Confocal Microscopy Most Sensitive Diagnostic Technique for Some Microbial Keratitis

Doctor Testing Biological Specimen
Only bacterial infections are detected with greater sensitivity using corneal cultures, a report suggests.

While in vivo confocal microscopy (IVCM) may be the most sensitive technique for diagnosing Acanthamoeba keratitis (AK) and fungal keratitis (FK), corneal cultures continue to be the most accurate for bacterial keratitis (BK), according to a study published in Eye.

The study, a retrospective review, looked at polymerase chain reaction (PCR), IVCM, and culture results for microbial keratitis (MK) diagnosis for sensitivity and specificity at Moorfields Eye Hospital between August 2013 and December 2014. Researchers had PCR results for 259 MK patients with concurrent culture for 203/259 and IVCM for 149/259. “Sensitivities and specificities with 95% confidence intervals [95% CI] were calculated for AK and FK, by comparison with culture, for both IVCM and PCR,” according to the research team. 

The investigators calculated AK, FK, and BK sensitivities, and compared each diagnostic method with a composite reference standard (a positive result for one or more of culture, PCR or IVCM having a specificity of 100% by definition). For the latter, sensitivities with (95% CI) were: for AK, IVCM 77.1% (62.7-88.0%), PCR 63.3% (48.3-76.6%), culture 35.6 (21.9-51.2); for FK, IVCM 81.8% (48.2-97.7%), PCR 30.8% (9.09-61.4%), culture 41.7% (15.2-72.3%); for BK, PCR 25.0% (14.7-37.9%), culture 95.6% (87.6-99.1%).

Results of the study show a need for continued diagnostic result audits to improve PCR diagnosis. While FK and AK are common, “ophthalmology units need to have all these techniques available to optimise their MK management,” researchers explain. The study also shows that PCR is more sensitive for Acanthamoeba than it is for fungus.

The study’s limitations include the fact that clinical diagnosis was not used as a comparator — researchers could not be sure what caused keratitis in 49.4% of patients who had negative diagnostic tests. Another limitation is that the low total yield (50.6%) identified might bias the results.


Hoffman JJ, Dart JKG, De SK, Carnt N, Cleary G, Hau S. Comparison of culture, confocal microscopy and PCR in routine hospital use for microbial keratitis diagnosis. Eye. Published online November 5, 2021. doi:10.1038/s41433-021-01812-7