- For the initial evaluation of any adult who requires routine evaluation for TB exposure, administer a second TB skin test within 1 to 3 weeks if the first test result is negative.
- The workup for active TB consists of a chest x-ray film, a human immunodeficiency virus (HIV) test, and possibly sputum collection. HIV-negative persons with a normal chest x-ray result are unlikely to have pulmonary TB, and sputum collection is unnecessary.
- Contrary to common belief, there is no age cutoff for treating latent TB.
- Treat active TB with isoniazid, rifampin, and pyrazinamide. Add ethambutol if a patient is a current or former resident of an area in which bacterial resistance to isoniazid is greater than 5%.
- Refer all suspected cases of TB to your local public health department for the purpose of tracking contacts.
With the decline of tuberculosis (TB), physician familiarity with this disease has substantially diminished. Yet TB remains common in immigrants, individuals with HIV infection, and other high-risk populations. How do we remain vigilant for TB? How do we screen? How should the physician interpret TB test results? What are the preferred options for treatment? What are the public health implications of TB? This article offers an update on this recalcitrant public health problem.
Whom, and how, to screen
Two factors are key in remaining vigilant for tuberculosis: knowing who in your patient population is most at risk for exposure, and knowing who is most likely to develop active disease if infected.
Persons at risk for exposure to TB (Table 1) or at risk for developing active disease if infected (Table 2) should receive a TB skin test regularly, although the optimal frequency has not been determined. Routine testing is not indicated for others.
Proper technique. The TB skin test should be administered with intermediate-strength purified protein derivative (PPD), 0.1 mL injected intradermally, resulting in a raised bleb. The test should be read 48 to 72 hours later and the area of induration, not erythema, measured and recorded in millimeters. With no induration, the result should be recorded as 0 mm, not as “negative.”
Interpreting test results. Interpretation of the test results depends on a person’s risk factors and age. For those listed in Table 3, a 5-mm induration is considered positive; for those listed in Tables 1 and 2 who are not in Table 3 , a 10-mm induration is positive. For everyone else, 15 mm is positive.
Caveat. Multiple puncture tests, though easier to administer, do not inject a standardized amount of tuberculin into the skin; results are more difficult to interpret and, if judged reactive, must be confirmed with a PPD test. This option is not recommended for testing.
Prior receipt of the bacillus Calmette-Guerin (BCG) vaccine does not affect the interpretation of the TB skin test, nor should it affect decisions to treat latent TB. The effectiveness of BCG vaccine in preventing TB infection in highly questionable, and the reaction to PPD caused by BCG wanes after a few years.
Two-step testing. Two-step testing means administering a second TB skin test within 1 to 3 weeks if the first test result is negative. This procedure should be used for the initial evaluation of adults who require routine testing. If the second skin test result is positive, it indicates the person was infected with TB before, and that immunity has waned and was “boosted” by the first test.
Without the 2-step process, a positive result on repeat testing would suggest recent infection rather than prior exposure. This could have implications for the decision to accept or not accept treatment for latent TB.
TABLE 1
Conditions associated with high risk of exposure to tuberculosis
| Foreign-born in area with high tuberculosis rates |
| Use of illicit drugs |
Institutionalization or work in
|
| Work in health care facilities with high-risk clients |
| Socioeconomic disadvantage |
| Children of those at high risk |
TABLE 2
Conditions associated with high risk of developing active tuberculosis
| HIV infection |
| Recent infection with Mycobacterium tuberculosis |
| Chest x-ray indicating old tuberculosis |
Immunosuppression
|
| Leukemia and other cancers of blood, lymph, and bone marrow |
| Cancer of the head and neck |
| Gastrectomy or intestinal bypass |
| Chronic malabsorption |
| End-stage renal disease |
| Diabetes |
| Silicosis |
| Illicit drug use |
TABLE 3
How to interpret a tuberculosis skin test
| 5 mm considered positive in those with: |
|
| 10 mm is considered positive in: |
Work-up for suspected tuberculosis
For those suspected of having TB because of chronic cough, night sweats, fever, and weight loss, or because of a positive TB skin test result, the workup consists of a chest x-ray film, an HIV test, and possibly sputum collection for microscopic evaluation and culture.