Rapid and sensitive diagnosis of fungal keratitis with direct PCR without template DNA extraction

This study was aimed at developing a direct PCR assay without template DNA extraction for the rapid and sensitive diagnosis of infectious keratitis. Eighty corneal scrapings from 67 consecutive patients with clinically suspected infectious keratitis were analysed prospectively. Direct PCR was performed with all scrapings, with specific primers for fungi, bacteria, herpes simplex virus-1 (HSV-1) and Acanthamoeba simultaneously. The results were compared with those obtained from culture, smear, and confocal microscopy. Discrepant results were resolved according to the therapeutic effects of the corresponding antimicrobial drugs. The lowest detection limit of direct PCR was ten copies of each pathogen. Sixty-six scrapings yielded positive results with direct PCR, giving a total positive detection rate of 82.5% (66/80). For 34 patients with high suspicion of fungal keratitis, the positive detection rate of direct PCR was 84.8% (39/46). This rate increased to 91.2% (31/34) when repeated scrapings were excluded, and was significantly higher than the rates obtained with culture (35.3%, 12/34) and smear (64.7%, 22/34) (p <0.001), and was also higher than the rate obtained with confocal microscopy (74.1%, 20/27). The sensitivities for the diagnosis of infectious keratitis with direct PCR and culture were 98.0% and 47.1% (p <0.001), whereas the specificities were 81.8% and 100%, respectively. The time required to complete the entire direct PCR procedure was only 3 h. The direct PCR assay is a rapid diagnostic technique with high sensitivity and specificity for infectious keratitis, and it is expected to have an impact on the diagnosis and treatment of infectious keratitis in the future.