How To Establish The Diagnosis Of Tuberculosis?

As already explained, there are two entities of the disease, infection and clinical manifestations, i.e., when the disease is active. Hence, it is important to establish the activity of the disease, which is important for starting the treatment of tuberculosis. The treatment is a long one and needs a combination of 3-4 drugs in all the cases. The following tests help in establishing the activity of the disease. However, as one can see, all tests have their own limitations, and therefore, one cannot rely on a single test.

1. Sputum examination for tubercle bacilli

It should be carried out at least 3-4 times. Approximately 5,000 to 10,000 tubercle bacilli are required per ml of sputum for the detection of tubercle bacilli in a smear. Hence, repeated examinations of sputum are advised.

2. Sputum culture for tubercle bacilli

Although this test is more sensitive than the direct smear examination, it is more time-consuming, and requires about 68 weeks. Again, as in a direct smear examination, 3-4 cultures of tubercle bacilli may be required to obtain positive results. A well-equipped laboratory and trained staff is required for carrying out this test.

3. Chest X-ray

It should be carried out in all the suspected cases of tuberculosis. One should not rely solely on the chest X-ray report as proof of the activity of the disease. Other tests, including symptoms and signs of the disease, must be taken into account.

4. Tuberculin / Mantoux test It has a limited value. However, a negative tuberculin test may prove to be highly valuable in some of the cases, to exclude the diagnosis of tuberculosis in a particular case.

5. ELISA (enzyme-linked immunosorbent assay) test for tuberculosis

It is one of the serological tests and detects antibodies against tubercle bacilli. It is highly useful to detect hidden cases of tuberculosis, especially extra pulmonary i.e., when the lesion of tuberculosis is outside the lungs. The accuracy of this test is 90-95 per cent, and it does not become positive with the administration of the BCG vaccination.

However, like other tests, this test too has its own limitations. It may be false-positive, if the patient has an infection, with other types of organisms related to the group of tubercle bacilli, or due to some non-specific reaction. Similarly, the test may become false-negative when there are very low levels of circulating antibodies, due to suppression of the immune system of the body.

Hence, in view of the limitations of the test, this test should be interpreted keeping the clinical profile and other investigations in view.

What is the tuberculin skin test?

This test detects the 'infected' cases of tuberculosis by eliciting a hypersensitivity reaction in patients who are already infected with tubercle bacilli. In this test, more precisely the Mantoux test, purified proteins isolated from tubercle bacilli called 'tuberculins' are injected intracutaneously on the upper surface of the forearm. Out of several tuberculin tests, the Mantoux is most commonly used, since by and large its results are highly reliable. This test will have little value in places where there is a high incidence of tuberculosis, i.e., at such places almost every person can be expected to be infected with tubercle bacilli. However, it has value otherwise. Those cases which are tuberculin negative, may not be suffering from tuberculosis, and in such cases, one should think of other pathology in a concerned patient.

In about 48-72 hours, following the injection of the test, an allergic reaction occurs as a raised, hard red spot at the site of injection. The size of this area should be carefully measured, and an area of 10 mm, roughly about half an inch or more in diameter, indicates that the reaction is positive. Only the in duration (raised area) should be measured with regard to its width. The redness around the raised area must be avoided while taking measurements. Ideally, a trained and experienced technician, especially one who specifically deals in carrying out tuberculin tests, is required. An in duration of less than 7 mm is considered negative, while in duration between 7-10 mm, may be considered a borderline or a doubtful case of tuberculin positive, and such cases need further tests, for exclusion, or for establishing a diagnosis of tuberculosis.


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