Diagnosis of TB Infection
Tuberculosis diagnosis essentially depends on isolating the bacteria from tissue. Chest x-ray reveals cavitation, calcification (healed disease), and nodes in the upper lobes, but cannot confirm the diagnosis. Granulomas with caseation obtained in biopsy indicate the diagnosis, but if caseation is not present, other possible diagnoses include sarcoidosis and lymphoma.
Sputum smears and cultures are useful in diagnosing pulmonary tuberculosis. Usually, three early morning specimens of sputum are obtained on three different days. The specimen is prepared on a slide, stained with an acid-fast dye, and observed under a microscope. The slide may show the characteristic acid-fast bacilli (AFB). Unfortunately, this alone does not confirm the diagnosis, because many mycobacterium species have a similar appearance.
Definitive diagnosis requires growing the bacteria and confirming that the culture is M. tuberculosis with biochemical tests or DNA probes. This can take several weeks. Smears that give a negative result initially can produce a positive result in a culture weeks later.
Urine cultures sometimes provide a diagnosis of Mycobacterium tuberculosis, especially in the case of genitourinary TB.
In a gastric aspirates test, a small nasogastric tube is placed in the stomach early in the morning and gastric contents are suctioned and processed for smears and culture.
In lymph node biopsy an enlarged lymph node is removed and a portion is cultured. The remaining portion is sent for histology (staining) and examined under a microscope. Frequently, a caseating granuloma and sometimes an AFB are seen. Unfortunately, the presence of caseating granulomas without AFB is nonspecific and can indicate other diseases.
Culture of Body Fluids
Fluid drained from the pleural space, the pericardial space, or the peritoneal space (see Anatomy of the Respiratory System) may be positive for AFB on smear and positive for TB on culture. If the smears are not positive in the case of TB pleurisy (they are only positive 50% of the time), a biopsy of the pleural membrane may yield a diagnosis. In suspected pericardial TB, pericardial fluid is drained. Biopsies of the pericardial membrane may be taken at the same time.
Bronchoalveolar lavage is often the next diagnostic test performed in patients with suspected pulmonary TB when sputum smears are negative.
The fiberoptic bronchoscope, a flexible tube about as big around as a pencil, is slid into either the nose or the mouth and passed down the back of the throat, into the trachea, and into each lung. There are no pain fibers in the tracheobronchial tree, so the procedure does not hurt. Topical lidocaine is sprayed into the back of the throat to inhibit the gag reflex and light sedation is given. The patient may cough during the procedure, so small doses of lidocaine are sprayed into the lungs occasionally during the procedure to numb the cough receptors. The procedure can take from 5 minutes to an hour.
To sample material from the alveolar space, the bronchoscope is advanced as far as possible into the lungs. The tube forms a seal with the airway wall, so fluids cannot go behind the bronchoscope tip. Sterile saline is pushed through the bronchoscope to the alveolar space, where it flushes out bacteria and loose cells. The solution is then suctioned out, cultured, and stained. Bronchoscopy for tuberculosis may initially involve bronchoalveolar lavage only.
Biopsy is often performed if an abnormality is seen in the airways during bronchoscopy (uncommon with tuberculosis). A thin wire is threaded through the bronchoscope and small biopsy forceps attached to the end of the wire remove several tissue samples from the bronchi. The samples are then sent to a laboratory for analysis.
In some areas of the world, blood tests are sometimes used to diagnose active tuberculosis (TB). In July 2011, the World Health Organization (WHO) warned against using these tests, which are unapproved for active TB diagnosis, often inacurate, and can lead to misdiagnosis and improper treatment. According to the WHO, blood tests for latent TB diagnosis are under review.