Dr. Harold Kessler of Chicago's Rush Medical College presented data from a collaboration with colleagues at the University of Colorado in which they analyzed HIV isolates they obtained from 22 source patients of health care workers who sustained occupational exposures to evaluate the presence of antiretroviral resistance. They found that genetic evidence of resistance to AZT was present in 10 of the samples, and resistance to ddI in 5, but found no evidence of resistance of D4T. The patients whose cells contained HIV with resistance mutants were more immunosuppressed (median CD4 count of 9 cells/mm3) compared to those for whom they found no resistance mutations (median CD4 count 169 cells/mm3). The data suggested a high prevalence of antiretroviral drug resistance in HIV isolates from patients who were the source of health care worker occupational exposures. The importance of this finding is that current CDC guidelines suggest that the front line therapy of occupational exposures should include AZT+3TC +/- a protease inhibitor, but may need to be individualized if the patient is drug experienced. The current study suggests that resistance is common, so that often post-exposure prophylaxis will need to use combinations of D4T with either 3TC or ddI, i.e. drugs that are different from what the source patient was taking.