Utility of investigation for suspected microbial keratitis: a diagnostic accuracy study

Purpose The true disease status of a population with suspected microbial keratitis (MK) cannot be verified. There is not an accurate (gold) reference standard to confirm infection and inter-test comparisons of sensitivity and specificity therefore lead to bias with questionable estimates of test utility. We present an alternative method to report results. Methods We used a decision to treat as the definition for MK. We retrospectively compared the results of corneal culture and polymerase chain reaction (PCR) as these are objective tests available for the three principal groups of pathogens. We then estimated the potential contribution of positive results, either alone or in combination, to support the working diagnosis. Results We included 2021 (77.4%) eyes with suspected bacterial keratitis, 365 (14.0%) with suspected acanthamoeba keratitis, and 226 (8.6%) with suspected fungal keratitis, all treated between July 2013 and December 2019. In these groups, there were 51.6% positive culture and 6.5% positive PCR results for bacteria, 19.0% and 40.5% for acanthamoeba, and 28.3% and 15.0% for fungi. Between groups the differences in the proportions of positive results from culture and PCR was statistically significant ( P < 0.001). The added benefit of PCR to the result of culture in identifying a potential pathogen was 1.4% for bacteria ( P = 0.6292), 24.4% for acanthamoeba ( P = 0.0001), and 5.8% for fungi ( P = 0.3853). Conclusions For suspected MK a comparison of the test positivity rate is an easily comprehensible outcome measure of test utility.