In the new study, the researchers developed a technique called cell-free DNA test for detecting heart transplant rejection that works by measuring small differences in the DNA sequences of both the transplant donor and recipient.

In a study of 65 patients, including 21 children and 44 adults, the researchers found they were able to accurately detect the two main types of rejection, antibody-mediated rejection and acute cellular rejection, in 24 patients who suffered moderate to severe rejection episodes, one of whom required a second transplant.

They were also able to detect signs of rejection up to five months before the biopsies indicated anything troubling.

The cell-free DNA test is different from another blood test, AlloMap, used to detect rejection. The commercially available AlloMap uses a blood sample to analyze the expression of immune- system genes involved in rejection. The researchers said that the cell-free DNA test outperformed AlloMap by a substantial margin.

"We've found that this cell-free DNA assay is a very accurate way to diagnose acute rejection, sometimes weeks to months before a biopsy picks up any signs," said co-senior author Kiran Khush, assistant professor of medicine at Stanford. "This earlier detection may prevent irreversible damage to the transplanted organ."

The researchers said they are optimistic that biopsies will eventually be commercially used although much remains to be done. They are also hopeful it can be generalized to detect the rejection of organs other than the heart, and to diagnose other problems such as infections that plague organ transplant recipients.