Analytical framework to evaluate and optimize the use of imperfect diagnostics to inform outbreak response: Application to the 2017 plague epidemic in Madagascar

During outbreaks, the lack of diagnostic “gold standard” can mask the true burden of infection in the population and hamper the allocation of resources required for control. Here, we present an analytical framework to evaluate and optimize the use of diagnostics when multiple yet imperfect diagnostic tests are available. We apply it to laboratory results of 2,136 samples, analyzed with 3 diagnostic tests (based on up to 7 diagnostic outcomes), collected during the 2017 pneumonic (PP) and bubonic plague (BP) outbreak in Madagascar, which was unprecedented both in the number of notified cases, clinical presentation, and spatial distribution. The extent of these outbreaks has however remained unclear due to nonoptimal assays. Using latent class methods, we estimate that 7% to 15% of notified cases were Yersinia pestis-infected. Overreporting was highest during the peak of the outbreak and lowest in the rural settings endemic to Y. pestis. Molecular biology methods offered the best compromise between sensitivity and specificity. The specificity of the rapid diagnostic test was relatively low (PP: 82%, BP: 85%), particularly for use in contexts with large quantities of misclassified cases. Comparison with data from a subsequent seasonal Y. pestis outbreak in 2018 reveal better test performance (BP: specificity 99%, sensitivity: 91%), indicating that factors related to the response to a large, explosive outbreak may well have affected test performance. We used our framework to optimize the case classification and derive consolidated epidemic trends. Our approach may help reduce uncertainties in other outbreaks where diagnostics are imperfect.

The response to the 2017 plague outbreak in Madagascar was complicated by the lack of a perfect or "gold standard" diagnostic. This study shows how multiple, imperfect diagnostic tests can be used to improve the response to an outbreak.

Performance of plague rapid diagnostic test compared to bacteriology: a retrospective analysis of the data collected in Madagascar

Background

Plague is a highly fatal disease caused by Yersinia pestis. Late diagnosis hampers disease outcome and effectiveness of control measures, induces death and disease spread. Advance on its diagnosis was the use of lateral flow rapid diagnostic test (RDT).

Methods

We assessed the performance of the plague RDT based on Y. pestis F1 antigen detection more than 15 years after its deployment in Madagascar. We compared the RDT with bacteriological culture results, using data from plague notified cases collected during the periods for which both tests were performed independently and systematically.

Results

Used with bubonic plague (BP) patient samples, RDTs had a sensitivity of 100% (95% CI: 99.7–100%), a specificity of 67% (95% CI: 64–70%) with a good agreement between bacteriology and RDT results (86%; κ = 0.70, 95% CI 0.67–0.73). For pneumonic plague (PP), RDT had a sensitivity of 100% (95% CI: 91–100%) and a specificity of 59% (95% CI: 49–68%) and concordance between the bacteriological and plague RDT results was moderate (70%; κ = 0.43, 95% CI 0.32–0.55). Analysis focusing on the 2017–2018 plague season including the unprecedented epidemic of PP showed that RDT used on BP samples still had a sensitivity of 100% (95% CI: 85–100%) and a specificity of 82% (95% CI: 48–98%) with a very good agreement with bacteriology 94% (κ = 0.86, 95% CI 0.67–1); for PP samples, concordance between the bacteriological and plague RDT results was poor (61%; κ = − 0.03, 95% CI -0.17 – 0.10).

Conclusions

RDT performance appeared to be similar for the diagnosis of BP and PP except during the 2017 PP epidemic where RDT performance was low. This RDT, with its good sensitivity on both plague clinical forms during a normal plague season, remained a potential test for alert. Particularly for BP, it may be of great value in the decision process for the initiation of therapy. However, for PP, RDT may deliver false negative results due to inconsistent sample quality. Plague diagnosis could be improved through the development of next generation of RDTs.