What is injected in tuberculin test
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Here is a complete answer based on authoritative medical textbooks:
Tuberculin Test - What is Injected?
The Substance: Purified Protein Derivative (PPD)
PPD (Purified Protein Derivative) of tuberculin is injected in the tuberculin (Mantoux) test. It consists of heat-stable proteins liberated from Mycobacterium tuberculosis into liquid culture media.
Two tuberculins are accepted as standard by the WHO:
- PPD-S (used internationally)
- PPD-RT 23 (used in India, with Tween 80 added as a detergent to prevent adsorption on glass/plastic surfaces)
1 TU of PPD-RT 23 is equivalent to 5 TU of PPD-S.
The Injection (Mantoux Method)
| Parameter | Detail |
|---|---|
| What | PPD tuberculin |
| Dose (India) | 1 TU of PPD-RT 23 in 0.1 mL |
| Dose (International) | 5 TU of PPD-S in 0.1 mL |
| Route | Intradermal |
| Site | Flexor (volar) surface of the left forearm, midway between elbow and wrist |
| Needle angle | 5-15 degrees, bevel facing upward |
| Expected result | A pale wheal 6-10 mm in diameter at the injection site |
How it Works (Mechanism)
PPD triggers a Type IV (delayed-type) hypersensitivity (DTH) reaction mediated by T-cells. In individuals previously sensitized to M. tuberculosis (through infection or BCG vaccination), CD4+ T-cells recognize the PPD antigens and release cytokines, causing local induration at the site within 48-72 hours.
Reading the Result
The result is read at 72 hours (48-96 hours acceptable) by measuring the transverse diameter of induration (not erythema) in mm.
| Induration | Interpretation (general population) |
|---|---|
| < 6 mm | Negative |
| 6-9 mm | Doubtful (may be due to atypical mycobacteria) |
| ≥ 10 mm | Positive |
| ≥ 5 mm | Positive in immunocompromised (HIV, transplant, steroids) |
Important Limitations
- False negatives: immunocompromise, malnutrition, chronic renal failure, improper technique, up to 25% of active TB cases test negative
- False positives: prior BCG vaccination, exposure to environmental non-tuberculous mycobacteria (NTM) - because PPD is a crude extract containing antigens shared across mycobacterial species
- An alternative modern test is the IGRA (Interferon-gamma Release Assay) - QuantiFERON-TB Gold or T-SPOT.TB - which uses M. tuberculosis-specific synthetic peptides and is unaffected by BCG vaccination
Sources: Park's Textbook of Preventive and Social Medicine; Fishman's Pulmonary Diseases and Disorders; Sherris & Ryan's Medical Microbiology, 8th Ed.; Harriet Lane Handbook, 23rd Ed.
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