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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