But two studies discussed at the oncology meeting found that patients who were considered Her2-negative even using both tests benefited from Herceptin.  Both studies reanalyzed tumor samples from earlier clinical trials showing that Herceptin, if used after a tumor is removed by surgery, cuts the risk of the cancer’s recurring by half. For a woman to have entered those trials, her tumor had to be classified as Her2-positive by a local clinical laboratory.  But scientists have now gone back and retested those preserved tumor samples and found that as many as 20 percent of them were actually Her2-negative. Yet the women with those tumors also experienced a reduction in cancer recurrence from Herceptin, in some cases as great as that in the Her2-positive women.  “This is a revolution compared to what we believed before,” said Dr. Edith A. Perez of the Mayo Clinic, who presented one of the studies. She said the findings raised questions of whether women who were Her2-negative should be tested again.  Some experts were skeptical, saying the number of patients in the two studies was too small to draw firm conclusions. Also, they said, it was not clear if those women were truly Her2-negative, since they had tested positive by the local laboratory.  Dr. Daniel F. Hayes, a breast cancer specialist at the University of Michigan who helped develop guidelines for Her2 testing, said it would be unwise to start giving Herceptin to Her2-negative women because the drug was expensive and raised the risk of heart failure.  But he said the studies called attention to the inconsistent quality of Her2 testing in many small laboratories. Laboratories can use commercially available tests or develop their own.  Dr. Soonmyung Paik, who presented the second study at the cancer conference, said the problem might lie not in sloppy testing but rather in the cutoff used to determine which women get Herceptin.  Dr. Paik, who is with the National Surgical Adjuvant Breast and Bowel Project, said that about 40 percent of women had intermediate levels of Her2. They are now classified as negative but might still derive some benefit from the drug. On the other hand, he said, many women who are Her2-positive do not benefit from Herceptin. So better ways are needed to determine who should be treated.  “To me, the take-home message is that we don’t have a perfect test, unfortunately,” Dr. Paik said.  Dr. Pamela M. Klein, an executive at Genentech, the manufacturer of Herceptin, said the company was continuing to explore how to best identify patients for the drug.  The fact that this uncertainty is occurring so long after the 1998 approval of Herceptin — the paragon of “personalized medicine” — suggests that it will not be so easy to tailor other drugs to patients based on gene or protein tests.  It left some doctors at the conference incredulous and uncertain how to treat their patients.  “Here we are, 10 years into it,” said Dr. Marc L. Citron, an oncologist in Lake Success, N.Y., “and we don’t know how to test for it.”  A version of this article appears in print on , on Page F5 of the New York edition with the headline: Cancer Drug May Elude Many Women Who Need It. Order Reprints| Today's Paper|Subscribe
New evidence is raising questions about the tests used to prescribe the breast cancer drug Herceptin, which was initially seen as a model for tailored medicine.