The clinical diagnosis of microbial keratitis.

PURPOSE: To evaluate the ability of ophthalmologists to predict the laboratory results of presumed microbial keratitis and to explore which findings may influence diagnostic prognostication.
DESIGN: Prospective, cross-sectional study.
METHODS: Fifteen ophthalmologists completed study forms at the initial presentation of patients with presumed microbial keratitis. After predicting the category of microbial recovery, clinicians submitted corneal scrapings for masked laboratory processing. The relative effects of ocular inflammatory signs on correct microbial diagnosis were explored with Poisson regression.
RESULTS: Clinical examiners correctly predicted the presence or absence of microbial recovery in 79 (76%) of 104 ulcerative keratitis cases and successfully distinguished among bacterial, fungal, and amebic keratitis for 54 (73%) of 74 culture-positive infections, although only 31 (42%) were subcategorized properly. The positive predictive value of clinical diagnosis was 65% (95% confidence interval [CI], 43% to 84%) for 20 eyes with Pseudomonas keratitis cases, 48% (95% CI, 32% to 63%) for 38 other bacterial keratitis, 45% (95% CI, 17% to 77%) for 13 fungal keratitis, and 89% (95% CI, 52% to 100%) for nine Acanthamoeba keratitis cases. The recognition of Pseudomonas keratitis significantly improved by the occurrence of a larger infiltrate (P = .02), and correctly predicting Acanthamoeba keratitis was enhanced by observing a ring infiltrate (P < .001). Antimicrobial use before referral significantly attenuated clinical diagnosis (P = .03) and hampered microbial recovery (P = .004).
CONCLUSIONS: Established Pseudomonas keratitis and Acanthamoeba keratitis can be suspected before laboratory confirmation, but overlapping inflammatory features and recent empiric antimicrobial treatment limits etiologic recognition of most microbial corneal infections.