Diagnostic Procedures for Mycobacterium tuberculosis Infection

Detection of Mycobacterium tuberculosis, the causative agent of tuberculosis (TB), requires various diagnostic procedures to confirm infection and determine its activity. These methods aim to identify the presence of the bacteria, assess the extent of infection, and evaluate drug susceptibility.

Tuberculin Skin Examination (TST)

This procedure, also known as the Mantoux examination, involves the intradermal injection of purified protein derivative (PPD) derived from M. tuberculosis. A localized reaction, measured in millimeters of induration (firm swelling) 48-72 hours after injection, indicates prior exposure to the bacteria. Interpretation of the result depends on individual risk factors, such as recent contact with a TB patient, HIV status, and the presence of other medical conditions. A positive result signifies sensitization to TB antigens but does not differentiate between latent and active disease.

Factors Affecting TST Results

• Prior BCG vaccination
• Immunocompromised status
• Incorrect administration
• Reader variability

Interferon-Gamma Release Assays (IGRAs)

IGRAs are blood assays that measure the immune system's response to M. tuberculosis antigens. These assays quantify the amount of interferon-gamma (IFN-γ) released by T cells when exposed to specific TB antigens. Unlike the TST, IGRAs are not affected by prior BCG vaccination. Common IGRAs include the QuantiFERON-TB Gold In-Tube assay and the T-SPOT.TB assay.

Advantages of IGRAs

• Less affected by BCG vaccination
• Requires only one patient visit
• More objective interpretation

Microbiological Examinations

These examinations are crucial for confirming active disease and determining drug susceptibility.

Sputum Smear Microscopy

Microscopic examination of sputum samples stained with acid-fast stains (e.g., Ziehl-Neelsen or Kinyoun stain) can identify acid-fast bacilli (AFB). A positive AFB smear suggests active TB, but it cannot differentiate between M. tuberculosis and other mycobacteria. Sensitivity depends on the bacterial load in the sputum.

Sputum Culture

Culture of sputum samples on specific media (e.g., Lowenstein-Jensen or Middlebrook agar) allows for the growth and identification of M. tuberculosis. Culture is more sensitive than smear microscopy and can also be used for drug susceptibility . Liquid culture systems can improve sensitivity and reduce turnaround time.

Nucleic Acid Amplification Assays (NAATs)

NAATs, such as polymerase chain reaction (PCR), detect M. tuberculosis DNA in clinical samples. NAATs are highly sensitive and specific and can provide rapid results. Some NAATs can also detect mutations associated with drug resistance, particularly rifampicin resistance. The GeneXpert MTB/RIF assay is a widely used NAAT for rapid diagnosis and drug resistance detection.

Imaging Studies

Chest radiographs (X-rays) are commonly used to evaluate for pulmonary TB. Findings may include infiltrates, cavities, and lymphadenopathy. Computed tomography (CT) scans can provide more detailed information about the extent of lung disease.