371 Patients receiving hemodialysis (HD) are at increased risk of developing active disease after primary tubercu- losis (TB) infection. 1-4 The greater risk of TB is caused by an increase in the likelihood of progression from latent infection to active disease, most likely because of the impaired cell-mediated immunity associated with end-stage renal disease. 5 The high incidence of extra- pulmonary disease may be a significant factor in the delay in diagnosis of TB in these patients. Consequently, routine TB screening of this population is recommend- ed on an annual basis in the dialysis units. 6 The tuber- culin skin test (TST) remains the most useful screening tool, 7,8 and it is also considered the “gold standard” for latent TB infection. 9 Repeating TST of uninfected persons does not sensi- tize them to tuberculin. However, delayed-type hyper- sensitivity to antigens like tuberculin resulting from mycobacterial infection or bacille Calmette-Guérin (BCG) vaccination may gradually wane with years. Although subsequent initial skin testing may be nega- tive for TB, the stimulus of a first test may boost or increase the size of the reaction to a second test admin- istered 1 week to 1 year later and thus may suggest an apparent-but-false tuberculin conversion. 8 This phe- nomenon has come to be known as the booster effect. 10 The significance and frequency of the booster phenomenon in sequential testing of HD patients have not been determined. Therefore, we examined the fre- quency and significance of the booster phenomenon in 2-step tuberculin testing of these patients. SUBJECTS AND METHODS Sixty-four patients were treated with regular HD between January 1 and December 31, 1999, at the Hacettepe University School of Medicine, Hemodialysis Unit, in Turkey. Testing was done between August and October 1999. Our dialysis patients had similar social backgrounds, and the majority was from the moderate social class. All underwent dialysis for an average of 4 to 5 hours, 3 times a week. The patients were given an educational handout on TST the week before the testing started, and verbal consent for testing was obtained. Patient charts were reviewed for demographics (age, sex, duration of hemodialysis, cause of renal failure), medical history, and laboratory values (hemoglobin, The booster phenomenon in 2-step tuberculin skin testing of patients receiving long-term hemodialysis Ali Akcay, MD, Yunus Erdem, MD, Bulent Altun, MD, Celalettin Usalan, MD, Erhan Agca, MD, Unal Yasavul, MD, Cetin Turgan, MD, and Sali Caglar, MD Ankara, Turkey Background: Tuberculosis remains a significant health problem for patients receiving long-term hemodialysis (HD). The tuberculin skin test (TST) is an important method for detecting Mycobacterium tuberculosis infection. This study examined the significance and frequency of the booster phenomenon in serial TST of HD patients. Method: Fifty-three outpatients in a hospital-based HD center in Turkey were screened for tuberculosis with the TST between August and October 1999. To determine the frequency of booster phenomenon, patients with less than 10 mm indurations to the initial TST were given a second test 7 days later. Results: Nineteen (35.8%) of 53 patients had a significant tuberculin reaction (≥ 10 mm) on the initial TST. The booster effect was detected in 10 (29.4%) of 34 patients who had a negative reaction (< 10 mm) to the initial test. Overall, 29 (54.7%) patients showed a significant reaction on both tests. Conclusions: These results showed significant rates of TST positivity and the booster effect in this HD center. (Am J Infect Control 2003;31:371-4.) From the Hacettepe Universty Faculty of Medicine, Department of Internal Medicine, Division of Nephrology. Reprint requests:Ali Akcay, MD, Oguzlar Mahallesi 2. Cadde, 35. Sokak 17/7, Balgat,Ankara,Turkey. Copyright © 2003 by the Association for Professionals in Infection Control and Epidemiology, Inc. 0196-6553/2003/$30.00 + 0 doi:10.1067/mic.2003.38