Prior BCG vaccination have variable effect on PPD results. It varies with age at vaccination, number of years since the BCG vaccination, number of times vaccinated, and number of PPD performed. When evaluating Positive PPD among immigrants, disregard the history of BCG vaccination.
BCG vaccination is given in the areas where tuberculosis is prevalent. The World Health Organization recommends BCG vaccination at birth, without booster doses, to prevent childhood complications of TB infection ; however, the vaccineâ€™s efficacy is known to be inconsistent. Though BCG vaccine given at birth can decrease the risk of miliary TB and TB meningitis among children, estimates of its effectiveness in preventing adult pulmonary TB range widely from 0% to 80%.
BCG vaccine may confound PPD readings, but several studies indicate that PPD can still be a useful screening tool for tuberculosis infection after vaccination. In a Quebec cohort of 1198 foreign-born children and young adults, prior BCG vaccination could account for 50% of PPDs with induration of 5 to 9 mm, but only 4% of reactions 10 mm or greater. This study also showed that patients from countries with a high or moderate incidence of TB were more likely to have reactive PPDs than those from countries of low incidence, suggesting that exposure to TB accounts for some of the positive PPDs .
The USPSTF further notes that reactions >10 mm should not be attributed to prior BCG vaccine . The Centers for Disease Control and Prevention (CDC) and American Thoracic Society joint statement recommends against altering guidelines for testing and interpretation among BCG recipients . BCG vaccine does not always protect people from getting TB .
- Fine P, Carnelro IA, Milstien JB, Clements CJ. Issues relating to the use of BCG immunization programmes. WHO discussion document. V&B 99.23. Available at: http://who.int/vaccine_research/documents/en/bcg_vaccines.pdf. Accessed on September 3, 2006.
- Rowland K, Guthmann R, Jamieson, B. How should we manage a patient with a positive PPD and prior BCG vaccination? Clinical Inquiries. The Journal of Family Practice. Vol 55, No. 8, August 2006.
- US Preventative Services Task Force. Screening for tuberculosis infection, including Bacille Calmette-GuÃ©rin immunization. Guide to Clinical Preventative Services; 1996. Available at: www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=hstat3.section.10931#13112.
- American Thoracic Society and Centers for Disease Control and Prevention. Targeted tuberculin testing and treatment of latent tuberculosis infection. Am J Respir Crit Care Med 2000;161:1376â€“1395.
- Latent TB Infection. Division of Tuberculosis Elimination. http://www.cdc.gov/nchstp/tb/faqs/qa_latenttbinf.htm#Latent3.