Magazine article USA TODAY

Tb Easier to Transmit Than Standard Test Reveals

Magazine article USA TODAY

Tb Easier to Transmit Than Standard Test Reveals

Article excerpt

The standard test for gauging whether a tuberculosis patient is infectious misses half the people capable of transmitting the disease, a Stanford (Calif.) University Medical Center study found. After using DNA fingerprinting to track TB spread in San Francisco, the researchers concluded that patients classified as non-infectious by the test nevertheless gave rise to one-sixth of new cases. "Only half of TB patients are detected by this test, yet the other half are still infectious and continue to propagate this epidemic," notes Marcel Behr, lead author of the study.

For diagnosing tuberculosis and determining the severity of a patient's illness, TB control programs worldwide rely on a test called the acid-fast bacilli (AFB) smear. In this fast, cheap, and simple method, a sample of sputum (better known as phlegm)is inspected under a light microscope for the presence of tuberculosis bacteria. Positive results mean the patient should immediately be placed in isolation in a hospital, because every cough could launch enough bacteria to infect many other people. Negative results have generally been interpreted to mean that the patient is non-infectious and requires no special precautions.

However, theoretical and experimental results have cast doubt on the ability of this workhorse test to detect infectious patients, maintains Peter Small, assistant professor of medicine. Although as few as five TB bacteria bedding down in the lungs can start a new infection, a sample must contain 5,000-10,000 bacteria per milliliter to reach the test's threshold of detection. Epidemiological investigations show an elevated rate of TB among people exposed to patients who tested negative.

To verify suspicions that so-called smear-negative patients are infectious, Small, Behr, and their colleagues mined a database holding records of all TB patients diagnosed in San Francisco since 1991. Among the information amassed by this joint project of Stanford; the University of California, San Francisco; and the San Francisco Department of Health are results from smear tests and DNA fingerprinting analyses of each patient's bacteria.

DNA fingerprinting affords a means of retracing a disease's movement from person to person in a community. …

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