TB Skin Test Interpretation

Determine likelihood of TB based on tuberculin skin test


A TB Skin Test (TST) is performed through the intradermal injection of 0.1 mL of tuberculin purified protein making a wheal of 6-10 mm in size.

Possible causes of false positive results include: prior BCG vaccine, infection with non-tuberculous mycobacterium, incorrect interpretation of reaction, incorrect administration of TST or antigen.

Possible causes of false-negative reactions include: anergy, recent TB infection (within 8-10 weeks), old TB infection, young age (<6 months), recent live-vaccine, overwhelming TB disease, viral illnesses, incorrect administration or interpretation of TST.


Countries listed as having a high burden of tuberculosis inclucde Bangladesh, DPR Korea, Pakistan, Phillipines, Russia Federation, Viet Nam, Brazil, Central African Republic, Congo, Lesotho, Liberia, Namibia, Tanzania, Zambia, Angola, China, Ethiopia, India, Indonesia, Kenya, Mozambique, Myanmar, Nigeria, South Africa, Thailand, Zimbabwe, Cambodia, Sierra Leone.

Learn more here: https://www.who.int/tb/publications/global_report/high_tb_burdencountrylists2016-2020.pdf

Conditions placing individual at high risk: substance abuse, silicosis, diabetes mellitus, severe kidney disease, low body weight, head and neck cancer, medical treatment with corticosteroids, specialized treatment for rheumatoid arthritis or Crohn’s disease.

Learn more here: https://www.cdc.gov/tb/topic/basics/risk.htm


The TB Skin Test Interpretation calculator is created by QxMD.

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1. Size?

More Information

The test should be read 48-72 hours after administration. The area of induration (not redness) should be measured across forearm (i.e. perpendicular to long axis).

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