What do I do with a positive Tuberculin Skin Test (TST)?

Management of a Positive Tuberculin Skin Test (TST)

A positive tuberculin skin test (TST) requires evaluation for active tuberculosis disease followed by treatment for latent tuberculosis infection (LTBI) if active disease is ruled out. 1

Initial Evaluation After a Positive TST

• Perform a thorough evaluation to rule out active tuberculosis disease, including:

  • Medical history focusing on TB symptoms (cough, fever, night sweats, weight loss)
  • Physical examination
  • Chest radiography
  • Collection of sputum specimens if respiratory symptoms are present 1

• A positive TST (≥5 mm induration) should prompt further evaluation, but cannot distinguish between LTBI and active TB 1

• For individuals with a positive TST, consider confirmatory testing with an Interferon-Gamma Release Assay (IGRA) such as QuantiFERON-TB Gold or T-SPOT.TB, especially if:

  • The person has received BCG vaccination previously 1, 2
  • The person is from a low TB-incidence country 2
  • There is concern about a false-positive TST result 1

Interpretation of TST Results

• A positive TST is defined as:
  • ≥5 mm induration for:
    • HIV-infected persons
    • Recent contacts of TB cases
    • Persons with fibrotic changes on chest radiograph
    • Immunosuppressed patients 1
  • ≥10 mm induration for:
    • Recent immigrants from high-prevalence countries
    • Injection drug users
    • Residents of high-risk congregate settings
    • Persons with certain medical conditions (diabetes, silicosis, end-stage renal disease) 1, 3
  • ≥15 mm induration for persons with no known risk factors 1

Management After Ruling Out Active TB

• If active TB is ruled out, treatment for LTBI is strongly recommended for all persons with a positive TST, unless medically contraindicated 1, 3

• Treatment regimens for LTBI include:

  • Isoniazid for 9 months (preferred for most patients)
  • Isoniazid for 12 months for HIV-infected persons or those with fibrotic lesions on chest X-ray
  • Isoniazid plus rifampin for 4 months for persons with fibrotic pulmonary lesions 3

• Treatment is particularly important for:

  • Recent converters (within 2 years)
  • Children under 5 years of age
  • Immunocompromised individuals
  • Those with radiographic evidence of prior TB 1, 3

Special Considerations

• For BCG-vaccinated individuals, IGRAs are preferred over TST due to higher specificity and fewer false-positive results 2, 4

• Studies show that only 30-49% of BCG-vaccinated individuals with positive TST results have positive IGRA results, suggesting many TST positives may be false positives 2, 5

• For individuals from high TB-incidence countries, the likelihood of a true positive test is higher (46% vs. 9% IGRA positivity compared to those from low-incidence countries) 2

• Strong predictors of true infection (positive IGRA after positive TST) include:

  • TST induration ≥16 mm
  • Abnormal chest radiograph consistent with healed TB
  • Birth in a high TB-incidence country 2

Follow-up

• No routine follow-up TST or IGRA testing is needed after treatment for LTBI 1

• Annual TB education should be provided to all individuals with LTBI, including information about symptoms of active TB and when to seek medical attention 1

• For contacts of persons with infectious TB, follow-up testing 8-10 weeks after exposure is recommended if the initial test was negative 1

By following this algorithm, you can appropriately manage patients with positive TST results, minimize unnecessary treatment, and prevent progression to active TB disease.