Fellow's quick read · Infectious Diseases
Tuberculosis diagnostics
ATS / IDSA / CDC 2017 (Lewinsohn, Clin Infect Dis 2017;64:e1–e33) · reviewed 2026-07-09
Personal study digest for a new ID fellow. Recommendations are from the 2017 ATS/IDSA/CDC diagnosis guideline (23 recommendations — 6 strong, 17 conditional; GRADE). It targets low-incidence, high-resource settings like the US. The "What's changed since 2017" section is reviewer synthesis of newer evidence — mostly WHO diagnostic policy the US guideline predates — each claim cited. Not a substitute for the full guideline.
In one line
Two separate questions: test for infection (LTBI) — IGRA vs TST, and test for disease — smear + culture + NAAT, then rapid molecular resistance testing. Two rules a fellow never forgets: neither IGRA nor TST distinguishes active from latent, and no negative test (smear or NAAT) excludes TB — culture is still the gold standard.◆Testing for infection (LTBI): which test?
- Prefer an IGRA over TST in anyone ≥5 y who is likely infected and at low/intermediate risk of progression. This is a STRONG recommendation when the patient is BCG-vaccinated or unlikely to return to have a TST read; conditional otherwise. A TST is an acceptable alternative when an IGRA is unavailable, too costly, or too burdensome.
- High risk of progression (≥5 y): no preference — TST or IGRA is acceptable as the first-line test.
- Children <5 y: the guideline prefers a TST over IGRA (conditional, very low). Some experts use IGRAs over age 3.
- A positive IGRA/TST proves infection, not disease. Before treating LTBI, exclude active TB — screen for symptoms and get a chest radiograph; sample if the film suggests active disease.
- Do not test people at low risk of infection and progression. If testing is nonetheless obliged (law/credentialing): use an IGRA, and confirm a positive with a second test — call it infected only if both are positive.
◆Testing for pulmonary TB disease
- AFB smear microscopy on every patient with suspected pulmonary TB (STRONG). 3 specimens is US normative practice; request ≥3 mL sputum (optimal 5–10 mL); concentrated specimens + fluorescence microscopy preferred. A negative smear does not exclude TB and a positive smear does not confirm it.
- Culture is the gold standard — perform both liquid and solid culture on every specimen (liquid at minimum). Culture also yields the isolate for phenotypic drug-susceptibility testing.
- NAAT on the initial respiratory specimen (conditional). Smear-positive + negative NAAT → TB unlikely. Smear-negative + intermediate/high suspicion + positive NAAT → presumptive TB; a negative NAAT can never exclude it. (Named assays: Hologic Amplified MTD, Cepheid Xpert MTB/RIF.)
- Can't expectorate, or smear-negative → sputum induction before bronchoscopy; collect a post-bronchoscopy sputum in anyone who is scoped.
- Children: obtain mycobacterial culture of respiratory specimens in all suspected pulmonary TB.
◆The rapid-resistance rule (a strong rec worth memorizing)
- Perform rapid molecular DST for rifampin (± isoniazid) on the respiratory specimen of anyone who is AFB smear-positive OR Hologic MTD-positive AND meets ≥1 of: (1) prior TB treatment; (2) born in or lived ≥1 y in a country with ≥moderate TB incidence (≥20 / 100 000) or high primary MDR prevalence (≥2%); (3) contact of MDR-TB; (4) HIV-infected (STRONG). Don't wait weeks for phenotypic DST in these patients.
◆Extrapulmonary TB
- Mycobacterial culture on all extrapulmonary specimens (STRONG). Also send AFB smear, NAAT, and histology. For any of these, a positive result rules in (false-positives are unlikely) but a negative result never excludes extrapulmonary TB. NAAT on non-sputum specimens was an off-label use in 2017.
- Fluid work-up: cell counts + chemistries on amenable fluids (pleural, CSF, ascitic, joint); adenosine deaminase (ADA) on suspected pleural, TB-meningitis, peritoneal, or pericardial fluid; free IFN-γ on suspected pleural or peritoneal fluid.
- Genotyping: submit one culture isolate from every culture-positive patient to a regional genotyping lab (STRONG).
◆Which test tells you what
| Test | Best use | Caveat a fellow must hold |
|---|---|---|
| IGRA / TST | Infection (LTBI) | Cannot distinguish active from latent; exclude disease before LTBI therapy |
| AFB smear | Fast, cheap, infection-control triage | Low sensitivity; neither rules TB in nor out |
| NAAT | Rapid rule-in of TB | A negative NAAT never excludes TB |
| Culture (liquid + solid) | Gold standard + phenotypic DST | Slow (weeks); still the reference |
| Rapid molecular DST | Rifampin ± INH resistance, in days | Needs a smear-positive / MTD-positive specimen; trigger it on the risk criteria |
◆Key decisions a fellow owns
- Exclude active TB before every LTBI treatment — a positive IGRA/TST alone is never enough.
- A negative smear or NAAT does not stop the work-up — culture, and treat empirically when clinical suspicion is high.
- Fire off rapid molecular DST early whenever any MDR risk factor is present — it changes the empiric regimen while culture DST is pending.
- Get adequate specimens — three sputa, ≥3 mL each; induce before you scope.
What's changed since 2017
Reviewer synthesis of newer evidence — not the guideline. Most items are WHO diagnostic policy that the US-focused 2017 document predates; each claim cited.
- Still the operative US diagnosis guideline — no newer ATS/IDSA/CDC TB-diagnosis guideline since 2017. The US document lags WHO, which has re-tooled TB diagnostics repeatedly since.
- Xpert MTB/RIF Ultra replaced Xpert MTB/RIF — higher sensitivity (90.9% vs 84.7% vs culture), with the biggest gains where the old assay failed: smear-negative culture-positive (77.5% vs 60.6%) and PLHIV (87.6% vs 74.9%). The trade is lower specificity from "trace" calls (3–30% of results), especially with prior TB. Zifodya, Cochrane Database Syst Rev 2021;2:CD009593 (PMID 33616229; doi:10.1002/14651858.CD009593.pub5)
- Xpert Ultra is now the initial CSF test for TB meningitis — WHO-recommended over smear/culture; Ultra 70% sensitivity for probable/definite TBM vs 43% Xpert and 43% culture. A negative CSF result does not exclude TBM — treat empirically. Bahr, Lancet Infect Dis 2018;18:68–75 (PMID 28919338; doi:10.1016/S1473-3099(17)30474-7)
- Xpert MTB/XDR — rapid molecular second-line DST — one cartridge for isoniazid, fluoroquinolones, ethionamide, and injectables, extending the guideline's rifampin ± INH rule. Sensitivity 94% INH / 94% FQ (lower for injectables); specificity 98–100%; WHO-endorsed 2021. Penn-Nicholson, Lancet Infect Dis 2022;22:242–249 (PMID 34627496; doi:10.1016/S1473-3099(21)00452-7)
- Targeted next-generation sequencing (tNGS) for drug resistance — WHO-recommended (rapid communication Jul 2023; guidance 2024): a sequencing panel resolves resistance to multiple drugs in 3–5 days vs 4–6 weeks for culture DST. WHO Consolidated guidelines on TB, module 3 (2024)
- Urine LF-LAM (AlereLAM) for advanced HIV — a rare rule-in test the 2017 US guideline barely addresses; WHO 2019 recommends it in HIV-positive inpatients who are seriously ill, have advanced HIV, or CD4 <200, and outpatients with CD4 <100 or seriously ill. Low sensitivity but fast and point-of-care where sputum is hard to get. WHO LF-LAM policy update, Nov 2019
- AI chest-radiograph reading (CAD) — WHO 2021 accepts computer-aided detection as an alternative to a human reader for TB screening/triage in people ≥15 y (pooled sensitivity 0.87, specificity 0.74; CAD4TB, Lunit INSIGHT CXR, qXR). WHO CAD policy statement, 2021
- A new class between TST and IGRA — M. tuberculosis antigen-based skin tests (TBST: Cy-Tb, C-TST, Diaskintest) — WHO 2022 (conditional). Intradermal like a TST but ESAT-6/CFP-10–specific like an IGRA; not FDA-cleared / not in US practice.
- The US IGRA is now 4th-generation — QuantiFERON-TB Gold Plus (QFT-Plus, adds a TB2/CD8 tube) was FDA-approved June 2017 and replaced QFT-GIT; the guideline's "IGRA over TST" logic is unchanged.
- Recency-sensitive: FujiLAM (higher-sensitivity urine LAM) is not yet WHO-recommended — lot-to-lot variability is the barrier. Confirm current status. Lancet Glob Health 2022;10:e1600 (FujiLAM accuracy)
◆Anki cards minted this run
- Xpert Ultra vs Xpert MTB/RIF — higher sensitivity in smear-negative & PLHIV, at a specificity cost (trace).
- Xpert Ultra as the initial CSF test for TB meningitis — a negative result does not exclude TBM.
- Xpert MTB/XDR — rapid molecular second-line DST (INH, fluoroquinolones, injectables).
- Urine LF-LAM (AlereLAM) — WHO thresholds for advanced-HIV TB diagnosis (inpatient CD4 <200 / outpatient CD4 <100).
Sources: Guideline — Lewinsohn DM et al. Official ATS/IDSA/CDC Clinical Practice Guidelines: Diagnosis of Tuberculosis in Adults and Children. Clin Infect Dis 2017;64(2):e1–e33 (doi:10.1093/cid/ciw694). Currency — Zifodya, Cochrane 2021, PMID 33616229 (doi:10.1002/14651858.CD009593.pub5); Bahr, Lancet Infect Dis 2018;18:68–75, PMID 28919338 (doi:10.1016/S1473-3099(17)30474-7); Penn-Nicholson, Lancet Infect Dis 2022;22:242–249, PMID 34627496 (doi:10.1016/S1473-3099(21)00452-7); WHO Consolidated guidelines on tuberculosis (module 3, diagnosis; tNGS 2024; CAD 2021; TBST 2022; LF-LAM 2019); QFT-Plus FDA approval (Jun 2017); FujiLAM, Lancet Glob Health 2022;10:e1600. Literature via PubMed.