Diagnosis of latent TB

This is an automatically translated article.

The article was written by Doctor Nguyen Thi Thuy Hang- Doctor of Microbiology - Laboratory Department - Vinmec Times City International Hospital

The bacteria that cause TB in people with latent TB infection can become reactivated. The risk of latent TB infection is high in close contacts of patients with active TB, HIV infection, pneumococcal disease, immunosuppressive drug use, organ transplantation, malignancy, end-stage renal failure, people with diabetes...

1. Diagnosis of latent TB

Diagnosis of latent TB infection is important for comprehensive disease control and antituberculosis treatment, which greatly reduces the risk of developing active TB. The diagnosis of latent TB is based on two tests: a positive IGRA (Interferon-Gamma Release Assay) latent TB skin test (also known as TST-tuberculin skin test) or a positive IGRA (Interferon-Gamma Release Assays) latent TB blood test. Active TB was excluded by clinical examination, chest x-ray, sputum examination, or abnormalities in organs other than the lungs suspected of tuberculosis. For many years, the laboratory diagnosis of latent TB infection was based on the Mantoux test. Studies have shown that among people with a positive Mantoux test and no other risk factors, the chance of developing active TB is about 0.1% per year. However, this test has some limitations that affect its sensitivity and specificity: It can be false positive in atypical TB infection or with previous BCG vaccination, false negative in cases: immunocompromised or newly infected with TB under 8 months, infants under 6 months. Moreover, the patient must return to read the results after 48 hours, up to 72 hours.

2. Latent TB test

Recently, the IGRA latent TB blood test was performed to diagnose latent TB. The principle of the test is to detect TB infection based on the quantification of IFN-γ levels in peripheral blood after activation by TB-specific antigens, ESAT-6, CFP-10. There are currently two types of IGRA tests in use: the T-SPOT® TB test and the QuantiFERON®-TB Gold Plus (QFT-Plus) test. These tests have been shown to be more specific than the Mantoux test because it is not affected by BCG vaccination. The sensitivity of the IGRA test is similar (but higher in immunocompromised patients) as that of the Mantoux test and has been shown in some studies to correlate better with exposure. infected.
IGRA tests have been shown to have better predictive value for the development of active TB in people with latent TB infection. However, these tests also have some limitations such as the need to draw venous blood, the processing procedure is quite complicated, and the cost is higher than the Mantoux test. The most cost-effective use of the IGRA test is as a confirmatory test in patients with a positive Mantoux test, particularly in areas with high BCG vaccination rates. The World Health Organization recommends that either the Mantoux test or the IGRA test can be used to identify latent TB.

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