Evaluation of microbiological diagnostic techniques in postoperative endophthalmitis in the Endophthalmitis Vitrectomy Study
M. Barza, P. R. Pavan, B. H. Doft, S. R. Wisniewski, L. A. Wilson, D. P. Han and S. F. Kelsey
Tupper Research Institute, Department of Medicine, New England Medical Center, Boston, Mass, USA.
OBJECTIVE: To analyze the data for cultures and Gram stains prospectively
collected by protocol in the Endophthalmitis Vitrectomy Study. DESIGN:
Cultures of aqueous, undiluted vitreous, and (for patients who underwent
vitrectomy) vitrectomy cassette fluid obtained from 420 patients were
prepared on chocolate agar, in thioglycolate broth, and on Sabouraud
dextrose agar; Gram stains of the aqueous and undiluted vitreous were made.
Criteria were devised to distinguish true pathogens (confirmed positive
cultures) from contaminants. SETTING: Private and university-based
retina-vitreous practices and corresponding microbiology laboratories.
RESULTS: Compared with the aqueous, undiluted vitreous produced a higher
percentage of confirmed positive cultures and higher colony counts on
chocolate agar and was more frequently the only source of a positive
culture from the eye. Nevertheless, the aqueous and vitrectomy 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 laboratory-confirmed infection was not
statistically significantly higher in the vitrectomy group than in the tap
or biopsy group. CONCLUSIONS: The vitreous was a richer source of positive
cultures and high colony counts than was the aqueous, either because it is
more supportive of bacterial growth or because a somewhat larger inoculum
of the vitreous than of aqueous could be obtained. The result of Gram stain
should not determine the choice of antibiotic drugs in the treatment of
endophthalmitis. Vitrectomy, with culture of the vitrectomy cassette fluid,
did not produce significantly more positive cultures than tap or biopsy
material, and the procedure should not be performed to improve the
microbiological yield.
