Get montaux test done Send TB gold quantiferon with image
Reading File
Finding Sources
Finding Sources
Reading File
Reading File
Finding Sources
Reading File
Finding Sources
Searching the Web
QuantiFERON TB Gold test procedure image diagram how it works
Finding Sources
Searching the Web
QuantiFERON TB Gold Plus test tubes image result interpretation
Reading File
Reading File
Finding Sources
Generating Image
QuantiFERON TB Gold Plus test: medical diagram showing the 4 blood collection tubes (Nil/negative control, TB1 antigen tube, TB2 antigen tube, Mitogen/positive control), the overnight incubation step at 37°C, ELISA measurement of IFN-gamma, and result interpretation table (Positive/Negative/Indeterminate). Clean educational medical illustration with labels and arrows showing the workflow from blood draw to result.
Here is a complete, textbook-sourced overview of both tests:
Mantoux Test (Tuberculin Skin Test / TST)
Principle
The Mantoux test detects delayed-type hypersensitivity (Type IV) to Mycobacterium tuberculosis antigens. It uses purified protein derivative (PPD) injected intradermally, and the immune system's memory T-cells mount a local reaction if previously sensitized.
Procedure
- Inject 0.1 mL of PPD intradermally into the volar (inner) forearm using a 27-gauge needle at 5-15 degrees
- Read the result at 48-72 hours (results unreliable after 72 hours)
- Measure only induration (raised hardness) - NOT erythema or redness
- Sensitivity develops 2-10 weeks after infection
Interpretation (TABLE 67-2 - Tintinalli's Emergency Medicine)
| Induration Size | Positive In |
|---|---|
| ≥5 mm | HIV patients; close contacts of active TB; abnormal CXR (healed TB); immunosuppressed (prednisone >15 mg/day >1 month); organ transplant recipients |
| ≥10 mm | Injection drug users; high-prevalence groups (immigrants, long-term care residents); silicosis, diabetes, carcinoma of head/neck/lung; children <4 years |
| ≥15 mm | All others (low-risk individuals) |
Screening Conversions (New Infections)
- ≥10 mm increase within 2 years = positive if age <35 years
- ≥15 mm increase within 2 years = positive if age >35 years
Sensitivity by Induration Size
| Cut-off | Sensitivity |
|---|---|
| 5 mm | 98% |
| 10 mm | 90% |
| 15 mm | 50-60% |
False Positives
- Prior BCG vaccination (most common confounder)
- Exposure to non-tuberculous mycobacteria (NTM)
False Negatives
- Improper administration or reading
- Very early disease (window period)
- Recent MMR vaccination
- Profound immunosuppression (HIV - 20% of active TB patients test negative)
- Anergy states
Key note: A negative TST does NOT rule out TB. 4% of close contacts of infected patients test positive with no symptoms, and half have detectable disease within 1 month of exposure.
QuantiFERON-TB Gold (IGRA - Interferon-Gamma Release Assay)
Principle
QuantiFERON-TB Gold is an in vitro blood test measuring cell-mediated immunity (CMI). It measures IFN-γ released by sensitized memory T-cells when stimulated by M. tuberculosis-specific antigens:
- ESAT-6 (Early Secreted Antigenic Target 6)
- CFP-10 (Culture Filtrate Protein 10)
These antigens are absent from all BCG strains, which is why QFT-Gold has higher specificity than TST, especially in BCG-vaccinated populations.
Available Platforms (FDA-Approved)
| Assay | Tubes | CD4/CD8 |
|---|---|---|
| QFT-Gold (original) | ESAT-6, CFP-10 | CD4 only |
| QFT-GIT (Gold In-Tube) | ESAT-6, CFP-10, TB7.7 | CD4 only |
| QFT-Plus | TB1 (ESAT-6 + CFP-10) + TB2 (shorter peptides) | CD4 + CD8 |
QFT-Plus Tubes (4 Tubes)
- Nil tube - Negative control (no antigen, no stimulator) - measures background IFN-γ
- TB1 tube - ESAT-6 + CFP-10 antigens → stimulates CD4+ T cells
- TB2 tube - Shorter peptides of ESAT-6 + CFP-10 → stimulates CD4+ and CD8+ T cells
- Mitogen tube - Positive control (non-specific T-cell stimulator - confirms blood is viable)
Procedure
- Collect blood directly into the 4 antigen-coated tubes
- Incubate overnight at 37°C (within 16 hours of collection)
- ELISA measures IFN-γ concentration in each tube
- Software calculates result by subtracting background (Nil tube)
Result Interpretation
| Result | QFT Criteria |
|---|---|
| Positive | IFN-γ ≥ 0.35 IU/mL above Nil (in TB1 or TB2 for QFT-Plus) |
| Negative | IFN-γ < 0.35 IU/mL AND Mitogen response adequate |
| Indeterminate | Nil too high (>8 IU/mL) OR Mitogen response too low (<0.5 IU/mL) |
QFT-Plus is positive if EITHER TB1 or TB2 tube is positive.
Key Advantages Over TST
| Feature | TST (Mantoux) | QFT-Gold |
|---|---|---|
| Patient visits | 2 (placement + reading) | 1 |
| Result time | 48-72 hours | 24 hours |
| BCG interference | YES - false positives | NO |
| NTM interference | YES | Minimal |
| Sensitivity | ~70-90% | ~90-100% |
| Specificity | Lower | Higher |
Limitations of QFT
- Requires laboratory infrastructure and technical expertise
- Less reproducible on serial testing than TST (especially near the 0.35 IU/mL threshold - "borderline positives" may convert/revert spontaneously)
- Higher cost than TST
- Cannot distinguish active TB from latent TB (neither can TST)
- A negative result does not exclude TB - send cultures if clinical suspicion is high
CDC Guidelines: When to Prefer Which Test
(Murray & Nadel's Textbook of Respiratory Medicine)
| Situation | Preferred Test |
|---|---|
| BCG-vaccinated individuals | IGRA preferred |
| Groups with low return rates (homeless, drug users) | IGRA preferred |
| Children <5 years | TST preferred |
| All other situations | Either acceptable |
| Dual testing (IGRA + TST) | When initial test negative but high suspicion; OR to encourage treatment adherence in BCG+ with positive TST |
Sources:
- Tintinalli's Emergency Medicine, Block 6, p. 494 (Mantoux/TST)
- Rosen's Emergency Medicine, Block 24, p. 2713 (IGRA/QuantiFERON)
- Murray & Nadel's Textbook of Respiratory Medicine, Block 9, p. 933 (IGRA Platforms & Interpretation)
- Fishman's Pulmonary Diseases and Disorders, Block 26 (IGRA vs TST)
This is a shared conversation. Sign in to Orris to start your own chat.