The reason so many diagnostic method patents are challenged is one sentence of statute and one Supreme Court framework built on top of it. Eligibility begins at 35 U.S.C. 101, which provides that “Whoever invents or discovers any new and useful process, machine, manufacture, or composition of matter, or any new and useful improvement thereof, may obtain a patent therefor, subject to the conditions and requirements of this title.” The statute is broad, but the courts have read three implicit exceptions into it. The USPTO's Manual of Patent Examining Procedure, section 2106, states that the categories of invention “are limited to abstract ideas, laws of nature and natural phenomena (including products of nature),” citing Alice Corp. v. CLS Bank and, before it, Mayo Collaborative Services v. Prometheus Laboratories.

A diagnostic method sits directly in the path of the law-of-nature exception because what it usually claims is a relationship that exists in the body whether or not anyone patents it. In Mayo, the asserted patent was U.S. Patent 6,355,623, “Method of treating IBD/Crohn's disease and related conditions wherein drug metabolite levels in host blood cells determine subsequent dosage,” granted March 12, 2002. Its claims recited administering a thiopurine drug, then determining the resulting metabolite level, where a level below one threshold indicated a need to increase the dose and a level above another indicated a need to decrease it. The Supreme Court treated the metabolite-to-dosing correlation as a law of nature.

"The judicial exceptions (also called “judicially recognized exceptions”) are subject matter that the courts have found to be outside of, or exceptions to, the four statutory categories of invention, and are limited to abstract ideas, laws of nature and natural phenomena (including products of nature)."— USPTO, MPEP 2106, source

How does the Section 101 eligibility test work for a diagnostic claim?

MPEP 2106 sets out the analysis as a sequence of inquiries. First, the claim must fall within one of the four statutory categories (a diagnostic method is a process, so it clears this). Then comes the two-step framework the Office labels the Alice/Mayo test. Step 2A asks whether the claim is directed to a judicial exception — a law of nature, natural phenomenon, or abstract idea. The MPEP frames the controlling question this way: the claim “must not be directed to a judicial exception unless the claim as a whole includes additional limitations amounting to significantly more than the exception.” If the claim recites a natural correlation, it is directed to an exception, and the analysis moves to step 2B.

Step 2B asks whether the claim's additional elements add an “inventive concept” — something significantly more than the exception itself. In Mayo, the additional steps were “administering” the drug and “determining” the metabolite level. The Court found those steps to be well-understood, routine, and conventional activity that any treating physician already performed, so they did not supply the inventive concept the law requires. The correlation was the only new thing in the claim, and a correlation that exists in nature cannot itself be the basis of a patent. That is the structural reason diagnostic claims fail: the discovery is often genuine and valuable, but the claim captures the natural relationship rather than a specific, non-routine application of it.

It is worth being precise about what the law-of-nature exception does and does not reach. The exception is not a rule that anything involving biology is unpatentable; it targets claims whose substance is the natural relationship itself. A correlation between thiopurine metabolite concentration and the right dose is a fact about human physiology that existed before anyone wrote it down. A claim that recites measuring the metabolite and then concluding something about dosing does not invent that relationship — it observes it. MPEP 2106 frames this as the difference between claiming the exception and claiming a specific application of it, and the entire step-2B inquiry exists to test whether the surrounding claim limitations supply that application. Where the only non-natural elements are conventional data-gathering steps, the claim is treated as reaching the natural law in disguise.

What makes a diagnostic claim more likely to be eligible

MPEP 2106 and the case law it summarizes draw the line at application. A claim that does not merely state a correlation but applies it through a particular, non-conventional technique — a new assay, a specific treatment step that goes beyond what was already routine, or a transformation of a particular article — has a stronger eligibility posture because the additional limitations can amount to significantly more than the natural law. The Office's life-sciences eligibility examples walk through exactly this distinction, contrasting claims that recite a natural relationship in the abstract with claims that tie the relationship to a concrete, unconventional set of steps.

For patent drafters and litigators in the diagnostics space, the practical consequence is that the value of a diagnostic discovery and the eligibility of a claim covering it are two different questions. A correlation between a measured analyte and a disease state can be a major scientific advance and still produce an ineligible claim if the claim language reaches the correlation itself. The statute is the starting point — Section 101's broad “process” category — but MPEP 2106 and Mayo are where eligibility is actually decided, and they decide it on whether the claim, read as a whole, adds significantly more than the law of nature it rests on.

This is a report of what the statute, the MPEP, and the cited opinions state; it is not legal advice, and eligibility in any specific matter turns on the exact claim language and the full record before the examiner or court.