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Pulmonary & Critical Care Bulletin
Vol. VII, No. 3, July 15, 2001
In this issue :
From Editor's Desk
BCG TEST AND PEDIATRIC TUBERCULOSIS --- A Critical Evaluation
For diagnosis of tuberculosis in children, tuberculin test ( despite its limitations) is widely used all over the world. During the last few decades, accelerated BCG reaction (BCG Test) has been suggested as a diagnostic test for tuberculosis. In India, the credit for introducting this test goes to Udani, who in 1971 showed its validity in clinically suspected cases of tuberculosis, which were later proven to be so on autopsies. Udani reviewed the literature and concluded that BCG test was positive in 87.9% cases compared to 52.3% positive on ITU (PPD) reported in literature. Before accepting this higher sensitivity, we must take into consideration, a number of factors listed below:
1. Udani and other workers have demonstrated that some children, who do not react to 1 or 2 TU PPD with Tween 80, will react to the standard dose of BCG Vaccine. The reason for this appear to be that the BCG vaccine contains more tuberculin than 2 TU so that it will show a response in persons with weaker sensitivity. Indeed Choudhury et al (1974) estimated that BCG was equal antigenically to between 5 and 10 TU with Tween 80 (i.e. 25-50 TU without Tween 80). The response thus appears to be due to a larger test dose.
2. Because of the fundamental batch to batch variations in the number of organisms in the BCG vaccine, standardised material for elicitation of allergy is not available. BCG vaccine, because of the presence therein of live organisms, dead organisms, their debris (all these making the product impure) and metabolic products of the growth of the organism, is in reality a much stronger tuberculin and therefore gives much bigger reactions. This will in fact be a non-specific reaction to a variable high dose of unstandardised antigen rather than specific reaction for identification of tuberculous infection. Some persons, with vague unclassified symptoms when tested with BCG vaccine, show a big reaction due to the high strength of tuberculin contained in BCG vaccine and are wrongly labelled as tuberculous.
3. Udani and some other workers advocate the use of BCG in all children on the ground that if they are not infected they will be protected by the single injection. This, indiscriminate large scale application of BCG test in pediatric practice, will invalidale not only later BCG test but also future tuberculin testing, making a reevaluation of once (BCG) tested child almost impossible.
4. A number of workers have reported false positivity rates of 16 - 25% with BCG test in normal or non-tubercular controls. Due to this factor, BCG test, just like tuberculin test, cannot be interpreted as evidence of active disease. At best it can only serve as an indicator of prior tubercular infection, just like the tuberculin test.
5. Even though BCG test is more often positive in malnourished individuals (as compared to tuberculin test), it has been shown that in severe grades of malnutrition, even BCG test may be falsely negative.
In view of above mentioned factors, limitations and controversies surrounding BCG test, promotion of this test will only add to the already existing confusing state rather than simplifying the situation.
Regarding tuberculin test, care is always required in its interpretation. This aspect has been amply stressed by Edwards: "Like any diagnostic test, the test result must be considered in context. Test material, method of administration and the condition of the person tested are all of consequence to the reaction. Of what value an electro cardiogram would be if the clinical history were unknown, the leads improperly placed, the stylus faulty, or the reader inexperienced or improperly trained?"
1. Udani PM, Parikh UC, Shah PM, Naik PA, BCG test in tuberculosis. Indian Pediatrics, 1971, 8, 243.
Dr. Rajinder Singh Bedi, M.D.
Dr. Rajinder Singh Bedi, M.D.
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