Laboratory Diagnosis of Mycobacterium tuberculosis Infection

Mycobacterium tuberculosis (MTB) is the causative agent of tuberculosis (TB) in humans. It is a slightly curved rod arranged singly or in groups. Mycobacterium tuberculosis grows very slowly, taking up to 8 weeks for visible growth. The cell wall of Mycobacteria contains mycolic acid and gives acid-fast characteristics.

This page covers how TB is diagnosed. For the organism, how it causes disease, and the clinical picture, see the Mycobacterium tuberculosis and tuberculosis page, and for the genus as a whole, the Mycobacterium overview.

Robert Koch discovered the tubercle bacillus in 1882.

Figure: laboratory-diagnosis of Mycobacterium tuberculosis infection

Sample: Sputum, BAL, urine (in renal tuberculosis), blood, fluids, tissue, and gastric lavage (in case of children). The use of samples may be either directly (as in case of body fluids which is sterile) or after decontamination (as in sputum which is unsterile). Some samples are concentrated before use for culture, animal inoculation, and sometimes to yield better results in microscopy.

The diagnostic workflow, in order

No single test does everything, so TB diagnosis follows a sequence, and knowing why each step comes where matters more than the methods alone.

1. Acid-fast smear microscopy first: fast, cheap, same-day, and it flags the most infectious patients (those with many bacilli). But it needs large numbers of organisms and cannot tell M. tuberculosis from other mycobacteria.
2. Rapid molecular test (GeneXpert MTB/RIF) alongside or next: confirms M. tuberculosis and detects rifampicin resistance in under 2 hours, even when the smear is negative. It is now a frontline test, not a backup.
3. Culture as the reference standard: the most sensitive method, and the only one that allows full drug-susceptibility testing, but it takes weeks because the organism grows so slowly.
4. Drug-susceptibility testing on the cultured organism: essential given rising MDR-TB.

The tension that shapes everything: the fast tests (smear, GeneXpert) guide immediate treatment decisions, while the definitive tests (culture, susceptibility) take weeks. This is why TB treatment is often started before culture confirms it. The single most important interpretive point follows from step 1: a negative smear does not rule out TB, roughly half of culture-positive cases are smear-negative.

Direct Detection of Actively Growing Bacilli

Direct detection of infection includes microscopy, culture, antigen detection, and nucleic acid detection.

Microscopy

Microscopy is the most rapid diagnostic method for the detection of tubercle bacilli. Standard light microscopy and fluorescent microscopy are commonly used for detection. In light microscopy, M. tuberculosis appears as red-pink long or slightly curved bacilli when stained by AFB staining (Ziehl-Neelsen stain).

The acid-fast stain has low sensitivity; approximately 50% of “smear-negative” samples are found to be “culture-positive.”

Figure: Mycobacteria in Fluorescent Microscopy

When using fluorescent microscopy, acid-fast organisms will appear yellow or orange under ultraviolet light when stained with auramine-rhodamine stain and bright orange fluorescing bacilli in a pale green background when stained by acridine orange.

Culture

Culture is a more sensitive method for the detection of tubercle bacilli and is necessary for performing antimicrobial drug sensitivity testing and genotyping.

Figure: Typical M. tuberculosis colonies in agar media

After digestion of the specimen by treatment with NaOH and concentration by centrifugation, the sample is inoculated into a protein-rich medium (such as Lowenstein-Jensen medium or Middlebrook medium) and incubated at 37°C for up to 8 weeks. M. tuberculosis shows dry, rough, creamy, or buff-colored colonies in Lowenstein-Jensen medium.

On liquid medium, tubercle bacilli grow on the surface and form a wrinkled pellicle.

Mycobacteria grow more rapidly and reliably in a liquid culture medium compared with the solid medium.

Automated liquid culture systems (such as BACTEC MGIT) detect growth faster than solid media, often within 1 to 2 weeks, using a fluorescence-based indicator of oxygen consumption rather than the older radioactive method.

Presumptive colonies from culture medium are further identified by biochemical tests such as niacin test, nitrate reductase test, tween 80 hydrolysis, arylsulfatase test, and urease test. M. tuberculosis is niacin-positive, nitrate-reduction-positive, and urease-positive; niacin positivity is the classic feature distinguishing it from most non-tuberculous mycobacteria.

Molecular Diagnosis

Because of the slow growth rate of the M. tuberculosis complex (MTBC) bacilli, molecular techniques have become the cornerstone for the rapid detection of TB.

Nucleic Acid Amplification Tests (NAAT)

The nucleic acid amplification tests (NAAT) identify the etiological agent as well as provide information about drug resistance in a couple of days or even hours upon receipt of the specimen. PCR is the most common NAAT format. Other methods include ligase chain reaction, strand displacement amplification, loop-mediated isothermal amplification (LAMP), and transcription-mediated amplification.

Molecular tests that detect mutations in genes provide information about drug resistance;

• katG gene (catalase-peroxidase; mutations cause isoniazid resistance),
• rpoB gene (RNA polymerase; mutations cause rifampicin resistance).

Xpert MTB/RIF assay

Figure: GeneXpert MTB/RIF Assay

GeneXpert MTB/RIF assay is a nucleic acid amplification test which simultaneously detects DNA of Mycobacterium tuberculosis complex (MTBC) and resistance to rifampicin (RIF) (i.e. mutation of the rpoB gene) in less than 2 hours.

The GeneXpert MTB/RIF assay is covered in full on its own page.

Antigen detection methods

MTB antigen detection provides direct evidence of TB. Lipoarabinomannan (LAM), a cell-wall antigen, can be detected in urine. Urine LAM is recommended mainly for HIV-positive patients with advanced immunosuppression, in whom it improves rapid TB detection. Other mycobacterial antigens have also been studied as detection targets, but LAM (especially urine LAM in advanced HIV) is the one in current diagnostic use.

Indirect methods for the detection of MTB

It includes the detection of immune response by tuberculin skin testing (TST) and interferon-gamma release assays (IGRAs). These tests are positive in both latent disease and in active tuberculosis so any person with a positive test must be evaluated for the presence of active disease.

Tuberculin skin test or PPD test

Figure: Mantoux tuberculin test (Image source: CDC/PHIL)

The PPD (purified protein derivative) test determines if someone has developed an immune response to MTB. This response can occur if someone currently has TB, if they were exposed to it in the past, or if they received the BCG vaccination.

The standard method is the Mantoux test, in which PPD is injected into the top layers of the forearm skin and the induration (localized swelling) is read at 48 to 72 hours. The full procedure and how to interpret the induration size are on the tuberculin skin test page.

Interferon-Gamma Release Assays (IGRA)

Figure: Interferon-Gamma Release Assays (IGRA) Image

The interferon-gamma assay overcomes the difficulties faced with the tuberculin test interpretation. Two formats of the interferon-gamma release assay are available: QuantiFERON-TB Gold and T-SPOT.TB. The IGRA assay is based on the ability of the MTB antigens, such as Early Secretory Antigen Target 6 (ESAT-6) and Culture Filtrate Protein 10 (CFP-10) to stimulate the host for the production of interferon-gamma (IFN-γ).

The amount of interferon-γ released from the cells is measured. Results are interpreted both qualitatively (positive, negative, or indeterminate) and quantitatively. BCG immunization does not affect the test result of IGRA assay, as the antigens used here are specific for Mycobacterium tuberculosis and are not present in BCG vaccine.

Where students get confused

A negative smear does not rule out TB. Smear microscopy needs many organisms (roughly 10,000/mL), so about half of culture-positive cases are smear-negative. A negative AFB smear never excludes tuberculosis.

Smear positive is not the same as M. tuberculosis. Acid-fast bacilli on a smear could be M. tuberculosis or a non-tuberculous mycobacterium. The smear shows acid-fast organisms; culture or molecular tests identify the species.

GeneXpert is a frontline test, not a confirmation-only test. It detects M. tuberculosis and rifampicin resistance in under 2 hours, including in many smear-negative cases. It is used early, not only after culture.

Tuberculin and IGRA show infection, not active disease. A positive TST or IGRA means the immune system has met M. tuberculosis (latent or active infection, or, for TST, prior BCG). It cannot diagnose active TB on its own.

IGRA is not affected by BCG; the tuberculin test is. IGRA uses antigens (ESAT-6, CFP-10) absent from the BCG vaccine, so prior BCG does not cause a false-positive IGRA, unlike the tuberculin test.

Culture is slow but indispensable. It takes weeks, but it is the most sensitive test and the only route to full drug-susceptibility testing, which is essential given MDR-TB.

References and further readings

1. Tille, P. M. (2022). Bailey & Scott's Diagnostic Microbiology (15th ed.). Elsevier.
2. Procop, G. W., et al. (2017). Koneman's Color Atlas and Textbook of Diagnostic Microbiology (7th ed.). Wolters Kluwer.
3. World Health Organization. (2021). WHO consolidated guidelines on tuberculosis. Module 3: Diagnosis – Rapid diagnostics for tuberculosis detection (2021 update). World Health Organization.
4. Murray, P. R., Rosenthal, K. S., & Pfaller, M. A. (2020). Medical Microbiology (9th ed.). Elsevier.