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Researchers are reporting that they have taken the first step toward developing a reliable blood test for ovarian cancer, including cases that are still early enough to be curable.

A means of early detection is much needed, doctors say, because ovarian cancer is usually diagnosed late, and as a result is often fatal.

In a preliminary study in 116 women, 50 with ovarian cancer and 66 without, the new test looked promising. It missed none of the cancers, although it gave false-positive results - mistakenly diagnosing cancer in healthy women - in 5 percent of the cases.

The test, developed by scientists at the National Cancer Institute, the Food and Drug Administration, or FDA, and Correlogic Systems, a company in Bethesda, Maryland, requires only a drop of blood and gives results in 30 minutes. It is described in a report posted Friday on the Web site of the British medical journal The Lancet (www.thelancet.com).

The test is still experimental, and not available to the public outside of clinical trials. Its developers say it needs further study in many more women to determine whether the early findings hold up. If it does come to market, it will not be for several years, and its use might initially be limited to women at high risk.

Emanuel Petricoin, who helped create the test, said, "I'm all too aware as an FDA scientist of promising early results that start to fail as you go into the real world."

Ovarian cancer is not common, but it is often deadly. More than 80 percent of cases are advanced by the time of detection, and only about 25 percent of those patients survive five years or more. But when the cancer is found early, more than 90 percent of women live five years or more, and most are cured. In 2001, 23,400 American women developed the disease; 13,900 died of it.

Unlike most blood tests, which measure levels of single substances, the new test looks for patterns of proteins that set cancer patients apart. The blood is first analyzed with a technique called mass spectroscopy, which is used to sort proteins by their weight and electrical charge. The technique creates a spectrum, or graph, full of spiky-looking peaks and valleys that represent hundreds of proteins.

The spectrum is then analyzed by a computer program, known as an artificial intelligence algorithm, that is designed to recognize patterns.

Initially, the researchers had the computer analyze spectrums from 50 women who had ovarian cancer and 50 who did not. The program found a pattern of five unidentified proteins that existed only in the cancer patients.

Next, using the pattern information from the first 100 patients, the researchers analyzed 116 more spectra, from 50 women with cancer and 66 without it. The 66 women were known to be free of ovarian cancer because their blood had been drawn and frozen five years before, and their health monitored ever since. The diagnoses were concealed from the researchers until after the computer analysis was done.

The program correctly selected all 50 cases of cancer, including 18 that were Stage 1 and therefore highly curable. It also recognized that 63 of the 66 other women did not have cancer; in three of the healthy women, 5 percent, the computer mistakenly diagnosed cancer.

Dr. Lance Liotta, chief of pathology at the National Cancer Institute, who helped develop the test, said, "We were able to see a pattern in the blood of patients that reflected something going on in an internal body organ."

Most important, Dr. Liotta added, was that the pattern could be picked out even in early cancers - that is, those that were still limited to the ovaries and had not begun to spread. He said he and his colleagues expected to apply the same technique to other cancers.

Dr. Martee Hensley of Memorial Sloan-Kettering Cancer Center in Manhattan, who runs a clinic for women at high risk of ovarian cancer and who was not involved in the study, called the findings exciting and said, "I found it encouraging that the computer algorithm could find the early-stage cancers."

A blood test now used in high-risk women, for a substance called CA125, is much less reliable, Dr. Hensley said, giving high readings in only about half of patients with early ovarian cancers, and 80 percent with advanced disease.

But regarding the new test, she added: "There are certain caveats. The data are somewhat preliminary, and we don't know how this computer algorithm might work in a larger population."

A great concern with any screening test is the rate of false-positive results, because, in addition to frightening patients, mistaken findings can lead to unnecessary surgery.

The study on the new test was conducted in women who were at high risk for ovarian cancer because they had genetic mutations or a family history of the disease. In a general population with an average risk of ovarian cancer, the false-positive rate could be different. The only way to find out, the researchers said, is to study the test in larger groups, including those with different levels of risk.

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