The Tuberculin Purified Protein Derivative (PPD) test, also known as the Mantoux test, is a method used to determine if someone has been infected with the bacteria that cause tuberculosis (TB). A positive PPD test result indicates that you have been infected with TB bacteria, but it does not necessarily mean you have active tuberculosis disease. Understanding what a positive result means and what steps to take next is crucial for your health and the health of those around you.
Interpreting a Positive PPD Test: It’s Not Always Active TB
A positive reaction to a PPD test signifies that your body’s immune system has responded to the tuberculin protein, indicating a TB infection. This reaction is measured by the size of the firm swelling (induration) at the injection site, typically read 48 to 72 hours after the test is administered. It’s important to understand that a positive skin test only means you have been infected with TB bacteria at some point. It does not automatically mean you have active TB disease, which is when the bacteria are actively multiplying and causing illness. Further tests are required to determine if you have active TB disease.
Positive Result Criteria Based on Risk Factors
The size of the induration considered “positive” varies depending on individual risk factors. This is because certain conditions can weaken the immune system, making even a smaller reaction significant.
5 mm or More of Induration is Considered Positive in Individuals:
- Living with HIV/AIDS: Individuals with HIV/AIDS are at a significantly higher risk of developing active TB once infected.
- Organ Transplant Recipients: Post-transplant patients take immunosuppressant drugs to prevent organ rejection, which also increases TB risk.
- Immunosuppressed Individuals: This includes people taking steroid therapy (like prednisone, approximately 15 mg per day for 1 month or longer) or those with other conditions that weaken the immune system.
- Close Contacts of Active TB Cases: People who have recently been in close contact with someone diagnosed with active TB are at higher risk of infection.
- Chest X-ray Findings Suggestive of Past TB: If a previous chest X-ray shows signs of old, healed TB, a smaller reaction may be considered positive.
10 mm or More of Induration is Considered Positive in Individuals:
- Recent TB Test Converters: People with a known negative TB test within the past two years who now have a positive test. This indicates a recent infection.
- Individuals with Certain Medical Conditions: Conditions like diabetes, chronic kidney disease (kidney failure), and other conditions that increase the risk of progressing to active TB.
- Healthcare Workers: Due to potential exposure to TB in healthcare settings.
- Injection Drug Users: This population often has higher rates of TB and may have compromised immune systems.
- Recent Immigrants from High TB Prevalence Countries: Individuals who have immigrated within the last 5 years from countries with a high TB burden.
- Children Under 4 Years of Age: Young children are at higher risk of developing severe TB disease.
- Infants, Children, and Adolescents Exposed to High-Risk Adults: Exposure to adults with risk factors for TB increases the child’s risk.
- Residents and Employees of Congregate Settings: This includes prisons, nursing homes, homeless shelters, and other group living facilities where TB can spread more easily.
15 mm or More of Induration is Considered Positive in Individuals:
- Persons with No Known Risk Factors for TB: For individuals without any of the risk factors listed above, a larger reaction size is required to be considered positive.
False-Positive PPD Tests and the BCG Vaccine
It’s important to note that individuals who have received the Bacille Calmette-Guérin (BCG) vaccine, commonly given in many countries outside the United States to prevent childhood TB, may have a false-positive PPD test result. The BCG vaccine can cause a reaction to tuberculin, even if the person is not infected with TB bacteria. If you have received the BCG vaccine, it’s crucial to inform your healthcare provider before undergoing a PPD test, as further evaluation and alternative testing methods, such as a TB blood test (Interferon-Gamma Release Assay or IGRA), might be necessary to accurately determine if you have a TB infection.
What Happens After a Positive PPD Test?
If you have a positive PPD test, it is essential to consult with your doctor for further evaluation. The next steps typically involve:
- Chest X-ray: To check for signs of active TB disease in your lungs.
- Further Medical Evaluation: Your doctor will assess your medical history, risk factors, and symptoms to determine if you have active TB disease or latent TB infection.
- Testing for Active TB Disease: This may include sputum tests (testing phlegm for TB bacteria) and other clinical evaluations.
- Treatment for Latent TB Infection: If active TB disease is ruled out, but you have latent TB infection (meaning you are infected with TB bacteria but not currently sick and cannot spread the disease), your doctor may recommend treatment to prevent the infection from progressing to active TB disease in the future. Treatment for latent TB typically involves a course of antibiotics.
Understanding your PPD test results and following up with appropriate medical care is vital for managing TB infection and protecting your health and the community.
