The U.S. Supreme Court Wednesday heard a case that could determine if a pharmaceutical company could patent a method used to determine the effectiveness of a drug treatment is eligible for a patent.

The justices' ruling in Mayo Clinic v. Prometheus Laboratories could have major implications for drug diagnostic companies that are seeking ways to better tailor treatment to patients.

The patents from Prometheus cover a method for determining the optimal dosage of a drug that treats gastrointestinal diseases, such as Crohn's, and other autoimmune diseases.

Once the drug is administered, Prometheus' test measures how much of the drug appears in a patient's red blood cells after it is broken down in the body. This is known as the drug's metabolite level, which doctors can use to administer more or less of the drug to achieve the best results for a patient.

Minnesota-based Mayo Clinic had used Prometheus' test until 2004, but decided to use its own levels to determine a patient's optimal dosage. Prometheus sued Mayo for patent infringement, setting off the legal battle now before the Supreme Court.

Mayo contended that Prometheus-a unit of food and drink giant Nestle--patented natural phenomena, that is, the correlation between a drug's metabolites and its effectiveness. A federal court agreed, arguing that the final step in Prometheus' method--being warned that the amount of drugs must be adjusted--is a mental process, which is not a subject eligible for patenting.

The U.S. Court of Appeals for the Federal Circuit ruled in favor of Prometheus, deciding that Prometheus' treatment method applies the naturally-occurring correlation between the drug's metabolite levels and its efficacy.

For instance, Albert Einstein cannot patent e=mc2, but anyone that uses the principle in an invention could do so.

Mayo's attorney, Stephen Shapiro, said Prometheus is locking down metabolite levels used to decide the correct treatment for autoimmune diseases.

If you get the wrong number set in concrete for 20 years, that is a huge problem for patients, Shaprio said. And there are millions and millions of patients suffering from autoimmune disease.

Prometheus' attorney, Richard Bress argued that the company's patents cover a specific system of administering a drug that includes guidelines for doctors to better treat patients.

We are not trying to pass the general broad idea that you can use metabolite readings after you administer the drug to determine what the likely, what the best level of the next administration might be, Bress said.

Justice Elena Kagan, however, said that Prometheus could have patented a specific treatment based on the drug's metabolite level.

The difference that people are noting or some people are noting is that this is not a treatment protocol, it's not a treatment regimen, Kagan said. All you have done is pointed out a set of facts that exist in the world and are claiming protection for something that anybody can try to make use of in any way and you are saying 'You have to pay us.'