Evaluation of corneal scraping smear examination methods in the diagnosis of bacterial and fungal keratitis: a survey of eight years of laboratory experience.

Purpose: To determine the sensitivity, specificity, and predictive values of Gram and potassium hydroxide with calcofluor white (KOH+CFW) stains in the diagnosis of early and advanced microbial keratitis, a retrospective analysis of comparative data from a prospectively collected database was done.

Methods: Patients with nonviral microbial keratitis seen at L.V. Prasad Eye Institute between February 1991 and December 1998 were included in the study. The type of bacteria seen on Gram stain was determined from 251 corneal scrapings from patients with early keratitis and 841 corneal scrapings from patients with advanced keratitis. The presence of fungi in corneal scrapings was determined by KOH+CFW stain of 114 and 363 scrapings from patients with early and advanced keratitis, respectively. The smear findings were compared with culture results to analyze specificity, sensitivity, and predictive values of the staining techniques.

Results: The sensitivity of Gram stain in the detection of bacteria was 36.0% in early and 40.9% in advanced keratitis cases; however, the specificity was higher in both groups (84.9% and 87.1%, respectively). Comparatively, the sensitivity and specificity of fungal detection were higher using KOH+CFW in early (61.1% and 99.0%, respectively) as well as advanced keratitis (87.7% and 83.7%, respectively). Predictive values were high for KOH+CFW in fungus detection, while they were poor for Gram stain in bacteria detection. In advanced keratitis cases, the false positives were higher in fungal detection (16.3%) than in bacterial detection (10.3%), while the false negatives were significantly higher in bacterial detection compared with fungal detection (59.1% versus 12.3%, p< 0.0001). In early keratitis, on the other hand, both false positives and false negatives for bacterial detection were significantly higher than fungal detection.

Conclusions: Decisions can reliably be based on KOH+CFW stain of corneal scrapings for initiation of antifungal therapy in mycotic keratitis. The results of Gram stain, on the other hand, have limited value in therapeutic decisions for bacterial keratitis. Therefore, the search for a better modality for early and efficient diagnosis of bacterial keratitis needs to continue.