Tuberculosis laboratory findings
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Mashal Awais, M.D.[2];Sophia Saad, Associate Editor - WikiDoc [3] Alejandro Lemor, M.D. [4]
Laboratory findings
Overview
This microchapter covers laboratory diagnostics for active tuberculosis disease, including acid-fast bacilli (AFB) smear microscopy, mycobacterial culture, nucleic acid amplification tests (NAATs), molecular drug susceptibility testing, body fluid analysis (cerebrospinal, pleural, peritoneal, pericardial), urine lipoarabinomannan (LAM) assays, histopathology, and routine laboratory abnormalities. LTBI screening tests (TST, IGRAs) are covered in the Screening microchapter; treatment-related monitoring laboratories are covered in Medical Therapy.
Routine laboratory abnormalities
Routine blood tests are nonspecific but may provide supportive evidence:
- Anemia: Mild normocytic anemia occurs in up to ~50% of patients with active TB, particularly with chronic or disseminated disease
- Elevated inflammatory markers: ESR and CRP are frequently elevated but nonspecific
- Leukocyte count: Usually normal; mild leukocytosis or leukopenia may occur; pancytopenia is rare and associated with miliary TB or bone marrow involvement
- Hyponatremia: Common in TB meningitis (present in ~45% of cases), caused by SIADH (most common, ~78%) or cerebral salt wasting (~18%); associated with worse prognosis and higher mortality; also occurs with adrenal or pituitary involvement
- Hypercalcemia: Occurs via granuloma-mediated 1,25-dihydroxyvitamin D production; rare in miliary TB
- Hepatic abnormalities: Mild elevation of alkaline phosphatase and transaminases may reflect hepatic granulomas or disseminated disease
- Adrenal insufficiency: Low cortisol and aldosterone when adrenal glands are involved (Addison disease from TB)
AFB smear microscopy
The ATS/IDSA/CDC 2017 guidelines recommend AFB smear microscopy for all patients suspected of having pulmonary TB (strong recommendation, moderate-quality evidence).[1]
Key points:
- Specimen collection: Three sputum specimens are recommended; optimal volume is 5–10 mL (minimum 3 mL). If the patient cannot expectorate, induced sputum is preferred over gastric washings (higher sensitivity). Bronchoscopy with BAL is an alternative.
- Staining method: Fluorescence microscopy (auramine-rhodamine staining) is preferred over conventional Ziehl-Neelsen staining due to higher sensitivity.
- Sensitivity and limitations: Sensitivity is approximately 50–80% for pulmonary TB and is lower for extrapulmonary and paucibacillary disease. A negative AFB smear does not exclude TB. A positive AFB smear does not confirm TB, as nontuberculous mycobacteria (NTM) are also acid-fast.
- Differentiation from NTM: When smear is positive, NAAT or gene probe testing should be performed to distinguish M. tuberculosis from NTM — this is particularly important in settings like the United States where NTM prevalence exceeds TB prevalence.
Mycobacterial culture
Culture remains the gold standard for TB diagnosis and is essential for phenotypic drug susceptibility testing (DST). The ATS/IDSA/CDC 2017 guidelines recommend culture on all specimens regardless of smear result.[1]
- Culture systems: Both liquid (e.g., MGIT/BACTEC) and solid (e.g., Löwenstein-Jensen, Middlebrook 7H10/7H11) media should be used (conditional recommendation, low-quality evidence). Liquid culture is faster (1–3 weeks) than solid media (3–8 weeks) and has higher sensitivity.
- Species identification: All positive cultures must be identified to species level per CLSI and ASM guidelines to distinguish M. tuberculosis complex from NTM.
- Reporting requirements: In the United States, positive smear and culture results must be reported within 24 hours to the treating clinician and to state/local TB control programs.
- Extrapulmonary specimens: Most extrapulmonary TB is paucibacillary; culture yield is lower, and a negative culture does not exclude TB.
- MODS assay: The microscopic-observation drug-susceptibility assay is a liquid culture method that simultaneously detects M. tuberculosis and resistance to isoniazid and rifampin, with results in a median of 7–14 days; it has been validated primarily in resource-limited settings.
- Mycobacterial blood cultures: Indicated in suspected disseminated or miliary TB, particularly in HIV-positive patients with advanced immunosuppression (CD4 <100 cells/μL). Dedicated mycobacterial blood culture systems (e.g., BACTEC Myco/F Lytic) should be used; standard blood culture bottles have poor sensitivity for mycobacteria.
Nucleic acid amplification tests (NAATs)
NAATs provide rapid identification of M. tuberculosis complex (results within hours) and are recommended as adjuncts to — not replacements for — culture.
Xpert MTB/RIF Ultra
Xpert MTB/RIF Ultra is the most widely used WHO-recommended rapid diagnostic. The WHO recommends it as the initial diagnostic test for TB and rifampicin resistance detection, replacing smear/culture and phenotypic DST as the first test in the diagnostic algorithm (strong recommendation). Xpert MTB/RIF Ultra is not currently FDA-approved or available in the United States; US clinicians use the original Xpert MTB/RIF or the Hologic Amplified MTD test. The ATS/IDSA/CDC 2017 guidelines reference these FDA-approved NAATs. Xpert MTB/RIF was discontinued in most countries in 2023 and superseded by Xpert Ultra, but remains in use in the United States where Xpert Ultra lacks FDA approval. Xpert MTB/RIF has pooled sensitivity ~85% and specificity ~98% for pulmonary TB.
- Pulmonary TB performance (sputum): Pooled sensitivity ~88–91%, specificity ~95–96% (vs. culture reference standard)[2]; limit of detection 16 CFU/mL — approximately 7-fold more sensitive than the original Xpert MTB/RIF. Sensitivity is ~95% in smear-positive and ~82–88% in smear-negative specimens. Also detects rifampicin resistance (sensitivity ~96%, specificity ~98%).
- Extrapulmonary TB: Sensitivity varies by specimen type — highest in lymph node (87–95%), tissue (86–100%), and pus samples (97%); moderate in CSF (47–100% depending on setting and sample size); lowest in pleural fluid (47–78%). Specificity may be lower in extrapulmonary specimens (~67–97%), particularly for lymph node aspirates and pleural fluid.
- Trace results: A "trace" positive category exists in Xpert Ultra, indicating very low bacterial loads. In a 2025 Cochrane meta-analysis, only 38.8% of trace-positive results were true positives against a microbiological reference standard. Trace results may represent dead bacilli (e.g., in patients with prior TB treatment) and should be interpreted cautiously; reclassifying traces as negative modestly improved specificity. Specificity of Xpert Ultra is also notably reduced in patients with prior TB (86.2% vs. 94.8% overall). WHO considers Xpert Ultra trace results as bacteriologically positive in children and people living with HIV; however, in other populations, clinical judgment is required.
The ATS/IDSA/CDC 2017 guidelines recommend performing a NAAT on the initial respiratory specimen in patients suspected of pulmonary TB (conditional recommendation, low-quality evidence).[1] Interpretation depends on smear status and clinical suspicion:
- Smear-positive, NAAT-positive: TB presumed; start treatment
- Smear-positive, NAAT-negative: TB unlikely; consider NTM (repeat NAAT on second specimen; check for PCR inhibitors)
- Smear-negative, NAAT-positive (intermediate/high suspicion): Presumptive evidence of TB
- Smear-negative, NAAT-negative (intermediate/high suspicion): Cannot exclude TB; await culture
- Low clinical suspicion: NAAT generally not performed (false-positive risk unacceptably high)
Xpert MTB/XDR
A reflex test performed on specimens already confirmed to contain M. tuberculosis; detects resistance to isoniazid (sensitivity 94%, specificity 98%), fluoroquinolones (sensitivity 93%, specificity 98%), ethionamide, and amikacin, with results in <90 minutes. WHO-recommended for use in bacteriologically confirmed TB. Xpert MTB/XDR is also not currently FDA-approved or available in the United States.
Other NAATs
- Truenat MTB/MTB Plus: WHO-recommended low-complexity automated NAAT suitable for decentralized settings; sensitivity 73–80%, specificity 96–98% for pulmonary TB; also detects rifampicin resistance via a separate Truenat MTB-RIF Dx cartridge.
- Stool Xpert: WHO recommends Xpert MTB/RIF Ultra on stool specimens as a non-invasive alternative to respiratory specimens in children unable to produce sputum (conditional recommendation). Sensitivity is lower than for sputum (~50–70%) but specificity is high (~97–99%).
Drug susceptibility testing
The ATS/IDSA/CDC 2017 guidelines recommend that all initial M. tuberculosis isolates be tested for drug susceptibility.[1] Both genotypic and phenotypic methods have essential roles.
Phenotypic DST
Culture-based phenotypic DST remains the reference standard and should be performed on all positive cultures. Results typically take 4–6 weeks. It is essential for confirming genotypically indeterminate results and for drugs where molecular prediction is unreliable (notably pyrazinamide and ethambutol, for which phenotypic testing itself has suboptimal reproducibility).
Line probe assays (LPAs)
First-line LPAs (GenoType MTBDRplus, Nipro NTM+MDRTB): Detect rifampicin and isoniazid resistance from smear-positive specimens or culture isolates. WHO-endorsed.
Second-line LPAs (GenoType MTBDRsl): Detect fluoroquinolone and injectable agent resistance; WHO-recommended for rifampicin-resistant TB.
Targeted next-generation sequencing (tNGS)
WHO endorsed tNGS in 2024 (conditional recommendation) as a reflex test following initial NAATs, with the 2025 WHO consolidated guidelines prioritizing tNGS for comprehensive DST. However, tNGS does not replace current WHO-recommended initial diagnostics (e.g., Xpert), and phenotypic DST is still required per WHO guidance. tNGS can be performed directly on clinical specimens without culture, simultaneously predicting resistance to multiple first- and second-line drugs including bedaquiline, linezolid, clofazimine, delamanid, pretomanid, and fluoroquinolones. Commercial platforms include Deeplex Myc-TB (GenoScreen). Interpretation is guided by the WHO catalogue of M. tuberculosis mutations (2nd edition). Sensitivity is excellent for fluoroquinolones (≥95%) but lower for bedaquiline (~68%) and linezolid (~69%).
Whole-genome sequencing (WGS)
WGS provides the most comprehensive genotypic resistance profiling but typically requires culture isolates and reference laboratory infrastructure. The 2023 TBnet/RESIST-TB consensus recommends that mutations recognized as resistance markers in the WHO catalogue should not require confirmatory phenotypic DST, whereas uncharacterized mutations should be confirmed by culture-based DST. WGS is also used for molecular epidemiology and outbreak investigation.
Body fluid analysis
Cerebrospinal fluid (TB meningitis)
- Pleocytosis: Usually 5–300 WBC/μL; lymphocyte-predominant (early disease may show mixed or PMN-predominant pattern)
- Protein: Elevated, typically 100–200 mg/dL
- Glucose: Low; CSF:plasma glucose ratio <0.5 is characteristic
CSF Xpert MTB/RIF Ultra sensitivity is approximately 53–98% depending on setting and specimen volume; large-volume CSF sampling (≥6 mL) improves yield. CSF ADA at a threshold of 8–10 U/L has sensitivity ~59–93% and specificity ~80–96% depending on threshold used.
Pleural fluid
TB pleural effusions are typically lymphocyte-predominant exudates with low glucose and elevated LDH.
- ADA: Threshold ~40 U/L — sensitivity 89–99%, specificity 88–97% in meta-analyses. ADA <30 U/L effectively excludes TB pleural effusion.[1]
- Unstimulated IFN-γ: Sensitivity ~89%, specificity ~97%. The ATS/IDSA/CDC 2017 guidelines conditionally recommend IFN-γ measurement in pleural fluid (conditional recommendation, low-quality evidence).[1]
Peritoneal fluid
Lymphocytic exudate with protein >3.0 g/dL and elevated LDH ADA threshold ~36–40 U/L; sensitivity ~100%, specificity ~97% in one meta-analysis of 4 studies.[1] Free IFN-γ is conditionally recommended (sensitivity ~93%, specificity ~99%).[1]
Pericardial fluid
ADA threshold of 40 U/L — sensitivity 88%, specificity 83%.[1] The ATS/IDSA/CDC 2017 guidelines do NOT recommend free IFN-γ measurement in pericardial fluid due to insufficient data.[1]
Urine lipoarabinomannan (LAM) assay
The Alere Determine TB LAM Ag lateral flow assay is WHO-recommended for TB diagnosis in people living with HIV:
- Sensitivity: ~52% in inpatients, ~29% in outpatients; increases with lower CD4 counts
- Mortality benefit: RCTs have demonstrated reduced mortality when LAM is incorporated into inpatient diagnostic algorithms in HIV-positive patients
- Parallel testing: WHO recommends concurrent use of urine LAM + respiratory NAAT for TB diagnosis in adults/adolescents with HIV, as combined sensitivity exceeds either test alone
- Limitations: False-positive results may occur with disseminated NTM infection; limited sensitivity in HIV-negative individuals
- FujiLAM (Fujifilm SILVAMP TB LAM): A next-generation urine LAM assay with substantially higher sensitivity than Alere LF-LAM. Meta-analytic pooled sensitivity in PLHIV is ~63% overall and ~76–87% at CD4 ≤100 cells/μL, with specificity ~90%. An updated version (FujiLAM II) is under clinical evaluation. WHO recommendation is anticipated.
Histopathology
Caseating granulomas on tissue biopsy support the diagnosis of TB but are not pathognomonic. AFB staining and culture/NAAT of biopsy material increase diagnostic yield. Renal TB (urinalysis): Sterile pyuria (persistent pyuria with negative routine bacterial cultures) with microscopic hematuria is the classic finding; should prompt AFB cultures of early morning urine specimens on 3 consecutive days.
References
- ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 Lewinsohn DM, Leonard MK, LoBue PA; et al. (2017). "Official ATS/IDSA/CDC Clinical Practice Guidelines: Diagnosis of Tuberculosis in Adults and Children". Clinical Infectious Diseases. 64 (2): e1–e33. doi:10.1093/cid/ciw694.
- ↑ Horne DJ, Zifodya JS, Shapiro AE; et al. (2025). "Xpert MTB/RIF Ultra Assay for Pulmonary Tuberculosis and Rifampicin Resistance in Adults and Adolescents". Cochrane Database of Systematic Reviews. doi:10.1002/14651858.CD009593.pub5.