A tuberculosis (TB) test is a medical diagnostic procedure used to determine whether an individual has been infected with the bacteria that cause tuberculosis. This test is essential in detecting early signs of TB, which can be crucial for effective treatment and preventing the spread of the disease. In this article, we will provide an in-depth explanation of how a TB test is performed, including the different types of TB tests and the steps involved in each.
There are primarily two types of TB tests:
the Tuberculin Skin Test (TST), commonly known as the Mantoux test, and the Interferon-Gamma Release Assay (IGRA). Both these tests have their own merits and are performed differently. Let’s now delve into each of these tests, discussing their procedures, advantages, and limitations.
1. The Tuberculin Skin Test (TST):
The Tuberculin Skin Test is the traditional method used to diagnose TB. It is based on the principle that when an individual is infected with Mycobacterium tuberculosis, the bacteria that cause TB, their immune system reacts producing an immune response. This immune response is manifest a delayed hypersensitivity reaction when the individual is exposed to certain proteins derived from the TB bacteria.
To perform the TST, a healthcare professional administers an intradermal injection of purified protein derivative (PPD) into the inner surface of the forearm. The PPD contains a small amount of proteins from the TB bacteria. After the injection, the site will be gently examined to ensure the proper administration of the test. It is important to note that the test is not infectious and cannot transmit TB.
The next step in the TST procedure involves waiting for a defined period, typically 48 to 72 hours, to evaluate the test results accurately. During this waiting period, it is crucial for the individual to avoid scratching or otherwise irritating the injection site as it may lead to false readings.
After the specified waiting time, the healthcare professional will examine the test site for any signs of induration, a localized skin reaction. Induration is measured in millimeters and is essential in determining the test result. Induration occurs due to the interaction between the PPD proteins and the individual’s immune system cells at the injection site. The presence of induration does not necessarily indicate an active TB infection; it is merely a sign of exposure to the bacteria.
The size of induration determines the interpretation of the test result, which can be classified as positive, negative, or inconclusive. Interpretation guidelines vary slightly based on risk factors such as age, immunosuppression, and prior BCG (Bacillus Calmette-Guérin) vaccination.
A positive TST result generally indicates a current or past TB infection. However, it is important to consider the individual’s medical history, exposure risks, and other diagnostic tests to confirm the presence of active TB disease. Furthermore, a positive TST result does not differentiate between latent TB infection (LTBI) and active disease.
On the other hand, a negative TST result suggests the absence of a TB infection or an infection that is too recent to induce an immune response. However, the test has limitations, and false negative results can occur, especially in individuals with weakened immune systems or during the early stages of infection.
Inconclusive results may occur if the TST is neither clearly positive nor negative. In such cases, additional tests, medical history evaluations, and clinical examinations are necessary to determine the best course of action.
It is important to note that the TST may also yield false positive results, indicating a TB infection when there is actually none. False positive results can occur in individuals who have previously received the BCG vaccine, which is used in many countries to prevent severe forms of TB disease in children. Other factors, such as cross-reactivity with non-tuberculosis mycobacteria or recent exposure to other infections, can also lead to false positives.
Despite its limitations, the TST remains a valuable tool in diagnosing TB, especially in countries with high TB prevalence or limited access to more advanced diagnostic techniques.
2. The Interferon-Gamma Release Assay (IGRA):
The Interferon-Gamma Release Assay, or IGRA, is a newer diagnostic test used to detect TB infection. Unlike the TST, which relies on the analysis of the immune response at the injection site, the IGRA measures the release of interferon-gamma (IFN-γ), a substance produced immune cells when exposed to specific TB proteins.
To perform the IGRA, a healthcare professional draws a blood sample from the individual, usually using a needle and syringe. This sample is collected in a special tube containing antigens derived from TB bacteria. There are two common types of IGRA tests:
the QuantiFERON-TB Gold (QFT-G) and the T-SPOT.
TB.
After the blood sample is drawn, it is transported to a laboratory where it undergoes a series of steps to detect and quantify IFN-γ production. This typically involves incubating the blood sample with the TB antigens and control substances, allowing the immune cells to interact with the antigens. Following incubation, the level of IFN-γ released is measured, and the result is reported as either positive, negative, or indeterminate.
A positive IGRA result suggests that the individual has been infected with the TB bacteria. As with the TST, a positive result does not indicate active TB disease but rather the presence of a TB infection. Clinical evaluation and additional tests are necessary to make an accurate diagnosis.
A negative IGRA result indicates the absence of a TB infection or an infection that has not elicited a detectable IFN-γ response. However, as with the TST, false negatives can occur, particularly in individuals with immune system deficiencies or during the early stages of infection.
Indeterminate results may occur if the test neither clearly confirms nor excludes a TB infection. Several factors, such as problems with the test procedure or a low response to control substances, can lead to indeterminate results. In such cases, repeating the test or using an alternative diagnostic method may be necessary.
Compared to the TST, the IGRA offers several advantages, including higher specificity, meaning fewer false positives. It is not affected prior BCG vaccination, reducing the likelihood of false positives in vaccinated individuals. Additionally, the IGRA requires only one visit to a healthcare facility, as it does not require a follow-up examination of the injection site.
However, it is important to note that the IGRA is more expensive than the TST and may not be widely available in all healthcare settings. Furthermore, the IGRA requires a laboratory with specific equipment and trained personnel to perform the test accurately, which may limit access in resource-limited areas.
Both the Tuberculin Skin Test (TST) and the Interferon-Gamma Release Assay (IGRA) are diagnostic tests used to detect TB infection. The TST relies on the measurement of induration, a skin reaction, at the injection site, while the IGRA measures the release of interferon-gamma (IFN-γ) in response to TB proteins. Each test has its own advantages and limitations, and healthcare professionals consider multiple factors, including medical history, exposure risks, and other diagnostic tests, when interpreting the results.