Exploratory use of fluorescent SmartProbes for the rapid detection of microbial isolates causing corneal ulcer

Abstract
Purpose: To explore the use of optical SmartProbes for the rapid evaluation of corneal scrapes from patients with suspected microbial keratitis, as a clinical alternative to Gram stain.

Design: Experimental study with evaluation of a diagnostic technology.

Methods Corneal scrapes were collected from 267 patients presenting with microbial keratitis at a referral cornea clinic in South India. Corneal scrapes were flooded with SmartProbes (BAC One or BAC Two) and evaluated by fluorescence microscopy (without the need for sample washing or further processing). The SmartProbe labelled samples were scored as bacteria/fungi/none (BAC One) or gram-negative bacteria/none (BAC Two) and compared to Gram stain results.

Results: Compared to Gram stain, BAC One demonstrated sensitivity and specificity of 80.0 and 87.5 percent respectively, positive and negative predictive values (PPV, NPV) of 93.8 and 65.1 percent, and an accuracy of 82.2. BAC Two demonstrated sensitivity and specificity of 93.3 and 84.8 percent respectively, a NPV of 99.2 percent and an accuracy of 85.6 percent. When the corresponding culture results were compared to the Gram stain result, the sensitivity and specificity were 73.4 and 70.7 percent, the PPV and NPVs were 86.5 and 51.0 percent, and an overall accuracy of 72.6.

Conclusions: Fluorescent SmartProbes offer a comparative method to Gram stain for delineating gram-positive or gram-negative bacteria or fungi within corneal scrapes. We demonstrate equivalent or higher sensitivity, specificity, PPV and NPVs, and accuracy than culture to Gram stain. Our approach has scope for point-of-care clinical application to aid in the diagnosis of microbial keratitis.