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Evaluation of the
Digene Hybrid Capture II Assay with the Rapid Capture System for
Detection of Chlamydia trachomatis and Neisseria
gonorrhoeae.
Van der Pol, B,
Williams JA, Smith NJ, Batteiger BE, Cullen AP, Erdman H, Edens T,
Davis K, Salim-Hammad H, Chou VW, Scearce L, Blutman J and Payne JW.
J. Clin Microbiol 2002;40(10):
3558-3564.
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Helen H. Lee
Associate Professor of Medical Biotechnology
Diagnostics Development Unit
Cambridge University, United Kingdom |
Automated screening procedures for
infectious agents become practical especially if testing becomes
obligatory or recommendatory for a given (large) population. As Chlamydia
trachomatis (CT) screening becomes recommendatory for sexually active
women under the age of 26, automated testing such as the Digene CT/GC
Rapid Capture System (HC2-RCS) for CT and Neisseria gonorrhoeae (GC),
becomes practical and therefore its reproducibility and reliability need
to be assessed against the "gold standard" for CT testing. As
the authors have mentioned, automation minimizes errors during sample
analysis and reduction in man-hours. However, these advantages can easily
be upset by the high capital outlay for equipment, and therefore, it would
have been more meaningful if comparative cost per test had been presented
to further aid decision-makers to consider shifting from the traditional
manual screening procedures to automated testing. This may however be
beyond the objectives set by the authors, as they clearly stated at the
start that their objective was to compare the HC2-RCS assay with the same
assay performed manually (HC2-M), the COBAS Amplicor CT/NG polymerase
chain reaction (PCR) assay, and culture.
The experiment was carefully designed to meet the objectives, minimize
biases that may arise from sampling, and minimize equivocal results. By
comparing HC2-RCS with HC2-M, PCR and culture, it is possible to see how
the automated system fares not only relative to the manual method, but
also compared to PCR, which is more sensitive than culture. Selection of
participants is clearly described, although there is no mention of
calculations regarding the chosen sample size and the level of precision,
in terms of the confidence interval range, it provides. Reasons for
exclusion of samples from analysis, as well as uninterpretable results are
however provided. The discussion makes good reference to other
publications, and outlines perceived limitations of the study, namely the
difficulty of defining infected patients, and problems associated with
storage and handling of the samples.
The authors have defined infection as positive only if it is culture
positive or HC2 and PCR-positive. There was >99% correlation between
the manual and automated platforms of the HC2 assay indicating that
automation did not affect sensitivity nor specificity of the assay. There
were two discrepant results for CT-positive samples: PCR-positive but
HC2-RCS negative. Furthermore, when confirmed by CT-ID, one sample turned
out to be false-negative by HC2-RCS since this particular sample tested
positive by CT-ID. There were no false-positives for CT by the HC2-RCS
assay. There were also discrepant results between PCR and HC2 for GC and
these were difficult to resolve because they were also culture negative.
The discrepancy between the two nucleic acid (NA)-based assays (PCR and
HC2-RCS) would probably have been resolved if an internal control for the
latter assay has been part of the test. Although the HC2-RCS assay is not
a DNA amplification assay but a signal amplification assay, there may be
inhibitors to the formation of DNA-RNA hybrids, or in the signal
amplification, which needs to be addressed in the chemistry of the HC2
assay, whether manual or automated.
As expected, the sensitivity was lower in culture than in the NA-based PCR
and HC2 assays because the former only detects active infection. The
results would have been more meaningful had the authors mentioned the
antibiotic status of the patients, especially those attending the STD
clinic. If they had excluded patients on antibiotics, the results may have
been easier to interpret. Furthermore, "CT-negative" cultures
should have been incubated further or blind-passaged before reporting them
as negative. Where results were culture negative (for CT) but were HC2-RCS
and PCR-positive, the authors may have benefited from describing the
NA-based positive results as "weak", or "moderately"
positive in terms of the HC2 or PCR results.
The authors were able to demonstrate that the automated version of the HC2
assay was equally as sensitive and specific as the manual platform of the
assay (with insignificant false-negative result with CT). This therefore
indicates, as noted by the authors, that laboratories now have the option
to choose which is the most cost-effective method for them, based on the
prevalence of disease and the throughput rate at the clinic, without
needing to be concerned about effects on test performance. However, the
automated COBAS Amplicor PCR showed better sensitivity for CT, which is
insignificant as the authors indicated, but may give a different picture
with bigger sample size. It would be interesting to have a comparative
study of all automated screening assays currently available in the market
in terms of sensitivity, specificity, cost and robustness.
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