Novel Approaches to TB Diagnosis  
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Current Diagnostic Techniques    

A range of techniques are currently available for diagnosis of M. Tuberculosis infection and differentiation between active and latent infection. The Tuberculin Skin Test is the most widely used for broad identification of infection, while chest xrays and bacteriological cultures are employed when active infection is suspected.

 
Tuberculin Skin Test
   

The Tuberculin Skin Test (TST) involves the injection of tuberculin or PPD (purified protein derivative), a mixture of purified proteins from the M. Tuberculosis bacterium, under the skin. The test is designed to measure delayed-type hypersensitivity reactions. In infected individuals, the body will recognize the antigens in the Tuberculin serum and cause a local reaction at the site of injection. The degree of reaction (size of the induration created) is read 42 to 78 hours after injection. False positive results may occur with recent BCG vaccination (within the past five years). The test will not detect infection in individuals with recently infected or in those with compromised immune systems, such as HIV positive, those on certain immunosuppressive drugs (e.g. beta blockers used in treatment for xxxxx), or . The most widely used form of the test is the Mantoux Skin Test, which consists of an intradermal injection on the underside of the forearm. The earlier forms of the TST, the Time Test, involving multiple small punctures on the underside of the forearm, may still be used in some areas.

 

Induration created by the Mantoux Skin Test. The size of a positive test result depends on the exposure history and health status of the individual and a measure of the actual induration rather than the erythema produced:

Patient Status
Positive Result
HIV +
>5mm
Healthy individuals with exposure history or risk factors
>10mm
Healthy individuals with no exposure history
>15mm
 
Chest Radiograph
   

Chest radiographs can be used to detect active TB infection in the lungs, the most common site of infection. Active pulmonary infection will produce lesions in any part of the lungs. While chest radiographs cannot actually be used as positive diagnosis, Chest radiographs are used typically to rule out active TB infection in cases of positive TST. Abnormalities are typically noted in the apical or posterior portions of the upper lobe or the superior portions of the lower lobe.

 

Chest Radiograph showing consolidation in superior portion of upper right lobe, a typical site of Tuberculosis associated pulmonary abnormalities.

 
Bacteriological Smears and Cultures
   

A sputum smear (or other sample) can be tested for the presence of Acid-Fast-Bacilli, and a negative result will rule out TB infection. The most definative diagnosis will come from culture of lung secretions or other samples to detect the presence of M. tuberculosis.

 

Positive Acid-Fast Stain of Tuberculosis.

M. Tuberculosis culture.

 

Clinical Observation

   

Clinical signs of active infection may be used make a diagnosis, and include fatigue, fever, unexplained weight loss, night sweats. Symptoms of pulmonary TB include productive cough longer than 3 weeks, hemoptysis, and chest pain. Extrapulmonary TB will have a range ofsigns and symptoms depending on the site of primary infection.

   
   


 Images from:

CDC at the Saskatchewan Lung Association: http://www.lung.ca/tb/tbtoday/tbdiagnosis/further_test.html

TILS Online: www.bioon.com/TILS/ Highlight/200504/99811.html