Infectious Tuberculosis Tests (Infectious Disease)

Tuberculosis (TB) testing checks whether you have been infected with Mycobacterium tuberculosis and helps determine if you have latent infection (no symptoms, not contagious) or active TB disease (symptomatic and potentially contagious). Because cough, fever, weight loss, and night sweats can come from many causes, lab testing is essential for accurate answers and next steps.

A proactive plan usually begins with a TB blood test (IGRA)—QuantiFERON-TB Gold Plus or T-SPOT.TB—to detect TB infection. If the result is positive, your clinician evaluates for active disease using chest imaging and microbiologic tests on sputum: acid-fast bacilli (AFB) smear, nucleic acid amplification test (NAAT/PCR), and culture with drug-susceptibility. If the IGRA is negative after a recent exposure, repeat testing 8–10 weeks after the last contact. These tests support screening, diagnosis, and monitoring, but do not replace a clinician’s evaluation or urgent care for severe symptoms.

Signs, Symptoms & Related Situations

• Common symptoms of active TB: cough >3 weeks, chest pain, coughing blood, fever, night sweats, fatigue, loss of appetite, weight loss.

• Who should test: close contacts of a TB case; healthcare and congregate-setting workers; people starting immunosuppressive therapy (e.g., TNF-α inhibitors); persons born in or who frequently travel to higher-incidence regions; residents of shelters, prisons, or long-term care.

• BCG-vaccinated: prefer IGRA (unaffected by BCG).

• When to seek urgent care: severe trouble breathing, coughing blood, confusion, or signs of dehydration/weakness.

Symptoms and exposures need review by a qualified clinician.

Why These Tests Matter

What testing can do

• Detect TB infection (latent or active) with IGRA blood tests.

• Confirm or rule in active TB disease with sputum AFB smear, NAAT/PCR, and culture with susceptibility.

• Provide documentation for work, school, or treatment clearance and guide public-health steps.

What testing cannot do

• An IGRA alone cannot diagnose active TB or determine contagiousness.

• No single test rules out disease immediately after exposure—window periods exist; retesting may be needed.

• Replace a clinician’s exam, chest imaging, or public-health guidance.

What These Tests Measure (at a glance)

• IGRAs (Interferon-Gamma Release Assays):

  • QuantiFERON-TB Gold Plus: measures interferon-gamma after TB-specific antigen stimulation; reports Positive/Negative/Indeterminate with internal controls (Nil, Mitogen).

  • T-SPOT.TB: counts IFN-γ-producing T-cells (“spots”); reports Positive/Negative/Borderline.
    Why use them? One-visit blood tests, unaffected by BCG, preferred for adults.

• Tuberculin Skin Test (TST/PPD): intradermal injection read at 48–72 hours; interpretation (5/10/15 mm) depends on risk factors; can be affected by BCG.

• Sputum AFB Smear: microscopy for acid-fast organisms; quick but less sensitive; smear-positive cases are often contagious.

• TB NAAT/PCR: detects M. tuberculosis complex DNA rapidly; supports early diagnosis and identifies rifampin resistance in some methods.

• Mycobacterial Culture & Drug-Susceptibility: gold standard to confirm disease and define resistance to key drugs (e.g., isoniazid, rifampin).

• Chest Imaging (clinical): X-ray/CT supports diagnosis and assesses extent; not a lab test but part of the work-up.

Quick Build Guide

How the Testing Process Works

1. Choose your test: most adults begin with QuantiFERON-TB Gold Plus; consider T-SPOT.TB if you’ve had indeterminate results or have significant immunosuppression.

2. Provide the sample: one blood draw for IGRA; no fasting required. If disease is suspected, your clinician will arrange sputum collections.

3. Review results: IGRA results post to your secure account, typically within a few days.

4. If IGRA is positive: your clinician evaluates for active disease with chest imaging and sputum AFB smear/NAAT/culture; isolation may be advised while assessing contagiousness.

5. If IGRA is negative after recent exposure: retest in 8–10 weeks post-exposure to account for the window period.

Interpreting Results (General Guidance)

• IGRA Positive: indicates TB infection (latent or active). Requires clinical evaluation; active disease is confirmed with imaging + sputum AFB/NAAT/culture.

• IGRA Negative: infection unlikely, but early testing, immune suppression, or specimen issues can cause false negatives—repeat after the window period if risk is high.

• Indeterminate/Borderline IGRA: often due to low immune response or handling issues—repeat the test or use the alternative IGRA.

• Sputum AFB/NAAT/Culture: a positive NAAT or culture confirms active TB disease; culture provides drug-susceptibility to guide therapy.
  Always interpret results with a qualified healthcare professional and follow local public-health guidance.

Choosing Panels vs. Individual Tests

• Screening (most adults): order a single IGRA (QuantiFERON-TB Gold Plus or T-SPOT.TB).

• Diagnostic work-up for symptoms: pair IGRA with chest imaging and sputum AFB/NAAT/culture (arranged clinically).

• Post-exposure protocol: baseline IGRA now and repeat at 8–10 weeks if initially negative.

FAQs

Do I need to fast for TB testing?
No. IGRAs do not require fasting.

Which is better—QuantiFERON or T-SPOT?
Both are guideline-accepted. Many programs use QuantiFERON; T-SPOT may help if you’ve had indeterminateresults or low lymphocyte counts.

Does the BCG vaccine affect results?
BCG can cause false-positive TST but does not affect IGRAs.

How soon after exposure should I test?
Test now for a baseline and retest in 8–10 weeks after the last exposure if the first test is negative.

If my IGRA is positive, do I have active TB?
Not necessarily. It shows infection. You’ll need a clinician evaluation with imaging and often sputum tests to determine disease and contagiousness.

What does “indeterminate” mean?
Controls did not perform as expected (e.g., low immune response). Your clinician will repeat the test or use the alternative IGRA.

Can these tests monitor treatment?
Sputum smear and culture track response in active disease. IGRA is not recommended for monitoring therapy.

Related Categories & Key Tests

• Infectious Disease Tests Hub

• Immunity & Titer Tests • Employment & Deployment Testing • Respiratory Infection Tests • HIV & Hepatitis Screening

• Key Tests: QuantiFERON-TB Gold Plus (IGRA) • T-SPOT.TB (IGRA) • Tuberculin Skin Test (TST/PPD) • Sputum AFB Smear • TB NAAT/PCR • Mycobacterial Culture & Drug-Susceptibility • Chest X-ray (clinical)

References

• Centers for Disease Control and Prevention — TB testing and IGRA guidance; evaluation of suspected TB disease.
• U.S. Preventive Services Task Force — Screening for Latent TB Infection in Adults.
• ATS/IDSA/CDC — Clinical Practice Guidelines for Diagnosis of Tuberculosis in Adults and Children.
• World Health Organization — Recommendations on screening, diagnosis, and drug-resistant TB testing.
• APHL — Best practices for IGRA specimen handling and mycobacterial testing workflows.
• Clinical microbiology texts — AFB smear, NAAT, and culture interpretation and limitations.

Available Tests & Panels

Your Tuberculosis (TB) Tests menu is pre-populated in the Ulta Lab Tests system. Select QuantiFERON-TB Gold Plus (or T-SPOT.TB) for infection screening. If you have symptoms or a positive screen, your clinician may order chest imaging and sputum AFB/NAAT/culture to confirm disease and guide management. Follow specimen instructions carefully and review results with your clinician to plan next steps.