Free tool · CIC® exam topic
TB skin test interpretation
Enter the induration in millimeters and choose the person’s risk group. The tool applies CDC’s 5, 10 and 15 mm cutoffs for a positive tuberculin skin test.
Result
- TST result
- Positive
- Cutoff applied
- 12 mm ≥ 10 mm cutoff for this group
A positive TST means TB infection is likely. It does not show whether the person has latent TB infection or TB disease — further evaluation (symptoms, chest x-ray) is needed.
CDC’s TST interpretation cutoffs
≥ 5 mm is positive
- People with HIV
- Recent contacts of people with infectious TB disease
- People with chest x-ray findings suggestive of previous TB disease
- People with organ transplants
- Other immunosuppressed people
≥ 10 mm is positive
- People born in countries where TB disease is common
- People who misuse drugs and alcohol
- People who live or work in high-risk congregate settings
- Mycobacteriology laboratory workers
- People with certain medical conditions
- People with a low body weight
- Children younger than 5 years of age
- Infants, children, and adolescents exposed to adults in high-risk categories
≥ 15 mm is positive
- People with no known risk factors for TB
Reading the test
- Read between 48 and 72 hours after placement, by a trained reader.
- Measure the diameter of induration (firm swelling) in millimeters, across the forearm (perpendicular to its long axis). Do not measure erythema.
- Record 0 mm when there is no induration.
Test yourself: a CIC-style question
A healthcare worker who is a recent contact of a patient with infectious TB disease has a tuberculin skin test reaction of 7 mm induration. How is this result classified?
- Negative — positive requires at least 10 mm
- Negative — positive requires at least 15 mm
- Positive — recent contacts use a 5 mm cutoff
- Uninterpretable — the test must be repeated
Show answer
C. Positive — recent contacts use a 5 mm cutoff
CDC classifies a reaction of 5 mm or more as positive in recent contacts of people with infectious TB disease, people with HIV, organ transplant recipients and other immunosuppressed people.
Questions like this come up in the Employee/Occupational Health domain of the CIC® exam. The 654 questions in the app each come with a written explanation.
FAQ
- When should a TB skin test be read?
- Between 48 and 72 hours after it is placed, by a health care worker trained to read TST results. Measure the induration (firm swelling) in millimeters across the forearm — not the redness (erythema).
- Does a positive TB skin test mean active TB?
- No. A positive TST means TB infection is likely, but it cannot tell latent TB infection from TB disease. Further evaluation, such as a symptom review and chest x-ray, is needed.
- What cutoff applies to healthcare workers?
- Use the group the person belongs to. Healthcare workers in high-risk congregate settings fall under the 10 mm cutoff; those with no known risk factors under 15 mm; those with HIV, recent contact or immunosuppression under 5 mm. CDC recommends two-step testing for baseline TST of U.S. healthcare personnel.
Sources
Checked against these sources on October 21, 2026. For education and CIC® exam preparation only — not a substitute for your facility’s policies, NHSN’s official reports, or clinical judgment. Calculations run in your browser; nothing you enter is sent anywhere.