Last Updated: May 23, 2017
Diagnosis of tuberculosis is based on a combination of clinical features, smear examination of sputum where available, tuberculin skin testing or Mantoux test, chest X-ray and Histopathologic / cytologic examination as appropriate. Common clinical features are weight loss, persistent fever and /or cough of more than 2 weeks duration and history of close contact with another Tuberculosis patient.
Mantoux test also known as Tuberculin skin test is used for the detection of infection by Mycobacterium Tuberculosis bacteria. The test consists of slowly injecting a liquid intradermally (superficially within the layers of the skin) on the forearm, leading to formation of a raised vesicle which should be easily visible to the naked eye. This liquid, used in India, is Tuberculin PPD RT 23 strain. Tuberculin is a glycerol extract of the Tuberculosis bacteria. This is one of the two WHO accepted standard tuberculins, apart from PPD-S. In India, where the prevalence of tuberculosis is high, the recommended dose of this liquid is 0.1 ml of 1 TU (TU stands for tuberculin units) .1 TU is the strength of the liquid mentioned on the vial. This liquid is usually administered by a very tiny syringe, usually of 26 gauge. (Needle diameter).
Please click on the link to watch a video demonstrating how this procedure is performed.
Test principle : The individual who has been infected with Tubercle bacilli responds with a hypersensitivity reaction at the test site in the form of induration. Induration is the palpable raised hardening of the skin, while erythema refers to redness of skin. Only induration is clinically significant and measured after 48 to 72hours following injection. Diameter of the Induration is measured by a scale in millimetres perpendicular to the long axis of the forearm.
Interpretation of the tuberculin test
- Size of induration 15 mm and above : - signifies infection with tubercle bacilli
- Size of induration 10-14 mm : - could be due to infection with tubercle bacilli, BCG induced sensitivity, or due to cross reaction with other environmental mycobacteria.
It is more likely to be attributable to infection with tubercle bacilli in case of history of contact with smear positive case of pulmonary TB, clinically confirmed TB OR X-ray consistent with active TB.
- Size of induration 5 -9 mm: usually non tuberculous in nature generally
- Size of induration <5 mm : indicates absence of any type of mycobacterial infection.
Precautions during this procedure :
- If a raised vesicle does not appear it means that the liquid has been injected too deeply, and the test should be repeated on the other arm. If the same arm is used the injection site should be should be separated at least 5 cm from the first injection site.
- Occasional patients may experience severe allergic reaction to the tuberculin PPD
- Larger the size of the induration, higher the probability of presence of infection with tubercle bacilli.
- Almost all reactions with induration more than 15 mm maybe considered attributable to infection with tubercle bacilli.
- Formation of necrosis (ulcer) at the test site, indicates high degree of tuberculin sensitivity and suggests presence of infection with tubercle bacteria.
- Tuberculin reaction may be suppressed (falsely low) in presence of immunosuppressive states. Like HIV, malnutrition, patient on steroid therapy etc.
- Mantoux test is generally not recommended for infants less than 12 weeks old. If the test turns out to be positive, then it is significant, but if the test is negative, then the test needs to be repeated again.
- The tuberculin test detects only the presence or absence of tuberculosis infection. The presence of infection is not synonymous with disease. It simply means, that even if you have tuberculosis infection in your body, as detected by positive Mantoux test, it does not mean you have active Tuberculosis in your body and require anti tuberculosis drugs in all cases. The clinician will correlate your mantoux test result with other tests ( as mentioned at the top of the blog) to give you a final picture.
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