Tb Easier To Transmit Than Standard Test Reveals - tuberculosis - Brief Article

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. Two patients whose bacteria have identical DNA fingerprints likely can trace their infection to the same ultimate source.

The researchers grouped 1,359 TB patients into clusters according to their bacterial DNA fingerprints. The first patient to be diagnosed in each cluster was considered the source--direct or indirect--for all other cases in the group. Of the 71 disease clusters, nearly 40% were initiated by a smear-negative patient. When these clusters were analyzed in detail, at least 17% of the patients had contracted the disease from a person with a negative smear test.

Small reaffirmed that because the test does identify the most infectious patients, it remains useful. On average, AFB smear-positive individuals are about four times more infectious than those found to be smear negative. By clarifying the limitations of the test, the findings call into question TB control procedures in developed and developing countries, Small says. In the U.S. and other industrialized nations, the test serves to identify those in need of immediate, aggressive intervention. Such patients have long been the focus of TB control programs, based on the rationale that they transmit nearly all new cases, but even where health officials take great pains to identify and treat smear-negative patients, it appears that a significant source of infection has been overlooked.

The study's results, if confirmed worldwide, could have grave implications for TB control in the developing countries, where shortages of money and facilities means that the smear test is often the sole means to diagnose TB. Unlike in the U.S., someone with a suspicious cough, but negative test results, is not given a further round of more sensitive tests. Instead, he or she is usually thought to be free of the disease and is sent home without treatment. Tuberculosis kills around 3,000,000 people each year, most of them in developing countries, so it is imperative that a new, affordable replacement test be devised quickly. "For any other disease, we wouldn't tolerate a diagnostic test that only picks up half of the cases." Behr emphasizes.

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