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The Supreme Court unanimously tossed out medical patent claims for Prometheus Laboratories on Tuesday for a test that could help doctors set drug doses for autoimmune diseases like Crohn's disease, a decision that could affect the burgeoning field of personalized medicine.

The justices unanimously agreed that the patents held by the company -- owned by Switzerland-based Nestle -- was invalid because they were based on the laws of nature, which are unpatentable.

The patent in question covers a blood test that helps doctors determine the proper dosage for a drug, thiopurine, to treat gastrointestinal and non-gastrointestinal autoimmune illnesses. The patent covers methods of administering thiopurine to a patient and then determining the levels of the drug or the drug's metabolites -- what's left after it breaks down in the body -- in the patient's red blood cells. That observation is used to adjust the amount of medicine needed for that patient.

"The question before us is whether the claims do significantly more than simply describe these natural relations," said Justice Stephen Breyer, who wrote the opinion. "To put the matter...