The TB diagnostic pathway is unique within clinical microbiology because of: the extreme biosafety requirements for growing M. tuberculosis in culture (BSL-3 work required for aerosol-generating procedures), the very slow growth rate of MTBC (doubling time approximately 20 hours, compared to 20 minutes for E. coli), the requirement for multiple complementary test types (smear, culture, molecular), and the critical importance of drug susceptibility testing (DST) for selecting appropriate treatment in an era of multidrug-resistant (MDR-TB) and extensively drug-resistant (XDR-TB).
Sample Types and Quality
The quality of the laboratory result starts with sample quality. Good TB diagnostics begin before the sample reaches the laboratory.
Sputum for pulmonary TB: the standard sample. Three early-morning sputum samples (collected on consecutive days, or two same-day samples plus one early morning) are collected for smear microscopy and culture. Each sample should be 2 to 10 mL of purulent material, not saliva. Poor-quality samples (predominately saliva) yield false negatives. Sputum induction (using hypertonic saline via nebuliser) improves sample quality in patients unable to expectorate spontaneously. Induced sputum is aerosol-generating and must be collected in a negative pressure room with appropriate PPE.
Bronchoalveolar lavage (BAL): for patients unable to provide sputum or when upper lobe disease is present. BAL has higher sensitivity than spot sputum for smear-negative pulmonary TB.
Gastric aspirate: used in children who swallow sputum. Children are often smear-negative and culture may be the only positive result.
Urine (3 early morning samples), cerebrospinal fluid, pleural fluid, peritoneal fluid, lymph node biopsy, bone biopsy, tissue from any suspected extrapulmonary site.
All samples for TB culture must be handled in a certified BSL-2 minimum (BSL-3 for aerosol-generating procedures) facility with appropriate PPE.
Smear Microscopy: Fast but Limited
Smear microscopy for acid-fast bacilli (AFB) is the simplest and oldest TB diagnostic test. The Ziehl-Neelsen (ZN) stain and auramine fluorescence stain detect AFB in sputum or other samples.
Smear positivity correlates with infectiousness and disease severity: patients with smear-positive pulmonary TB are the primary source of TB transmission. The WHO reporting scale: negative, scanty (1 to 9 AFB per 100 fields, should be confirmed by culture), 1+ (10 to 99 per 100 fields), 2+ (1 to 10 per field), 3+ (above 10 per field).
Limitations: sensitivity of direct smear for TB diagnosis is only 20 to 60 per cent, varying by disease type, sample quality, and laboratory technique. Smear microscopy cannot distinguish MTBC from non-tuberculous mycobacteria (NTM): any AFB-positive result requires culture and molecular confirmation to identify the species and determine drug susceptibility. Smear does not provide drug susceptibility information.
Concentrated smear (after decontamination and centrifugation) improves sensitivity by approximately 10 to 15 percentage points over direct smear.
Liquid Culture: The Gold Standard
Mycobacterial culture in liquid broth media (MGIT, Middlebrook 7H9-based automated systems such as BD BACTEC MGIT 960) is the gold standard for TB diagnosis. Liquid culture detects MTBC in 9 to 16 days median time to positivity for smear-positive samples, and 14 to 42 days for smear-negative samples, compared to 3 to 8 weeks for solid media (Löwenstein-Jensen).
Solid culture (LJ slopes) is used alongside liquid culture as a backup and for colony morphology and susceptibility testing confirmation. LJ media also supports growth of NTM that may grow poorly in MGIT.
Before culture: samples are decontaminated with N-acetyl-L-cysteine (NALC) and sodium hydroxide (NaOH) at a carefully controlled concentration (2 to 4 per cent NaOH) for a controlled time. Decontamination kills faster-growing non-mycobacterial bacteria that would overgrow the slow-growing mycobacteria in culture. Over-decontamination kills mycobacteria and reduces culture sensitivity; under-decontamination allows contaminant overgrowth. Contamination rate (from over-decontamination) should be below 2 to 5 per cent in a well-run laboratory.
All positive cultures must be identified to species to distinguish MTBC from NTM by molecular identification (Hain GenoType Mycobacterium line probe assay, MALDI-TOF with mycobacterial database, or 16S rRNA PCR and sequencing).
Molecular Diagnostics: Xpert MTB/RIF and Beyond
The WHO-endorsed Xpert MTB/RIF (GeneXpert, Cepheid) cartridge-based PCR is the most widely implemented molecular TB diagnostic test globally. It detects MTBC DNA directly from sputum and simultaneously identifies rifampicin resistance by detecting mutations in the rpoB gene, which are present in more than 95 per cent of rifampicin-resistant MTBC strains.
Xpert sensitivity for pulmonary TB: approximately 88 per cent (smear-positive cases) to 67 per cent (smear-negative cases) compared to culture reference standard. Specificity: above 98 per cent. The result is available in under 2 hours.
Xpert Ultra (Cepheid): the next-generation version with enhanced sensitivity for smear-negative, low-bacillary-burden TB (including meningeal TB where bacillary load is very low). Ultra detects IS6110 and IS1081 insertion sequences in addition to the rpoB gene region, improving sensitivity to approximately 73 per cent for smear-negative pulmonary TB.
Truenat MTB and LAMP-based assays: simpler, point-of-care molecular assays suitable for peripheral laboratories without full GeneXpert infrastructure. Endorsed by WHO for use in high TB burden, low resource settings.
Line Probe Assay (LPA, Hain GenoType MTBDRplus and MTBDRsl): detects mutations conferring resistance to isoniazid (inhA and katG mutations), rifampicin (rpoB mutations), fluoroquinolones, and second-line injectables directly from smear-positive sputum or culture. Used for rapid DST when Xpert rifampicin resistance is detected.
Whole genome sequencing (WGS): increasingly used in reference laboratories for complete drug resistance profiling, outbreak investigation and transmission cluster identification, and phylogenetic analysis. WGS can simultaneously identify all known resistance-conferring mutations for first and second-line drugs from a single MTBC culture isolate.
Drug Susceptibility Testing (DST)
DST determines whether an MTBC isolate is susceptible or resistant to the key anti-TB drugs. DST is essential for guiding appropriate treatment and for TB epidemiology and surveillance.
Phenotypic DST: the isolate is grown in liquid or solid media containing each drug at its critical concentration. Growth (resistance) or no growth (susceptibility) in the presence of the drug is read after 7 to 21 days (MGIT) or 3 to 4 weeks (LJ).
MDR-TB: resistance to both isoniazid and rifampicin (the two most effective first-line drugs). WHO data: approximately 3.3 per cent of new TB cases and 17 per cent of previously treated cases are MDR-TB globally. MDR-TB requires at least 6 months of second-line drug treatment with drugs including bedaquiline, linezolid, clofazimine, and delamanid.
XDR-TB (revised WHO 2021 definition): MDR-TB plus resistance to any fluoroquinolone AND to at least one of bedaquiline or linezolid. XDR-TB has very limited treatment options and poor outcomes without expert specialist management.
Frequently Asked Questions
What is the fastest way to diagnose TB?
Xpert MTB/RIF provides a definitive diagnosis of TB and simultaneous rifampicin resistance detection in under 2 hours directly from sputum, making it the fastest definitive TB diagnostic test available. Smear microscopy provides a preliminary result within hours but cannot distinguish MTBC from NTM and does not detect drug resistance.
Why does TB culture take so long?
M. tuberculosis has a doubling time of approximately 20 hours, compared to 20 minutes for E. coli. This slow growth reflects its highly adapted intracellular lifestyle. Even in liquid MGIT culture (faster than solid media), 9 to 16 days or more are required for smear-positive samples to produce detectable growth.
What is MDR-TB?
Multidrug-resistant TB is MTBC infection resistant to at least rifampicin and isoniazid, the two most effective first-line anti-TB drugs. MDR-TB requires treatment with second-line drugs including bedaquiline, linezolid, and clofazimine for a minimum of 6 months. Current WHO-recommended regimens for MDR-TB include the 6-month BPaLM regimen (bedaquiline, pretomanid, linezolid, moxifloxacin).
What does rpoB mutation mean?
rpoB is the gene encoding the beta subunit of bacterial RNA polymerase, the molecular target of rifampicin. Mutations in a specific region of rpoB (the rifampicin resistance-determining region, RRDR) prevent rifampicin from binding, conferring high-level resistance. Over 95 per cent of rifampicin-resistant MTBC clinical isolates carry an rpoB RRDR mutation, allowing PCR-based detection of rifampicin resistance via Xpert MTB/RIF.
What is latent TB?
Latent TB infection (LTBI) is infection with MTBC in a person who has no symptoms, no radiological evidence of active disease, and a negative sputum sample, but who carries a T cell immune memory response to MTBC antigens (detected by TST or IGRA). Approximately 25 per cent of the world population has LTBI. LTBI has a lifetime risk of reactivation to active TB of approximately 5 to 10 per cent, with higher risk in immunocompromised individuals. Preventive treatment (isoniazid preventive therapy, IPT, or rifampicin for 4 months) reduces reactivation risk.
What is IGRA?
Interferon-Gamma Release Assay (IGRA): a blood test for MTBC infection (both active and latent) based on measuring interferon-gamma production by sensitised T cells when stimulated with MTBC-specific antigens (ESAT-6, CFP-10, and TB7.7 in the QuantiFERON-TB Gold Plus assay). IGRA is more specific than the tuberculin skin test (TST) as ESAT-6 and CFP-10 are not expressed by BCG vaccine strains or most NTM, reducing false positives from BCG vaccination.
What is the difference between smear-positive and smear-negative TB?
Smear-positive TB has AFB visible on ZN or auramine stain of sputum, indicating high bacterial load and high infectiousness. Smear-negative TB has no AFB visible on smear but may be culture-positive or Xpert-positive, indicating lower bacterial load. Smear-negative TB is still infectious but less so than smear-positive. Smear-negative TB is more difficult to diagnose and requires molecular or culture confirmation.
What is XDR-TB?
Extensively drug-resistant TB (revised WHO 2021 definition): MDR-TB with additional resistance to any fluoroquinolone AND to bedaquiline or linezolid. XDR-TB has very limited treatment options and requires treatment in specialist centres with access to drugs including pretomanid, clofazimine, and delamanid. The global burden of XDR-TB is approximately 3,400 cases per year (WHO 2022 data).
What is whole genome sequencing used for in TB?
WGS of MTBC culture isolates provides: complete drug resistance profiling (identifying all known resistance mutations for first and second-line drugs simultaneously), lineage classification (phylogenetic placement of the isolate within the global MTBC phylogeny), and transmission cluster analysis (identifying isolates that are closely related by SNP-based clustering, used to detect ongoing transmission in outbreak investigations). WGS is increasingly replacing phenotypic DST panels in reference laboratories in high-income countries.
Why is TB a notifiable disease?
TB is a notifiable disease in most countries because of its transmissibility, the public health implications of identifying contacts, the need for contact tracing and testing, and the requirement for population-level surveillance data to guide TB control programmes. In the UK, all confirmed and probable TB cases must be notified to the local Health Protection Team within 3 days, allowing contact tracing, LTBI testing of close contacts, and treatment support.