A noninvasive urine test might offer doctors a simple way to tell if a kidney transplant patient is at risk of transplant rejection, a US study said Thursday.

The study published in the American Journal of Transplantation said low levels of a protein in the urine of kidney transplant recipients, known as CXCL9, could be used to diagnose and even predict transplant rejection.

Following a kidney transplant, patients must receive therapy with toxic side effects to prevent their immune systems from rejecting the new organ. Even with this immunosuppressive therapy, 10 percent to 15 percent of kidney recipients experience rejection during the first year after transplantation.

If signs of kidney injury are detected, doctors typically perform a kidney biopsy, in which a small piece of kidney tissue is removed to look for rejection-associated damage. Although this procedure is generally considered safe, it carries some minor risks, such as bleeding and pain, and does not always provide an accurate impression of the overall state of the kidney.

In the new study, researchers from the Icahn School of Medicine at Mount Sinai and the Case Western Reserve University periodically collected urine samples from 280 adult and child kidney transplant recipients for two years after transplantation.

After measuring the urinary levels of molecules that had previously been associated with rejection, the researchers identified CXCL9 protein as a potential biomarker, a molecule that indicates the effect or progress of a disease, for the diagnosis of rejection.

They found that transplant recipients with low levels of urinary CXCL9 protein six months after transplantation were unlikely to experience rejection or loss of kidney function over the next 18 months.

CXCL9 protein levels also may be useful for predicting and monitoring transplant rejection. The researchers said that urinary CXCL9 levels began to increase up to 30 days before clinical signs of kidney injury, which could allow doctors to intervene early to potentially avoid rejection-associated kidney damage. The protein levels began to drop after treatment for rejection, suggesting that the urine test could be used to monitor treatment progress.

"Development of noninvasive tests to detect immune activation before kidney damage occurs would help guide the care of kidney transplant recipients," said US National Institute of Allergy and Infectious Diseases Transplantation Branch Chief Nancy Bridges, a co-author of the paper.

"Clinical application of the findings from this study could help avoid unnecessary biopsies and excess immunosuppression," Bridges added.