Background Community-associated MRSA (CA-MRSA) has become an important clinical problem in many areas of the United States. Clindamycin is often used to treat soft tissue and musculoskeletal infections due to CA-MRSA in infants and children. However, clindamycin susceptibility testing is complicated by the presence of inducible clindamycin resistance (clinda-R) in some erythromycin resistant (erythro-R) CA-MRSA isolates. Standard laboratory susceptibility test methods do not detect this phenomenon. The D test is a simple disk diffusion method which reliably identifies inducible clinda-R.
Methods The Microbiology Laboratory computer logbook was utilized to ascertain the antibiotic sensitivity test results of all SA isolated from clinical specimens collected from children seen in the LSUHSCS-S Pediatric Outpatient Clinic from January 2002 to the end of September 2004. Of the erytho-R MRSA isolates, 218 were tested for inducible clinda-R by the D test, either during routine sensitivity testing (since May 2004) or from a frozen stock of saved isolates. The D test is performed as follows: erythromycin and clindamycin disks are placed adjacent to one another at a distance of 20 mm. Inducible clinda-R is identified by the formation of a blunted zone around the clindamycin disk in the area directly opposed to the erythromycin disk.
Results MRSA isolates/total number of SA isolates (%) from pediatric outpatients were 47/109 (43%) in 2002; 134/207 (65%) in 2003; and 106/152 (70%) in the first 9 months of 2004. Of these MRSA isolates, 8 were both erythro-R/clinda-R (2.8 %) and 244 were erythro-R/clinda-S (85%). Of erythro-R/clinda-S MRSA isolates tested, the number and percent found to have inducible clinda-R (D test positive) were 4/29 (14%) in 2002; 7/99 (7%) in 2003; and 7/90 (8%) in 2004; total sample 18/218 (8%).
Conclusions The prevalence of MRSA in our pediatric outpatients has increased to 70%; however, the rate of inducible clinda-R in erythro-R MRSA isolates is less than 10%. Therefore, clindamycin remains an acceptable initial therapy for suspected community-associated SA infections in our patients. We now perform the D test on the primary disk diffusion sensitivity plate for all Staphylococcus spp. isolated from pediatric patients. This enables the laboratory to report the clindamycin susceptibility result within 24 hours of the isolation of the organism.