What is injected in tuberculin test
Purified Protein Derivative (PPD) of Mycobacterium tuberculosis is injected in the tuberculin test. It is given as 0.1 mL (5 tuberculin units) intradermally into the volar surface of the forearm (Mantoux method).
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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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