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A mathematical
model can be used to estimate the sensitivity required of point of
care tests for N. gonorrhoeae and C. trachomatis that
will enable them to perform as well as gold standard tests in
different populations.
Sensitivity requirements for the point
of care diagnosis of Chlamydia trachomatis and Neisseria
gonorrhoeae in women.
Vickerman P, Watts C, Alary M, Mabey D,
Peeling RW.
Sexually
Transmitted Infections 2003;79:363-68.
Summary:
Question
How can the minimum required sensitivities of point of care STI tests,
which result in as many STIs being averted as if the gold standard test
had been used, be estimated?
Design
This study describes a mathematical model that estimates the minimum
required sensitivities of point of care tests (POC) that lead to more
gonococcal and chlamydial infections being averted than the gold standard
test by taking into consideration the delay between diagnosis using the
gold standard and treatment and the associated loss to follow-up and
transmission of infection.
Participants
The model was tested using behavioral, epidemiological, and clinical data
from six clinical settings that reflect differences in transmission during
a delay in treatment due to different levels of sexual activity and STI
prevalence. The settings included no transmission during delay in
treatment, transmission by female sex workers in Virginia, South Africa,
in Cotonou, Benin, and in London, UK, by women with long-term partners in
the Mwanza region of Tanzania, and by sexually active women attending a
GUM clinic in London.
Description of Tests and Diagnostic
Standard
Gift et al. showed that, for the number of infected people treated as a
result of a POC test to be greater than the number treated as a result of
the current gold standard test, the sensitivity of the POC test must
satisfy the equation: [Sr>μSg], where
Sr
is the sensitivity of the POC test, Sg is the sensitivity of
the gold standard test, and μ is the proportion of patients who return for
treatment. However, STI transmission may occur during the delay between
gold standard testing and treatment, and must also be taken into
consideration when comparing the use of the POC test to the gold standard
test. Therefore, the equation becomes: [Sr>μSg/1+Y],
where Y is the average number of sexual partners each infected person
infects. Y is estimated using an established mathematical equation that
describes the probability of STI transmission over a fixed period of time
and is dependent on the average number of sexual partners per unit time,
the proportion of sexual partners who are already infected with the STI,
the average number of sex acts per partnership per unit time, the STI
transmission probability per unprotected sex act, the condom efficacy per
sex act, the average consistency of condom use, and the delay in
treatment.
Values for the variables in the equation
for Sr were obtained from data collected in the six STI clinic settings.
The per sex act probability of N. gonorrhoeae and C. trachomatis
transmission from females to males was assumed to be 0.3 and 0.11,
respectively. Condom use was assumed to reduce the per sex act probability
of transmission by 90%. It was also assumed that there was a 10 day delay
in treatment when using the gold standard test, an 80% return for
treatment rate, and that the gold standard tests for C. trachomatis and
N.
gonorrhoeae were both 90% sensitive.
Main Outcome Measures
The minimal sensitivities required for POC tests to avert more STIs than
the gold standard tests, given the assumptions and data from the clinics,
were estimated for each clinical setting.
Main Results
From the equation, the required sensitivity of a POC test (Sr)
is low if the return rate (μ)
is low and /or there is STI transmission during the delay in treatment (Y).
The required sensitivity decreases for groups with a greater rate of
sexual partner change, higher frequency of sexual acts, and lower condom
use, all factors affecting Y. In settings where there is a
substantial potential for further STI transmission, such as among sex
workers, even a low sensitivity method of detecting STIs could have
greater impact than current gold standard tests if the return rate was
80%. In settings where there is little potential for further STI
transmission, the required sensitivity of the POC test is high and
dependent on the sensitivity of the gold standard test and the return
rate. The estimates of the required sensitivities of POC tests for C.
trachomatis and N. gonorrhoeae that will avert more STIs than
the gold standard tests are shown in the table for each of six clinical
settings.
Authors' Conclusions
The required sensitivity of a POC test is
highly dependent upon the proportion of women that return for the result
of a gold standard test, and the potential for further STI transmission
during the delay in treatment. In general, the required sensitivity of a
POC test increases proportionally to the sensitivity of the gold standard
test and the return rate and decreases proportionally to the inverse of
the delay in treatment. These results support the use of POC tests in
scenarios where it would be difficult to ensure a high return rate, and in
populations where there is potential for further STI transmission during
the delay in treatment from using laboratory STI tests.
Source of funding: UNDP/World Bank/WHO
Special Programme for Research and Training in Tropical Diseases (TDR),
and partly by the Wellcome Trust
For correspondence: Peter Vickerman,
London School of Hygeine and Tropical Medicine, Keppel Street, London
WC1E7HT, UK. E-mail address: peter.vickerman@lshtm.ac.uk.
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