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E. Y. Tu, C. E. Joslin, M. E. Shoff, G. C. Booton; Differential Rates of Isolation From Two Laboratories During an Ongoing Outbreak of Acanthamoeba Keratitis. Invest. Ophthalmol. Vis. Sci. 2007;48(13):753.
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Low rates of microbiologic isolation in previous outbreaks of Acanthamoeba keratitis (AK) has led to uncertainty concerning the accuracy of clinical and other evidence used to define disease as well as the value of superficial corneal cultures. An ongoing outbreak of 60 cases of AK in the Chicago Metropolitan Area from June 2003 through November 2006 was studied to determine the rate of Acanthamoeba isolation from superficial corneal scrapings. Results from a hospital-based and a research-based laboratory were compared to assess rates of isolation in comparison to historical rates.
60 incident cases of AK were identified utilizing clinical, confocal and Diff-Quick smear criteria at the University of Illinois Eye and Ear Infirmary from 06/2003 through 11/2006. Records were reviewed as to presentation, method of diagnosis, University of Illinois at Chicago Hospital clinical (UIC) laboratory and Ohio State University (OSU) research laboratory culture results. All cultures were obtained by superficial corneal scrapings with a Kimura spatula and dislodged into Paige’s saline for transport. Standard microbiologic methods for the isolation of Acanthamoeba were applied according to individual protocols at each institution.
42 cultures were performed at the UIC laboratory of which 16 (38%) were positive. No significant change in isolation rate was seen over the study period at UIC. Of 19 OSU samples completed, 19 (100%) successfully isolated acanthamoeba. Sufficient material allowed 15 samples to be submitted to both OSU and UIC laboratories with isolation rates of 100% (15/15) and 53% (8/15), respectively.
The high correlation of OSU positive cultures and clinical/confocal/smear evidence of disease supports our use of these criteria in the diagnosis of AK and the validity of our outbreak. Superficial corneal scrapings are of significant value in AK. Often relying on clinical microbiologic laboratories, rates of isolation in previous outbreaks of Acanthamoeba keratitis (AK) have been reported to be 50% or less, consistent with our UIC isolation rate of 38%. Comparison testing of independent samples from the same patients yielded a UIC laboratory sensitivity rate of approximately 53%. This supports the hypothesis that past studies reporting low rates of microbiologic isolation may be a result of individual laboratory expertise rather than a low specificity of clinical or other criteria utilized in the diagnosis of AK.
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