A positive PPD test indicates that a person has been exposed to tuberculosis (TB) bacteria, though it doesn’t necessarily mean they have active TB.
Understanding the PPD Test
The Purified Protein Derivative (PPD) test, also known as the tuberculin skin test (TST), is a medical tool used to detect whether someone has been exposed to the bacteria that causes tuberculosis (TB). It is not a test for active TB disease but rather an indication of prior exposure. A positive result suggests that your immune system has responded to the bacteria, but it does not differentiate between active and latent tuberculosis.
PPD testing has been used for decades as a way to screen for tuberculosis, especially in individuals at higher risk for the disease, such as healthcare workers, those living in close quarters, or individuals from areas with a higher prevalence of TB.
How the PPD Test Works
The test involves injecting a small amount of purified protein derivative under the skin, usually on the forearm. After 48 to 72 hours, a healthcare provider checks the injection site for a reaction, typically a raised bump. The size of the bump (in millimeters) helps determine whether the test is considered positive.
What Causes a Positive Result?
A positive PPD test means your body has encountered the TB bacteria in the past. It does not mean that you currently have an active infection. The body’s immune system reacts to the PPD because it has been sensitized to the bacteria—often years ago—by a previous exposure. However, it’s important to note that a positive test doesn't necessarily mean someone is infectious or has active TB.
Factors that could contribute to a positive PPD result include:
- Latent TB Infection (LTBI): The most common cause of a positive result. The bacteria remain dormant in the body without causing symptoms or spreading.
- Previous BCG Vaccination: The Bacille Calmette-Guérin (BCG) vaccine, given in many countries to prevent TB, can sometimes cause a false positive in those who have received it.
- Infection with Non-Tuberculosis Mycobacteria: Some bacteria related to TB can also lead to a positive PPD reaction.
- Exposure to Active TB: If someone has been in close contact with a person with active TB, it could cause a positive reaction.
The PPD Test and Latent TB
A positive PPD test is often an indication of latent tuberculosis infection (LTBI), a condition in which the person has the bacteria in their body but does not show symptoms and is not contagious. In this stage, the bacteria are inactive, and the person does not feel ill. However, without treatment, latent TB can develop into active TB later, which is a serious condition.
Not everyone with latent TB will develop active TB. Factors like a weakened immune system, such as from HIV or certain medications, can increase the risk of latent TB becoming active. Therefore, it’s crucial to follow up with additional tests, such as a chest X-ray or sputum test, to determine whether the bacteria are dormant or active.
Interpreting the Results
The interpretation of a positive PPD test depends on several factors, including the size of the induration (the raised bump) and the person’s risk factors. Here’s a general guideline:
|
Induration Size (mm) |
Risk Factor |
Interpretation |
|
5 or more |
High risk (e.g., HIV, close contact with active TB) |
Positive |
|
10 or more |
Moderate risk (e.g., recent immigration from TB-endemic area, healthcare workers) |
Positive |
|
15 or more |
Low risk (no risk factors) |
Positive if larger than 15mm, or considered negative if smaller |
The size of the bump can vary from person to person, and the test is often read in conjunction with the patient’s medical history and exposure risks.
Follow-Up After a Positive Test
If the PPD test is positive, additional tests are needed to confirm whether the person has active TB or latent TB. These follow-up tests typically include:
- Chest X-ray: This can show if there are any abnormalities in the lungs that might indicate active TB.
- Sputum Test: If a person has symptoms such as a persistent cough or chest pain, a sputum sample may be collected to look for the presence of TB bacteria.
- Blood Tests: In some cases, blood tests may be used to determine whether TB bacteria are active.
If the person is diagnosed with latent TB, treatment with medications such as isoniazid or rifampin may be recommended to prevent the bacteria from becoming active. Treatment for latent TB typically lasts several months.
Risks of Active TB
When TB becomes active, it can be serious and potentially fatal if left untreated. Symptoms of active TB include:
- Persistent cough (sometimes with blood)
- Chest pain
- Unexplained weight loss
- Night sweats
- Fever and chills
- Fatigue
If TB is active in the lungs, it is contagious and can spread through the air when the infected person coughs or sneezes. Active TB requires immediate medical treatment with a combination of antibiotics over several months. Treatment adherence is critical to prevent the development of drug-resistant TB.
Factors That Affect a Positive PPD Test
Several factors influence whether someone receives a positive PPD test result, and understanding them can provide clarity on how and why the test is conducted:
Age and Immune System Status
Younger individuals and those with compromised immune systems are at greater risk of developing TB, which can affect their PPD test outcomes. For example, individuals with HIV or those on immunosuppressive drugs may have a more pronounced reaction to a positive test.
Exposure History
Individuals who live in environments with a higher risk of TB, such as in crowded or healthcare settings, are more likely to have been exposed to the bacteria. Likewise, travel to countries where TB is more common can also increase the risk.
BCG Vaccination History
BCG vaccination can cause a false positive, particularly in countries where TB vaccination is common. While the BCG vaccine does provide some protection against severe forms of TB, it doesn’t guarantee complete immunity and may lead to confusion in interpreting the PPD test.
Recent Infection with Non-Tuberculosis Mycobacteria
There are other species of mycobacteria that cause infections similar to TB. These infections can sometimes lead to a positive PPD test result, although they are not typically contagious like TB.
Conclusion: What Does a Positive PPD Test Mean?
A positive PPD test does not necessarily mean you have active tuberculosis, but it does suggest that you’ve been exposed to the bacteria. It’s important to follow up with further testing to determine whether you have latent TB or active TB. If latent TB is diagnosed, treatment can help prevent it from developing into active TB, which is a more serious condition that requires immediate treatment. Always consult with a healthcare provider to understand the best course of action following a positive PPD result.