Comparative Analysis Of Bacterial Isolation Rates From Corneal Scrapes And Corneal Impression Membranes In Cases Suspected Microbial Keratitis

Corneal impression membranes (CIM) are an easier, safer, and quicker way to obtain samples in suspected microbial keratitis. This study compares the level of agreeability in the isolation rates of bacteria using CIM and corneal scrapes (CS) in patients with suspected microbial keratitis. 

Moorfields Eye Hospital, London, United Kingdom.

Retrospective review of 116 consecutive patients with suspected microbial keratitis requiring corneal sampling. CS was performed with sterile 21G needles; samples were inoculated onto chocolate, Sabaroud and non-nutrient agar, and a sample sent in brain heart infusion (BHI) broth. CIM was performed by placing a sterile 12mm polytetrafluoroethylene disc on the cornea for 3 seconds, and transferred to the laboratory in BHI broth which was vortexed for 5-10 seconds, and 10 µL of broth inoculated onto agar plates. The level of agreeability in bacteria isolation between the paired samples was compared using Cohen kappa.

The mean age of the patients (years) was 46.0 (+/-standard deviation 18.6). Micro-organisms was isolated in 74/116 (63.8%) CIM samples and in 86/116 (74.1%) CS samples (p=0.09). The four commonest micro-organisms were P. aeruginosa (19% CIM; 20.7% CS), S. epidermidis (19.8%; 25.0%), S. aureus (6.9%; 7.8%), and S. warneri (4.3%; 7.8%). The level of agreeability between CIM and CS was 0.91 for P. aeruginosa, 0.11 for S. epidermidis, 0.54 for S. aureus, and 0.31 for S. warneri.

CIM is an effective method in collecting corneal samples in patients with suspected microbial keratitis, with a high level of agreeability with CS in the isolation of P. aeruginosa, and a moderate level of agreeability in the isolation of S. aureus. The discordance between both methods of corneal sampling were mainly due to differences in coagulase negative Staphylococci, which can be of uncertain clinical significance. Our unit has now transitioned to using CIM alone for sampling in bacterial keratitis, but CS are still performed for suspected atypical keratitis.