Health care worker PPD test results were also reviewed over the same time periods. A total of 39 health care workers were previously PPD negative and tested (25 during the initial period, 17 during early follow-up, and 23 during the late follow-up). There was a total of 10 PPD conversions: 7/25 (28 percent) in the initial period, 3/17 (18 percent) in early follow-up, and 0/23 in late follow-up (P < 0.01). Of the three PPD conversions during the early follow-up period, two were linked to exposure to a patient with MDR tuberculosis who was not isolated on admission because of the fact that he was on therapy and had been AFB smear negative at the time of a recent hospital discharge. However, he subsequently proved to be smear positive. This led to an additional policy that any patient with a history of MDR tuberculosis would be isolated regardless of previous smear and treatment status.

The authors point out that it is difficult to know what components of the control measures were most important. However, the early implementation of administrative controls linked with beginning improvements in engineering controls led to a reduction in nosocomial transmission of MDR tuberculosis to other patients, as well as a reduction in PPD conversions in health care workers.

Similarly, Maloney and colleagues (15) detailed control methods implemented in June through October 1991 after an outbreak of MDR tuberculosis at Cabrini Medical Center in New York City. Control measures included improved isolation criteria (not detailed in the paper, but 90 percent of patients with MDR tuberculosis were isolated on admission, compared with isolation of 40 percent of patients preintervention) and molded surgical masks for employees (June): improved lab services (July); increase from 0/10 tuberculosis isolation rooms with negative pressure to 16/27 with negative pressure (September); and a chamber for sputum induction and pentamidine administration (October).

With the adoption of these measures the number of patients with MDR tuberculosis who had previously been admitted to Cabrini fell from 24 in the preintervention period (January 1990 through June 1991) to 6 in the postintervention period (July 1991 through August 1992). Three of the six postintervention patients also had documented nonhospital or preintervention exposures documented. In the postintervention period only 1 patient was found to have had a documented nosocomial exposure during a previous hospitalization, as opposed to 20/24 (83%) in the preintervention period.

Implementation of control measures led to no change in the overall rate of PPD conversions (~3 percent). However the rate of conversion on the HIV and medical wards fell from 16.7 percent during the preintervention period to 5.1 percent postintervention (p = 0.02), with no change on wards that did not usually house tuberculosis patients. The postintervention PPD



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