i need to diagnose a patient that potentially has TB
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tuberculosis diagnosis IGRA TST sputum
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Diagnosing Pulmonary TB: Interpreting TST, IGRA, Sputum & Culture
1. Clinical Suspicion First
Before ordering any test, the clinical picture must support TB. Symptoms that should raise suspicion include:
- Persistent cough (productive of mucus ± blood) — most common symptom; however, 10–25% of bacteriologically confirmed TB cases do not report cough
- Systemic symptoms: fever (35–80% of patients), night sweats, weight loss, malaise
- Hemoptysis — suggests extensive involvement but can also come from inactive TB (bronchiectasis, Rasmussen aneurysm, aspergilloma in old cavity)
- Risk factors: HIV, immunosuppression (TNF-α inhibitors, glucocorticoids, transplant), recent immigration from high-incidence countries, congregate living (prisons, shelters), injection drug use, close contact with a known case
A broadened symptom screen (any of: cough of any duration, fever, night sweats, or weight loss) has a negative predictive value of 97.7% for ruling out TB in HIV-positive persons. — Murray & Nadel's Textbook of Respiratory Medicine
2. Tuberculin Skin Test (TST / Mantoux / PPD)
How it works: 5 TU of PPD injected intradermally; read at 48–72 h by measuring induration diameter in mm (not erythema).
Limitations:
- Low specificity — cross-reacts with BCG vaccination and nontuberculous mycobacteria (NTM)
- False negatives in immunosuppressed patients, overwhelming TB, anergy
- Requires two patient visits
- "Boosting phenomenon" — a repeat TST 1–5 weeks later can spuriously increase reaction size (not true conversion)
TST Positive Cutoff by Risk Group
| Risk Group | Positive Threshold |
|---|---|
| HIV-infected persons | ≥ 5 mm |
| Recent close contacts of infectious case | ≥ 5 mm |
| Organ transplant recipients | ≥ 5 mm |
| Fibrotic lesions on CXR consistent with old TB | ≥ 5 mm |
| Immunosuppressed (glucocorticoids, TNF-α inhibitors) | ≥ 5 mm |
| Recent immigrants (≤5 years) from high-prevalence country | ≥ 10 mm |
| Injection drug users | ≥ 10 mm |
| Healthcare workers, residents of high-risk settings | ≥ 10 mm |
| Children <5 years; children/adolescents exposed to high-risk adults | ≥ 10 mm |
| Low-risk persons (testing not routinely recommended) | ≥ 15 mm |
Source: Table 183-5, Harrison's Principles of Internal Medicine 22E (2025)
BCG and TST: BCG given in infancy has limited effect on TST specificity. BCG given after 1 year of age can cause false-positive TST, but reactivity wanes — a positive TST ≥10 years after BCG should not be attributed to BCG.
3. Interferon-Gamma Release Assays (IGRA)
Two FDA-approved platforms:
| Platform | Assay | Mechanism |
|---|---|---|
| QuantiFERON-TB Gold (QFT-GIT / QFT-Plus) | Whole blood ELISA | Measures IFN-γ (IU/mL) after stimulation with ESAT-6, CFP-10 (± TB7.7 for GIT; ± CD8 peptides for Plus) |
| T-SPOT.TB | ELISPOT | Counts IFN-γ-producing T cells (spots) |
IGRA Interpretation
| Result | QFT (IFN-γ concentration) | T-SPOT.TB (spots above negative control) |
|---|---|---|
| Positive | ≥ 0.35 IU/mL above negative control | ≥ 8 spots |
| Negative | < 0.35 IU/mL | ≤ 4 spots |
| Borderline/Indeterminate | Inappropriate positive or negative control response | 5–7 spots (borderline); invalid = inappropriate controls |
Key advantages of IGRA over TST:
- Not affected by BCG vaccination — specificity ≥93% regardless of BCG status
- Only one patient visit needed; results in ~24 hours
- More specific — uses TB-specific antigens ESAT-6 and CFP-10 (absent in BCG strains and most NTM)
New option: Antigen-based skin tests (TBSTs) using ESAT-6/CFP-10 combine the convenience of TST with IGRA specificity, and WHO considers accuracy similar to IGRA — useful in BCG-vaccinated patients and PLWH.
CDC Guidance: When to Prefer IGRA vs TST
- Prefer IGRA: Prior BCG vaccination; high risk of loss to follow-up
- Prefer TST: Children <2 years
- Either acceptable: Children ≥2 years, most adults
- Dual testing (both TST + IGRA): Consider if initial test is negative but suspicion remains high, or for TST+ BCG-vaccinated individuals where IGRA can clarify whether to treat
⚠️ Critical: Neither TST nor IGRA diagnoses active TB. Both detect TB infection (latent or active). A positive result in a patient with pulmonary symptoms should prompt immediate microbiologic workup.
4. Sputum Testing for Active Pulmonary TB
Collect at least 3 sputum specimens (strongly recommended by CDC/ATS/IDSA). Specimens can be collected same-day (or early morning on consecutive days).
A. AFB Smear Microscopy
- Rapid, inexpensive; detects acid-fast bacilli
- Sensitivity ~45–80%, higher with cavitary disease
- Cannot distinguish M. tuberculosis from NTM
- Positive result = presumptive TB, but confirmation required
B. Nucleic Acid Amplification Testing (NAAT) — Xpert MTB/RIF
- WHO-recommended as initial test in HIV+ patients, MDR-risk patients, and seriously ill patients
- Detects M. tuberculosis complex and rifampin resistance in ~90 minutes
- Interpret with smear result:
| Smear | NAAT | Interpretation |
|---|---|---|
| Positive | Positive | Rapid confirmation of TB |
| Positive | Negative | Likely NTM — NAAT distinguishes Mtb from NTM |
| Negative (intermediate–high pre-test probability) | Positive | Presumptive evidence of TB |
| Negative (low pre-test probability) | Positive | Interpret cautiously — higher false-positive probability |
| Negative | Negative | Active TB not excluded — culture still required |
"Current NAATs do not possess sufficient sensitivity to replace mycobacterial culture." — Murray & Nadel's
C. Mycobacterial Culture (Gold Standard)
- Most sensitive method; required for definitive diagnosis
- Also required for drug susceptibility testing (DST)
- Takes 2–6 weeks on liquid media (MGIT) or up to 8 weeks on solid media
- Recommended even when NAAT is positive (especially if rifampin resistance detected) — needed for phenotypic DST
D. Drug Susceptibility Testing (DST)
- Ordered on all positive cultures
- Molecular rapid DST: detects resistance genes (rpoB for rifampin, katG/inhA for isoniazid, pncA for pyrazinamide, embB for ethambutol, gyrA/gyrB for fluoroquinolones)
- If rifampin resistance found on Xpert → MDR-TB protocol
5. Chest Imaging
Chest X-ray is essential but cannot provide a definitive diagnosis:
- Upper lobe infiltrates with or without cavitation — classic for reactivation/active TB
- Primary TB: parenchymal infiltrate + hilar/mediastinal lymphadenopathy ± pleural effusion ± miliary pattern
- Cavitation, nodules, bronchiectasis can mimic bacterial/fungal pneumonia, malignancy, or NTM
Chest CT: Best modality for full visualization; useful when CXR is inconclusive, to distinguish parenchymal vs pleural disease, and to guide bronchoscopy.
⚠️ Radiographic findings should always be paired with microbiologic evaluation.
6. Summary Algorithm
Clinical suspicion (symptoms + risk factors)
↓
Chest X-ray
↓
Collect ≥3 sputum specimens
→ AFB smear + NAAT (Xpert MTB/RIF) + Culture (all 3 specimens)
↓
Positive smear / NAAT → Presumptive TB → Start treatment
Negative smear / NAAT → Await culture; clinical judgment guides empirical therapy
↓
Culture positive → Definitive TB → DST (phenotypic + molecular)
↓
TST or IGRA: Used for TB infection screening, NOT for diagnosing active disease
Key Pitfalls
- TST/IGRA are positive in latent TB — don't diagnose active disease from them alone
- Smear-negative TB is common in HIV+ patients and early disease — culture is mandatory
- NAAT negative does not exclude TB — sensitivity is insufficient to replace culture
- BCG vaccination causes TST false positives; IGRA is not affected
- Never start treatment for latent infection (LTBI) without definitively excluding active TB first
Sources: Murray & Nadel's Textbook of Respiratory Medicine; Harrison's Principles of Internal Medicine 22E (2025, McGraw Hill); Goldman-Cecil Medicine
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