Comparative Analysis of Bacterial Isolation Rates From Corneal Scrapes and Corneal Impression Membranes in Suspected Microbial Keratitis

Purpose: Corneal impression membranes (CIMs) are an easier and safer method to obtain samples in microbial keratitis than corneal scrapes (CS). This study compares the level of agreement in the isolation rates of bacteria using CIM and CS in patients with suspected microbial keratitis. Methods: Retrospective review of 116 consecutive patients with suspected microbial keratitis requiring corneal sampling. Both CS and CIM were performed for each case. The level of agreement in microbial isolation rates between the paired samples was compared using Cohen kappa. Results: Microorganisms were isolated from 74/116 (63.8%) of CIM samples and in 84/116 (72.4%) of CS samples ( P = 0.12). The commonest bacterial isolates were Pseudomonas aeruginosa [20.0% (CIM) vs. 20.7% (CS); P = 0.87] and Staphylococcus species. The commonest coagulase-negative Staphylococcus species were Staphylococcus epidermidis (24.1% vs. 25.9%; P = 0.88) and Staphylococcus warneri (3.4% vs. 7.8%; P = 0.25). Staphylococcus aureus was isolated in 6.9% versus 7.8% ( P = 1.00). There was no isolation of fungi or acanthamoeba in any of the 116 CIM or CS samples. Mixed growth was found in 14.7% of CIM samples and 28.4% of CS samples ( P = 0.02). The level of agreement between CIM and CS was 0.89 ( P < 0.001) for Pseudomonas aeruginosa , 0.04 ( P = 0.71) for Staphylococcus epidermidis , 0.56 ( P < 0.001) for Staphylococcus aureus , and 0.11 ( P = 0.19) for Staphylococcus warneri. Conclusions: CIM is an effective method in collecting corneal samples in patients with suspected microbial keratitis, with a high level of agreement with CS in the isolation of Pseudomonas aeruginosa , and a moderate level of agreement in the isolation of Staphylococcus aureus .