What is the recommended initial test for Tuberculosis (TB)?

From the Guidelines

The recommended initial test for tuberculosis (TB) is either the tuberculin skin test (TST) or an interferon-gamma release assay (IGRA) blood test, with IGRA being preferred over TST in patients with cancer or those who have received the BCG vaccine. The choice between TST and IGRA depends on the patient's history, risk factors, and local testing availability 1.

• TST involves injecting a small amount of purified protein derivative (PPD) under the skin of the forearm and checking for a reaction 48-72 hours later.
• IGRAs, such as QuantiFERON-TB Gold or T-SPOT.TB, measure the immune response to TB proteins in a blood sample and are particularly useful for people who have received the BCG vaccine, as they don't cross-react with the vaccine like the TST can. For individuals with symptoms of active TB, additional tests are needed, including chest X-rays and sputum tests for acid-fast bacilli (AFB) smear and culture, with molecular techniques such as Xpert MTB/RIF or Line probe assays (LPA) being more sensitive 1.
• Healthcare providers should select the most appropriate initial test based on the patient's history, risk factors, and local testing availability, considering the limitations and advantages of each test, including sensitivity, specificity, and potential for cross-reactivity with the BCG vaccine 1. The initial screening tests are important because TB can remain latent without symptoms for years, and early detection allows for appropriate treatment before the disease progresses or spreads to others.
• In high TB prevalence countries, Xpert MTB/RIF testing may be suggested for initial diagnostic testing in patients at risk of pulmonary TB but at low risk of drug-resistant TB, along with chest X-rays when feasible and resources allow 1.

From the Research

Initial Test for Tuberculosis (TB)

The recommended initial test for Tuberculosis (TB) is a crucial aspect of diagnosing and managing the disease.

• The Xpert MTB/RIF assay is a rapid, automated test that can detect both TB and rifampicin resistance within two hours, with minimal hands-on technical time 2.
• This test has been shown to be sensitive and specific for diagnosing pulmonary TB and rifampicin resistance in adults and children 2, 3.
• The Xpert MTB/RIF assay can be used as an initial diagnostic test for TB detection and rifampicin resistance detection in patients suspected of having TB, MDR-TB, or HIV-associated TB 2.
• The Mantoux skin test is also used to aid in the diagnosis of tuberculosis (TB) disease and to identify persons with latent TB infection, but it is an imperfect test and approximately 10-25% of persons with microbiologically confirmed TB disease have a negative test result 4, 5, 6.
• A negative tuberculin skin test result is associated with an increased risk of death among persons with culture-confirmed TB disease 6.

Diagnostic Accuracy

The diagnostic accuracy of the Xpert MTB/RIF assay is as follows:

• Pooled sensitivity of 88% and pooled specificity of 98% when used as an initial test replacing smear microscopy 2.
• Pooled sensitivity of 67% and pooled specificity of 98% when used as an add-on test following a negative smear microscopy result 2.
• Pooled sensitivity of 94% and pooled specificity of 98% for rifampicin resistance detection 2.
• Pooled sensitivity of 65% and pooled specificity of 99% for diagnosing pulmonary TB in children 3.
• Pooled sensitivity of 94% and pooled specificity of 99% for detecting rifampicin resistance in children 3.