Is inclusion of Sabouraud dextrose agar essential for the laboratory diagnosis of fungal keratitis?

Abstract

Purpose: To determine whether the inclusion of Sabouraud dextrose agar (SDA) is essential in the diagnosis of fungal keratitis.

Materials and methods: Corneal scrapings of 141 patients with microbial keratitis were smeared and cultured. Sheep blood agar (BA), chocolate agar (CA), SDA, non-nutrient agar (NNA) with Escherichia coli overlay, and brain heart infusion broth (BHI) were evaluated for time taken for growth and cost. The media were also evaluated experimentally for rate of growth and time taken for identification.

Results: Twenty-six of 39 patients positive for fungus in corneal scrapings by microscopy were culture-positive. Fungus grew on BA in 22/39, on CA in 18/39, on SDA in 17/39, on NNA in 17/39, and on BHI in 13/39 cases. Growth on SDA was higher in ulcers with larger infiltrate (6/18 versus 9/13, P = 0.04). Estimated saving with inclusion of only BA/CA was Rs. 600 per patient. Performance of all media was similar in in vitro experiment although the characteristic spores and color were seen earlier on SDA.

Conclusion: Fungal keratitis can be reliably confirmed on BA or CA, which support growth of both bacteria and fungus.

Figure 1

Growth of Aspergillus flavus on (a) Blood agar after 3-days; (b) Chocolate agar after 3-days; (c) Sabouraud dextrose agar after 4-days; (d) Non-nutrient agar after 4-days (microscopic picture, ×400)