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Microbial keratitis audit: evaluating positive microbial culture rates in corneal scrapes taken by two different collection methods - comparing direct plating onto agar with inoculation of liquid culture medium

Poster Details

First Author: L. Hook UK

Co Author(s):    A. Khan   R. Stewart                 

Abstract Details


The traditional corneal scrape method of direct plating onto agar plates is an arduous task for the clinician and involves several (4 or 5) scrapes which can be difficult for patient compliance. A simpler method of direct inoculation into liquid transport medium (brain heart infusion broth) requires only a single sample and avoids the need for smearing onto plates by the clinician taking the sample. This audit compares the yield of positive cultures of both methods in suspected microbial keratitis and evaluates contamination rates. The secondary aim is to investigate the range of common causative organisms with each method.


Data were collected over a 9 month period in the Leeds Teaching Hospitals NHS Trust Ophthalmology department, a busy tertiary referral centre in West Yorkshire. Scrapes were taken across the acute referral and corneal clinics. The sampling method of plating changed to BHI in April 2020.


A database of consecutive corneal scrapes taken before or after April 2020 when the change from plates to BHI occurred was obtained from the Microbiology department. The electronic patient records were examined to obtain information on patient demographics, risk factors, clinical signs of infection, prior anti-microbial use, and the culture results and sensitivities. 65 scrapes were taken from 60 patients with 34 inoculated into BHI and 31 directly onto plates. Samples obtained during the month of April 2020 were excluded as it was uncertain which method was used during this month.


The mean age was 57, there were more males in the plates group (74% vs 39%) p<0.005. 41% of the BHI group and 52% in the plates group (p=0.4) had prior antimicrobial treatment. The overall positive culture rate was 40%. On plates the positive culture rate was 19% and BHI was 59% (p<0.005). 8% had polymicrobial growth, all in the BHI group. There was no growth on scrapes on the 11 taken from eyes with no clinical evidence of infection. 36 different organisms were cultured from 26 specimens. 69% were gram positive bacteria with similar rates in each group.


This audit shows that inoculation of BHI broth to collect corneal scrapes is as effective at yielding positive cultures as the traditional plating technique. There were significantly higher growth rates on broth. The 11 patients identified as having no evidence of infection did not yield any positive cultures so there is no evidence that the higher rates were due to contamination. The BHI technique has many advantages over plates including storage of media, fewer samples and no requirement for the clinician to plate the sample onto media. It is therefore easier, quicker and more comfortable for the patient.

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