If an isolate of an Enterobacteriaceae is resistant to one fluoroquinolone (i.e., ciprofloxacin), should it be considered resistant to all fluoroquinolones? How about Pseudomonas aeruginosa?

Answer:

The answer depends on the organism and the fluoroquinolone. NCCLS Table 1 and 1A in "Performance standards for antimicrobial susceptibility testing: eleventh informational Supplement, M100-S11" can assist in extrapolating the results from one fluoroquinolone to another. For example, in Table 1 on page 32, in the Enterobacteriaceae column under Group B, ciprofloxacin and levofloxacin are in the same box and connected with the word "or". This means that there is nearly complete cross resistance and cross susceptibility between these two agents. In contrast, in the Pseudomonas aeruginosa column, ciprofloxacin is in a box by itself and levofloxacin appears only in a separate box in Group U. Thus, you cannot predict the activity of ciprofloxacin based on levofloxacin results or vice versa for Pseudomonas (the agents must be in the same box with the word "or" to do so). If your results for ciprofloxacin and levofloxacin disagree with Enterobacteriaceae, make certain your quality control results are acceptable and the isolate identification and susceptibility results are correct. It would be inappropriate to extrapolate results for other fluoroquinolones unless there is sufficient documentation to support their relatedness, similar to that found in the NCCLS tables.