis of "active" syphilis was defined as laboratory RPR and
TPHA positive.
Main Outcome Measures
The sensitivity, specificity, positive predictive value (PPV), and
negative predictive value (NPV) of field RPR and RST tests were calculated
for the detection of cases of "active" syphilis as determined by
the results of samples tested in the laboratory by RPR and TPHA.
Main Results
The performance of the field RPR and RST tests against serologically
active syphilis (defined by laboratory RPR positive and TPHA positive) is
shown in the table. In the laboratory, 40 (3.0 %) of 1325 serum samples
were RPR and TPHA positive. In a direct comparison of RPR results on 1295
samples tested both in the field and in the laboratory, there was
agreement on 1207 negative and 35 positive samples, while 41 samples were
positive in the field but negative in the laboratory and 12 were negative
in the field and positive in the laboratory. A comparison of the results
of 1325 samples tested with RST and TPHA, which both use T. pallidum
antigens, showed agreement on 1204 negative and 51 positive samples, while
41 samples were positive by RST and negative by TPHA and 29 were negative
by RST and positive by TPHA.
| Performance of field
RPR and rapid syphilis tests in defining cases of "active"
syphilis |
| Test |
Number
tested |
Sensitivity
(%) |
Specificity
(%) |
PPV (%) |
NPV (%) |
| Field RPR |
1295 |
77.5 |
96.4 |
40.8 |
99.3 |
| Rapid
syphilis test |
1325 |
75.0 |
95.2 |
32.6 |
99.2 |
Authors' Conclusions
The new RST was easier to use and interpret
than the RPR test and performed similarly as a field screening test.
However, neither test predicted well the presence of active syphilis in
this low prevalence population. The data suggest that in a higher
prevalence population (10-15%), where laboratory facilities are limited, a
positive test for either field RPR testing or RST would imply a 65-80%
probability of active syphilis. In this case, 3-4% of subjects with a
negative test would escape detection.
Source of funding: Medical Research
Council, and Quorum Diagnostics, Ltd.
For correspondence: B. West, MRC
Laboratories, PO Box 273, Banjul, Gambia. E-mail address: bwest@mrc.gm
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