The isolates which were resistant to erythromycin were further tested by D-zone test which was performed as per CLSI guidelines by inoculating 0.5 McFarland bacterial suspensions on the Mueller-Hinton agar plates with the help of sterile swabs and placing the Erythromycin (E-15 [micro]g) and Clindamycin (CD-2 [micro]g) disks side-by-side with edge-to-edge distance of 15 mm.
All the erythromycin resistant isolates were subjected to D-zone test and the resulting distribution of S.
aureus isolates showed inducible Clindamycin resistance by giving a positive D-zone test; hence, its prevalence was found to be 21.00% (26/125) with percentage distribution of cMLSB phenotype and MS phenotypes in all S.
 The true sensitivity to Clindamycin can only be judged after performing D-zone test on the erythromycin resistant S.
All these were tested for D-zone test. Amongst them maximum isolates (40.29%, 35/88) were of cMLSB phenotype (D-zone test negative), followed by constitutive and inducible resistance phenotype.
The D-zone test for inducible clindamycin resistance was performed for isolates resistant to erythromycin and susceptible to clindamycin by single-agent testing (11).
Of all CC1 isolates, 16/25 (64%) were resistant to clindamycin; 14/16 (88%) were not susceptible by virtue of a positive D-zone test result.
This may explain the seemingly anomalous data from the USA400 era in Chicago that 31/33 (94%) of clindamycin-susceptible, erythromycin-resistant strains of MRSA were D-zone test positive (12).
A D-zone test confirmed the presence of iMLSB in the pre-treatment isolate.
Validation of the D-zone test starts with standard testing of both clindamycin and erythromycin disks by normal quality-control procedures to assure appropriate antibiotic concentration.
Inducible clindamycin resistance was determined for isolates with the erythromycin-resistant/clindamycin-susceptible phenotype by using the double-disk diffusion test (D-zone test) (13).
The 1 MRSA isolate that was not susceptible to clindamycin demonstrated inducible resistance by the D-zone test. In 4 cases, isolates were not available for testing at CDC.