EVALUATION OF MICROBIOLOGICAL DIAGNOSTIC-TECHNIQUES IN POSTOPERATIVE ENDOPHTHALMITIS IN THE ENDOPHTHALMITIS VITRECTOMY STUDY

Objective: To analyze the data for cultures and Gram stains prospectiv ely collected by protocol in the Endophthalmitis Vitrectomy Study. Des ign: Cultures of aqueous, undiluted vitreous, and (for patients who un derwent vitrectomy) vitrectomy cassette fluid obtained from 420 patien ts were prepared on chocolate agar, in thioglycolate broth, and on Sab ouraud dextrose agar; Gram stains of the aqueous and undiluted vitreou s were made. Criteria were devised to distinguish true pathogens (conf irmed positive cultures) from contaminants. Setting: Private and unive rsity-based retina-vitreous practices and corresponding microbiology l aboratories. Results: Compared with the aqueous, undiluted vitreous pr oduced a higher percentage of confirmed positive cultures and higher c olony counts on chocolate agar and was more frequently the only source of a positive culture from the eye. Nevertheless, the aqueous and vit rectomy cassette fluid were the only source of a positive culture from the eye in 4.2% and 8.9% of eyes, respectively. The overall yields of chocolate agar and thioglycolate broth were similar. A positive Gram stain from the aqueous or undiluted vitreous was highly predictive of a positive culture from the eye, but a negative Gram stain had little predictive value for the culture result. The overall rate of laborator y-confirmed infection was not statistically significantly higher in th e vitrectomy group than in the tap or biopsy group. Conclusions: The v itreous was a richer source of positive cultures and high colony count s than was the aqueous, either because it is more supportive of bacter ial growth or because a somewhat larger inoculum of the vitreous than of aqueous could be obtained. The result of Gram stain should nor dete rmine the choice of antibiotic drugs in the treatment of endophthalmit is. Vitrectomy, with culture of the vitrectomy cassette fluid, did not produce significantly more positive cultures than tap or biopsy mater ial, and the procedure should not be performed to improve the microbio logical yield.