Tuberculosis (TB) remains a significant global health challenge, making accurate and timely diagnosis crucial for effective control and treatment. Detecting Mycobacterium tuberculosis, the bacteria responsible for TB, is the first step in managing its spread and impact. Two primary methods are commonly employed for this purpose: the tuberculin skin test (TST) and blood tests, known as Interferon-Gamma Release Assays (IGRAs). Understanding the distinctions between these diagnostic tools helps in appreciating their roles in identifying TB infection.
Tuberculosis Skin Test
The tuberculosis skin test, also referred to as the Mantoux tuberculin skin test (TST), is a common method to determine if a person has TB bacteria in their body. This test involves injecting a small amount of purified protein derivative (PPD) tuberculin into the inner surface of the forearm, just under the top layer of the skin, creating a slight bump.
Following the injection, the patient must return to the clinic within 48 to 72 hours for the results to be read by a trained healthcare professional. The healthcare provider assesses the injection site by measuring the diameter of any firm swelling, or induration, in millimeters across the forearm. Redness at the site is not typically measured for interpretation.
A positive result, indicated by a firm, raised bump of a certain size, suggests that the immune system has reacted to the TB proteins, implying a past or present TB infection. The interpretation of the bump’s size depends on individual risk factors. Factors such as prior Bacille Calmette-Guérin (BCG) vaccination, a vaccine used in many countries to prevent TB, can influence TST results, potentially leading to false positives because the test cannot differentiate between an immune response from the vaccine and an actual infection.
Tuberculosis Blood Test
Tuberculosis blood tests, or Interferon-Gamma Release Assays (IGRAs), offer another approach to detect TB infection by measuring the body’s immune response to TB-specific proteins. These tests, such as QuantiFERON-TB Gold Plus (QFT-Plus) and T-SPOT.TB, involve a single blood draw from the patient. This collected blood sample is then sent to a laboratory for analysis.
In the laboratory, the blood sample is mixed with specific TB antigens. If a person has been infected with TB bacteria, their white blood cells will release interferon-gamma (IFN-γ) in response to these antigens. The IGRA measures the level of this IFN-γ, indicating the presence of TB bacteria.
Results are typically available within 24 to 72 hours, usually providing a clear positive, negative, or sometimes an indeterminate result. A positive result signifies exposure to TB bacteria, while a negative result suggests no evidence of infection. A notable advantage of IGRAs is their ability to differentiate between TB infection and prior BCG vaccination because the antigens used are absent from the BCG vaccine, reducing the likelihood of false positives.
Comparing the Tests
The tuberculin skin test and TB blood test differ in their procedures and how they are affected by external factors. The skin test requires two visits: one for the injection and a second visit 48 to 72 hours later for result interpretation. In contrast, the blood test is a single-visit procedure where a blood sample is drawn and sent to a lab, making it potentially more convenient for some individuals.
A significant distinction lies in the influence of the BCG vaccine. The skin test can produce false-positive results in individuals who have received the BCG vaccine, as it reacts to components present in both the vaccine and the TB bacteria. Conversely, TB blood tests are generally unaffected by prior BCG vaccination, offering more specific results in vaccinated populations.
Result interpretation also varies between the two tests. The skin test relies on a healthcare professional visually measuring the induration, which can introduce a degree of subjectivity. Blood tests, however, provide objective laboratory results based on the measured immune response, which can be reported directly. While both tests indicate the presence of TB bacteria, neither can distinguish between a latent (inactive) TB infection and active TB disease; further diagnostic tests like chest X-rays or sputum tests are needed for this differentiation.
Regarding accuracy and specificity, IGRAs are often considered more specific for TB infection, particularly in BCG-vaccinated individuals, and can be more accurate. The cost implications also differ; while the per-test cost of a skin test reagent might be lower, the overall cost of TST can increase due to the need for multiple visits and potential follow-up for false positives. Blood tests, while having a higher reagent cost, may lead to overall cost savings by reducing follow-up appointments and unnecessary procedures caused by false positives.
When to Use Each Test
The choice between a TB skin test and a TB blood test often depends on various factors, including patient history, likelihood of follow-up, and specific clinical circumstances. For individuals who have received the BCG vaccine, the blood test is generally preferred because it avoids the false-positive results associated with the skin test due to vaccination. This makes IGRAs a more reliable option for these populations.
Blood tests are also advantageous for those who may not return for a second visit, such as individuals experiencing homelessness or those with busy schedules, as they require only one appointment for blood collection. Healthcare providers may also opt for blood tests in individuals with compromised immune systems, as their immune response might be too weak to produce a reliable reaction to the skin test.
Despite the advantages of blood tests, the skin test may still be used in certain situations. It remains a common and acceptable alternative for children under five years of age, though some experts use blood tests in this group. In resource-limited settings where laboratory facilities for blood tests may not be readily available, the skin test often remains the more practical option. Ultimately, the decision rests on a healthcare provider’s judgment, considering the patient’s individual risk factors, medical history, and local guidelines to ensure the most appropriate and effective testing strategy.