The FDA labeling recommendations for carbamazepine were followed by the inclusion of similar information in the warning section of the product labeling for phenytoin highlighting the risk of severe and potentially fatal SJS or TEN.23 The FDA recommends genetic testing for the presence of the HLA-B*1502 allele in patients of Asian ancestry who are being considered for therapy with phenytoin. Testing recommendations for phenytoin are based on literature describing SJS and TEN, as well as the presence of an aromatic ring which has a similar structure to carbamazepine.33 However, the genetic association of HLA-B*1502 and SJS or TEN development may be more complex in the case of phenytoin. A case-control association study identified that additional variants (HLA-B*1301, Cw*0801, and DRB1*1602) may also be associated with development of phenytoin induced SJS and TEN.33 Sensitivity and specificity values for HLA-B*1502 testing from a meta-analysis of studies published as of 2012 were 36.6% and 87.2%, respectively, perhaps indicative of additional factors influencing phenytoin-associated risk for SJS and TEN.34
