Diagnosis of Tuberculosis

Diagnosis - Clinical - Skin Test - Tests

Those that reacted positive to the skin test will require further testing to verify the existence of tuberculosis. These further tests include a chest x-ray and the collection of a sputum sample for laboratory testing.

Chest x-ray

A chest x-ray is used to verify the existence of tuberculosis. Mass x-ray surveys were conducted in the past, but they are no longer considered useful as the yield for TB is low (sample x-rays).

Laboratory Sample

To complete a sputum test, the collection of 3 early morning specimens on three successive days is required. The sputum is obtained by having the patient cough deeply, so that a specimen can be obtained from deep inside the lungs—saliva is not acceptable.

Other method that could be used for further testing could be the collection of specimens from lung tissue, Gastric Washings, Bronchial Washings, Pleural Fluid, Lymph node tissue, bone marrow, Cerebrospinal fluid.

A sputum smear stained with auramine.

Once the specimen has been collected, it is sent to the laboratory, where bacteria are cultured from the sample, and a lab technician checks for the presence of the tubercle bacillus. A microscope smear is made from the specimen and stained with an acid-fast stain called auramine. The auramine stain enters the wall of the M. tuberculosis bacteria cells and makes them glow under a fluorescent light. If under a microscope, the rice-like bacilli are not seen, no tuberculosis is present in the sample.

If, however, lab technicians find the glowing white, rice-like, bacteria in the smear, the patient is given the title "Smear Positive", indicating that they have tuberculosis.

A different "acid-fast" stain can be used to look at TB bacteria. Note the small, purple, rod-shaped bacilli (click to enlarge).