Primary culture media for routine urine processing.

It has been recommended that routine microbiological processing of urine specimens include quantitative plating onto blood agar medium along with a selective and differential agar such as MacConkey agar for gram-negative organisms. Few data have been published to justify this combination. To evaluate the validity of this recommendation 2,553 midstream, clean-voided urine samples were quantitatively plated onto blood agar, MacConkey agar, and colistin-nalidixic acid agar, which is a selective medium for gram-positive organisms. The amounts of growth on each of the three media were compared. Results indicated that the best medium combination was colistin-nalidixic acid agar and MacConkey agar. The use of colistin-nalidixic acid agar instead of blood agar increased the detection of significant growth of enterococci, lactobacilli, and Torulopsis glabrata.