From the Guidelines
A positive Mantoux (Tuberculin Skin Test) result after BCG (Bacillus Calmette-Guérin) vaccination indicates infection with M. tuberculosis, especially in individuals from countries with a high prevalence of tuberculosis 1.
Key Points
- BCG vaccination can cause a positive tuberculin skin test result, but it does not necessarily indicate protection against tuberculosis 1.
- The specificity of the test can be improved by progressively increasing the cut point for positivity, but this may decrease the sensitivity of the test 1.
- A positive reaction to 5 TU of PPD tuberculin in BCG-vaccinated persons is usually considered indicative of infection with M. tuberculosis, especially among persons from countries with a high prevalence of tuberculosis 1.
- Previous BCG vaccination does not reliably distinguish tuberculin reactions caused by vaccination with BCG from those caused by natural mycobacterial infections 1.
- The interpretation of the TST is risk-stratified, with different cut points for positivity depending on the individual's risk factors, such as close contact with tuberculosis cases, immunosuppression, or medical risk factors that increase the probability of progression from LTBI to TB 1.
Important Considerations
- False-positive reactions can occur in persons who have been infected with nontuberculous mycobacteria or who have received BCG vaccine 1.
- False-negative reactions can occur in infants and young children, early after infection, in persons with clinical conditions associated with immunosuppression, or in association with treatment with immunosuppressive drugs 1.
- Repeated administration of TSTs cannot induce reactivity, but a repeat TST can restore reactivity in persons whose TST reactivity has waned over time 1.
From the FDA Drug Label
BCG vaccination results in tuberculin skin test reactivity Tuberculin skin test reactivity as a result of BCG vaccination cannot be readily differentiated from reactivity following exposure to tuberculosis.
A positive Mantoux (Tuberculin Skin Test) test result after BCG (Bacillus Calmette-Guérin) vaccination may indicate either persistent post-vaccination sensitivity or a virulent superinfection.
- It is usually not possible to clearly distinguish between the two causes of a positive test result.
- A sharp rise in the tuberculin reaction since the latest test should be further investigated (except in the immediate post-vaccination period) 2.
From the Research
Interpretation of Positive Mantoux Test Results
A positive Mantoux (Tuberculin Skin Test) test result after BCG (Bacillus Calmette-Guérin) vaccination indicates a delayed hypersensitivity reaction to tuberculin, which can be caused by either the BCG vaccination or exposure to Mycobacterium tuberculosis 3. The test result is not always a clear indicator of tuberculosis infection, as the sensitivity and specificity of the test are imperfect 3.
Factors Influencing Test Results
Several factors can influence the interpretation of a positive Mantoux test result, including:
- Age of vaccination: The younger the child is, the lower the reaction is 3
- Infection prevalence rate in the group considered: The lower the prevalence, the higher the criteria may be used for induration > 15 mm 3
- BCG vaccination: A history of BCG vaccination at birth does not interfere with TST results in children >3 years old, but may cause a false-positive result in children under 3 years old 4
- Stay or work in a high TB prevalence country: Increases the likelihood of a positive Mantoux result 5
- Previous TST: Increases the likelihood of a positive Mantoux result 5
Limitations of the Mantoux Test
The Mantoux test has several limitations, including:
- Variability in placement and interpretation 6
- Use of crude protein preparations, which can lead to non-specific reactions 6
- Limited ability to distinguish between BCG vaccination and tuberculosis infection [(4,7)]
Alternative Testing Methods
In vitro assessment of cellular immunity to mycobacteria offers several advantages over skin testing, including greater specificity and the ability to use discrete mycobacterial antigens 6. Interferon-gamma release assays (IGRAs) may also be used to distinguish between BCG vaccination and tuberculosis infection, particularly in children under 3 years old 4.