U.S. researchers said Tuesday they have developed a potential gene test for identifying acute rejection in kidney transplant patients, a finding that could eventually replace the need for biopsies.

Acute rejection after kidney transplantation occurs in about 15 percent to 20 percent of patients, even when they are treated with immunosuppressive medications.

Rejection is usually heralded by an increase in the patient's serum creatinine, a marker of kidney function, and a kidney biopsy is then performed to confirm whether rejection is taking place.

However, elevated creatinine is not sufficiently sensitive to identify all early rejection or specific enough to prevent some unnecessary kidney biopsies, so a noninvasive means of identifying acute rejection is needed, according to researchers from the University of California San Francisco.

For the new study, the researchers used an assay called quantitative polymerase chain reaction to measure the expression of 43 genes whose expression levels change during acute kidney rejection in blood samples collected from patients who had had a kidney transplant.

Using a training set of 143 samples and statistical analyses, the researchers identified a 17-gene set called kSORT that discriminated between patients with and without acute rejection detected by kidney biopsy.

The 17-gene set correctly identified 39 of the samples taken from 47 patients with acute rejection as being from patients with acute rejection, and 87 of 96 samples from patients without acute rejection as being from patients without acute rejection.

The researchers also found that the gene set was able to predict acute rejection up to three months before detection by biopsy, independent of age, time after transplant and sample source.

"The kSORT assay has the potential to become a simple, robust, and clinically applicable blood test," the researchers concluded.

The study was published in the U.S. journal PLOS Medicine.