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When doing the Dtest for clindimycin does the zone have to have a blunt side or is there an indeterminate result possible when there is growth between the Clin disk and the Eryth disk,(when using a 0.5 Mcfarland dilution)?
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The D test should only be performed if the organism is resistant to erythromycin and susceptible to clindamycin. If there is a large zone (>22mm) around the erythromycin disk, the organism is susceptible to erythromycin and the D test is not needed. If there is a small zone around the clindamycin disk, (<21mm), the organism is either intermediate or resistant to clindamycin and the D test is not needed. If there is a large zone around the clindamycin disk (>21 mm), and no zone or a small zone around the erythromycin disk, the D test should be performed, because the organism looks susceptible to clindamycin but may become resistant on therapy. When the test is performed, the disks are placed 15 to 20 mm apart. The observation of a flattening of the round zone around the clindamycin disk (forming a D rather than a circle), adjacent to the erythromycin disk is a positive result. This result means that it is likely that the organism will become resistant to clindamycin on treatment with the drug, even though it is not truly resistant now. It is recommended that the isolate be called “resistant.” Either a strain is D test positive or it is not. There is no indeterminant result. If the strain does not produce a “D”, but produces a round circular zone >21 mm in diameter, it is susceptible to clindamycin.
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