Evaluation of the 4-hour RapID NF plus method for identification of 345 gram-negative nonfermentative rods

Abstract

The ability of the RapID NF Plus system (Innovative Diagnostic Systems, Inc., Atlanta, Ga.) to identify 345 nonfermentative gram-negative rods was evaluated. Kits were inoculated with no. 1 McFarland suspensions, and reactions were interpreted after a 4-h incubation at 35°C. Overall, the method correctly identified 311 strains (90.1%) without additional tests and 21 strains (6.1%) with additional tests, and 13 strains (3.8%) were misidentified. Five of 13 misidentified strains were Alcaligenes faecalis- Alcaligenes odorans misidentified as Alcaligenes xylosoxidans; however, all strains were xylose negative but nitrate positive and could have been A. faecalis group I-Alcaligenes piechaudii. The system does not differentiate between Pseudomonas fluorescens and Pseudomonas putida, and all Acinetobacter species are identified as Acetinobacter calcoaceticus. Additionally, no subspecies differentiation is made between A. xylosoxidans subsp. xylosoxidans and A. xylosoxidans subsp. denitrificans. All strains of the former Flavobacterium group IIb are identified as Flavobacterium indologenes- Flavobacterium gleum, and no species identification of the genus Methylobacterium is attempted. The system is easy to set up and interpret and provides an accurate commercial nonautomated method for same-day identification of gram-negative nonfermenters.