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Since 1958 the World Health Organization (WHO) has recommended the use of RT-23 (2 tuberculin unit [TU] purified protein derivative [PPD]) instead of the standard 5 TU PPD to establish the prevalence of tuberculosis (TB) around the world. Before starting a hospital program to control the transmission of TB among health care workers at the National Institute of Nutrition (Mexico City), we compared the tuberculin product commonly used in Mexico (RT-23) with that used in the United States (5 TU PPD). In this trial the Mantoux test was performed on 80 health care workers from various areas of the hospital: 5 TU PPD and RT-23 were simultaneously inoculated in the left forearm and in the right forearm, respectively. The test results for both tuberculin products were read 72 hours later. When a 10-mm induration was used as the cutoff for reactivity, the specificity with use of RT-23 was 100%, but the sensitivity was only 57%. However, when a 5-mm induration was used as the cutoff, the sensitivity with use of RT-23 was 90%. Because of these findings, we advise that every country using RT-23 either consider changing the cutoff for reactivity or consider using 5 TU PPD for reassessment of the data on the incidence and prevalence of TB that were obtained with use of RT-23 (the product recommended by WHO).

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