WASHINGTON - An experimental blood test for prostate cancer may help eliminate tens of thousands of unnecessary biopsies at the same time that it detects many tumors that are now missed by the test commonly in use, its developers said yesterday.

PSA, the current test, measures a protein normally produced by the prostate, while the experimental one, called EPCA-2, detects a chemical made principally in cancerous tissue.

Prostate cancer, the most common malignancy in men, can be difficult to detect.

Today, about 80 percent of prostate biopsies find no tumor - a percentage that is rising as physicians search more aggressively for the disease.

"We hope this will minimize the number of unnecessary biopsies," said Robert Getzenberg, a molecular biologist at Johns Hopkins Hospital who developed the new test, which is still under study and not yet commercially available. A description of it appears today in the journal Urology.

Gerald Andriole, chief of urology at Washington University's medical school, said: "If the data hold up, this marker will be a substantial improvement over PSA."

The PSA test casts a net that is too big and too full of holes. Finding a replacement that catches fewer healthy men, but more of those who do have cancer, would help settle one clinical conundrum.

The test is being developed by researchers at Johns Hopkins and a Seattle biotech company called Onconome. It could be commercially available in 2008.

About 230,000 American men are diagnosed with prostate cancer each year, and about 30,000 die of it. The death rate is 2.5 times higher in blacks than in whites.

Men are screened in two ways - by rectal exam and by the PSA (prostate-specific antigen) test. If a doctor feels a lump or the PSA is above 2.5 (nanograms per milliliter of plasma), most physicians will suggest a biopsy.

EPCA-2 is a protein that is part of the "nuclear matrix," the scaffolding inside a cell's nucleus that helps it copy its genes. The Hopkins researchers tested it on 30 men with PSA readings above 2.5 in whom biopsies found no cancer. All had normal EPCA-2 readings (below 30 nanograms per milliliter). This suggested that the test might eliminate many of the "false-positive" PSA results.

On the other hand, EPCA-2 appears able to detect cancer even when the tumor is small. It identified 36 out of 40 men who had cancer confined to the prostate gland and 39 out of 40 men in whom the tumor had spread. It also detected many men - 14 out of 18 - who had cancer but whose PSAs were normal.

This last group is especially worrisome. A study published three years ago found that about 12 percent of men with normal PSAs have cancer.

The new test is not perfect, though. Getzenberg and his colleagues tried it on 35 men with severe "benign prostatic hypertrophy" - enlargement of the prostate that sometimes makes the PSA go up but isn't cancer. In eight of them the EPCA-2 was high, suggesting that the EPCA-2 test would flag some who turn out not to have cancer - although probably not as many as PSA does.