Prometheus Laboratories holds patents for how to determine the proper dosage of drugs that treat gastrointestinal and other autoimmune diseases. The problem for the company, defending itself Wednesday at the Supreme Court against a patent challenge from the Mayo Clinic, was that its procedures don’t sound like much of a discovery.

You administer the drug. You take a blood sample. If the level of the drug or its components is too low, you increase the dosage. If too high, you decrease.

Justice Elena Kagan summed up the argument for those who wonder whether Prometheus had actually invented anything.

“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.’ ”

Richard P. Bress, an attorney for Prometheus, said the company had gone to great expense to determine the proper levels and gave doctors valuable information. Before the test, he said, “doctors had no way to tailor for each individual, based on their metabolism, the right dosage of these powerful but potentially toxic drugs.”

The fight between Prometheus and the Mayo Clinic, which for the past seven years has wanted to market its own test, could hold great importance in the burgeoning field of “personalized medicine.”

The great medical advances of this century, Prometheus told the court in its brief, will likely concern “uniquely targeted treatments” based on an individual’s genetic makeup.

The case presented the justices with the question of whether Prometheus had done more than recognize a natural phenomenon.

As Justice Stephen G. Breyer put it: “What has to be added to a law of nature to make it a patentable process?”

Not surprisingly, such a question led to a slew of hypotheticals designed to defeat any easy test. Breyer wondered about patenting the discovery that too little fertilizer on a field made no impact, but too much burned the crops.

Chief Justice John G. Roberts Jr. offered another:

“You take wood, you put it on a grate, you light it, and you get heat,” Roberts said. Such a process “recites a series of acts performed in the physical world that transforms the subject of the process, the wood, to achieve a useful result, which is heat. So I can get a patent for that?”

No, Solicitor General Donald B. Verrilli Jr. said, because it would not meet the patent law requirement of being novel.

The federal government sided with the Mayo Clinic in the suit, saying that “you can’t get a patent by tacking a mental step onto an utterly conventional process for administering drugs and testing their effects.”

Prometheus’s test involves the optimal dosage of thiopurine drugs, which are used for all sorts of ailments, including Crohn’s disease, some skin diseases and lupus. After the drug is administered, a blood test measures the amount of metabolites resulting from the dosage. Those levels are then compared with predetermined ranges that indicate whether the dosage needs to be increased or decreased.

The Mayo Clinic said Prometheus’s patent is so broad it prevents others from offering a better test.

In effect, said lawyer Stephen M. Shapiro, it keeps patients from getting a “second opinion” and using “a better metabolite test with more accurate numbers.”

Bress said others might be able to improve on Prometheus’s work, but that does not mean what it discovered was not patentable.

“The idea that we are not novel because people took some of the same steps along the way to invention that we actually succeeded in is wrong,” he said.

Breyer was one of the most active questioners in the case but almost had to sit it out. Prometheus did not inform the court until Tuesday night that it had been purchased, after the court agreed last term to hear the case, by Nestle, a company in which Breyer’s wife, Joanna, owned stock. She sold it Wednesday morning before the arguments.The case is Mayo Collaborative Services v. Prometheus .