If the researchers are right, their test could help doctors do a better job of adjusting medications for people with hepatitis B, a deadly type of liver disease.

"About half to two-thirds of patients will basically fail the therapy after two to three years because they acquire drug resistance," says study co-author Brent Korba, a professor of microbiology at Georgetown University Medical Center. "It would be nice if you could predict those who are most likely to fail so you don't treat them (with the drug)."

Hepatitis B is one of several types of liver diseases that share little but the same name. An estimated 1.25 million Americans are infected with chronic cases of hepatitis B, which is typically spread through sex, needle-sharing and transmission from mother to baby during birth. Liver disease kills as many as one in four of those who suffer from the chronic form of the disease.

Hepatitis B, like AIDS, responds to antiviral drugs. One of them, called lamivudine, is commonly used to keep hepatitis B under control. "It's probably the first treatment choice for chronic hepatitis B today," Korba says.

But in many patients, the hepatitis B virus mutates into forms that become immune to lamivudine and other drugs. For that reason, many doctors are hesitant about giving patients the drug until they are in the later stages of infection.

Korba and his colleagues decided to study the hepatitis B virus's genetic makeup in 26 patients who had undergone treatment with lamivudine to see if there were any early warning signs of trouble. They reported their findings Monday at the 16th International Conference for Antiviral Research in Savannah, Ga.

The researchers found DNA "markers" in the genetic makeup of the virus in the patients who either developed an immunity to lamivudine or never responded to it at all. They couldn't find the markers in the virus in patients who responded well to the drug, says study co-author John Gerin, a professor of microbiology and immunology at Georgetown University Medical Center.

The test for the DNA markers is cost-effective, he adds. Alternatives to lamivudine are scarce, however. Only two other drugs are licensed for use in hepatitis B patients.

But Gerin says new drugs are on the horizon: "We do hope there will be options."

Frank Myers, an epidemiologist at Scripps Mercy Hospital in San Diego, says the findings are promising, especially if they can be extended to predict potential drug resistance in other diseases such as AIDS.

"We can not know if this new technique could be used with other viruses like HIV, but if it could, the benefits could be enormous," Myers explains.

More information

To learn more about hepatitis B, visit the U.S. Centers for Disease Control and Prevention. Or try the Hepatitis B Foundation.