Should you perform Mycobacterium tuberculosis (MTB) Polymerase Chain Reaction (PCR) and Acid-Fast Bacillus (AFB) smear on tissue samples when diagnosing tuberculosis (TB)?

From the Guidelines

Yes, performing both Mycobacterium tuberculosis (MTB) PCR and Acid-Fast Bacillus (AFB) smear on tissue samples is recommended when diagnosing tuberculosis. These tests should be conducted alongside tissue culture, which remains the gold standard for TB diagnosis, as suggested by the official American Thoracic Society/Infectious Diseases Society of America/Centers for Disease Control and Prevention clinical practice guidelines 1. AFB smear provides rapid preliminary results within hours, detecting acid-fast bacilli through microscopic examination, though it has limited sensitivity and cannot distinguish between MTB and non-tuberculous mycobacteria. MTB PCR offers higher sensitivity and specificity with results available within 24-48 hours, directly detecting MTB DNA and some assays can identify rifampin resistance. Using both tests together enhances diagnostic accuracy, as PCR can detect TB even when bacterial load is low and smear is negative. Some key points to consider when performing these tests include:

• Testing of 3 specimens is considered the normative practice in the United States and is strongly recommended by the Centers for Disease Control and Prevention and the National Tuberculosis Controllers Association in order to improve sensitivity given the pervasive issue of poor sample quality 1.
• Providers should request a sputum volume of at least 3 mL, but the optimal volume is 5–10 mL, and concentrated respiratory specimens and fluorescence microscopy are preferred 1.
• At least liquid culture should be done on all specimens as culture is the gold standard microbiologic test for the diagnosis of TB disease 1. For optimal results, proper tissue handling is essential - samples should be collected before antibiotic administration, placed in sterile containers without formalin, and promptly transported to the laboratory. This combined approach helps clinicians initiate appropriate treatment earlier, improving patient outcomes and preventing disease transmission. It is also suggested that both liquid and solid mycobacterial cultures be performed, rather than either culture method alone, for every specimen obtained from an individual with suspected TB disease, although this is a conditional recommendation based on low-quality evidence 1. Additionally, performing a diagnostic nucleic acid amplification test (NAAT), such as MTB PCR, on the initial respiratory specimen from patients suspected of having pulmonary TB is suggested, rather than not performing a NAAT, as it can provide valuable information for the diagnosis of TB disease 1.

From the Research

Diagnostic Approaches for Tuberculosis

• Mycobacterium tuberculosis (MTB) Polymerase Chain Reaction (PCR) and Acid-Fast Bacillus (AFB) smear are two diagnostic approaches used to detect tuberculosis (TB) in tissue samples.
• MTB PCR is a molecular technique that detects the genetic material of MTB, while AFB smear is a microscopic technique that detects the presence of acid-fast bacilli in tissue samples.

Sensitivity and Specificity of MTB PCR and AFB Smear

• Studies have shown that MTB PCR has higher sensitivity and specificity compared to AFB smear in detecting TB in tissue samples 2, 3, 4, 5.
• A study published in 2021 found that MTB PCR was positive in 100% of confirmed TB cases, while AFB culture was positive in 73% of cases 2.
• Another study published in 2011 found that real-time MTB PCR had a sensitivity of 74.6% and specificity of 98.5% in detecting TB in formalin-fixed, paraffin-embedded (FFPE) tissue specimens 3.

Comparison of MTB PCR and AFB Smear with Other Diagnostic Techniques

• MTB PCR has been compared to other diagnostic techniques such as Xpert MTB/RIF real-time PCR and culture, and has been found to have higher sensitivity and specificity 2, 4.
• A study published in 2005 found that MTB PCR had higher sensitivity and specificity compared to smear and culture in detecting extrapulmonary TB 4.
• Another study published in 2012 found that tissue PCR had a sensitivity of 87.5% and specificity of 86.7% in detecting genitourinary tract TB, and was found to be a useful adjunct to histopathological examination 5.

Clinical Utility of MTB PCR and AFB Smear

• MTB PCR and AFB smear can be used to diagnose TB in tissue samples, especially in cases where other diagnostic techniques are negative or inconclusive 6, 5.
• A study published in 2023 found that histopathological analysis of tissue samples, including AFB smear, can contribute to the diagnosis of TB, especially in extrapulmonary forms 6.
• MTB PCR can provide rapid and accurate diagnosis of TB, which can aid in the timely initiation of treatment and improve patient outcomes 2, 3, 4, 5.