TUBERCULOSIS: DIAGNOSIS

Tuberculosis is diagnosed based on clinical symptoms, sputum smear microscopy (SSM), chest x-rays, culture and tuberculin skin test (TST). The clinical symptoms of active pulmonary Tuberculosis disease are cough for three or more weeks, spitting up blood in the sputum, weight loss, fever or night sweats for three or more weeks, fatigue, feeling unwell, and loss of appetite, chest pain or discomfort, shortness of breath or difficulty in breathing.

 

Sputum smear microscopy (SSM): The gold standard diagnosis for tuberculosis is sputum smear microscopy (SSM). It is a simple technology that allows visual identification of the tuberculosis bacilli. It is very specific and most reliable way of identifying the presence of tuberculosis bacilli. Sputum smear microscopy is used to detect acid-fast bacilli (AFB) in the sputum. The appearance of AFB in the sputum (smear positive) indicates transmittable pulmonary disease. Currently, sputum smear microscopy is the only diagnostic tool that is affordable for low-income countries.

Chest radiography has been used usually in the past to diagnose tuberculosis; it is however much less specific when compared to sputum smear microscopy.

Culture is a tool for tuberculosis diagnosis but is not used as a part of routine tests. Culture is a more sensitive laboratory technique and is usually done when the symptoms are strongly suggestive of tuberculosis and sputum smear microscopy/SSM is negative. (Example: Culture is recommended in cases of tuberculosis in children and HIV/AIDS patients, because in these cases the number of tuberculosis bacilli in the sputum is often reduced and sputum smears are less likely to be found positive.)  Growth on solid culture media takes three to four weeks and requires special laboratories while growth in liquid culture is much faster. The average time for detection is 10 to 14 days. This system is known as Mycobacteria Growth Indicator Tube (MGIT)-tuberculosis culture and can be used to diagnose multidrug resistant tuberculosis (MGIT-DST).

MODS (Microscopic Observation Drug Susceptibility) are an alternative liquid culture system for case detection and drug susceptibility testing which is still under research.

Tuberculin skin test (TST): A small amount of fluid – tuberculin – is injected in the skin in the lower part of the arm. A positive test is measured by the size of the indurations (hardness, bump, and swelling) at the injection site two to three days post-injection. In developing countries this test is not used as a diagnostic test. In the presence of prior history of BCG vaccination the (TST) test may be positive. Persons with a positive skin test reaction are in high-risk groups and are prescribed drug (isoniazid) daily, for nine months. Isoniazid kills tuberculosis bacteria that are inactive in the body and prevents the individual from developing active tuberculosis from latent infection.