Invalidity search: finding prior art to challenge a patent

When you need an invalidity search

The forum decides what you search for. A reference that would win in district court may be useless in an IPR, and a deadline missed at the EPO cannot be recovered.

United States: courts and the PTAB

A US patent, and each claim independently, is presumed valid (35 U.S.C. 282(a)), and in Microsoft v. i4i (June 9, 2011) the Supreme Court held that an invalidity defense must be proved by clear and convincing evidence. At the Patent Trial and Appeal Board the petitioner only needs a preponderance of the evidence (316(e)), which is the main reason accused infringers file IPRs.

An IPR is limited to novelty and obviousness based on patents and printed publications (311(b)). It must be filed within 1 year after the petitioner is served with an infringement complaint (315(b)), so the search has to start the week the complaint arrives. The Board institutes only if there is a reasonable likelihood of prevailing on at least one claim (314(a)), and the final written decision is due within 1 year of institution, extendable by up to 6 months (316(a)(11)). After a final written decision, the petitioner cannot argue in court that the claim is invalid on any ground it raised or reasonably could have raised in the IPR (315(e)(2)). That estoppel is a reason to do the full patent and publication search before you file, not after.

USPTO fees, from the fee schedule effective January 19, 2025, with no small entity discount:

• IPR: $23,750 request fee and $28,125 post-institution fee for up to 20 claims; $470 and $940 per claim above 20 (37 CFR 42.15(a)).
• PGR: $25,000 request fee and $34,375 post-institution fee for up to 20 claims; $595 and $1,315 per claim above 20 (42.15(b)).
• Ex parte reexamination: $6,775 streamlined or $13,545 non-streamlined at the standard rate (1.20(c)).

Institution is discretionary, and the USPTO's approach to discretionary denial has changed several times since 2025. In October 2025 it proposed rules (90 FR 48335) under which an IPR would not be instituted unless the petitioner stipulates not to pursue 102 or 103 invalidity in other forums, nor where a challenged claim has already been upheld in court, at the ITC or the PTAB, nor where a parallel trial or decision is more likely than not to come first. As of October 26, 2026 no final rule has been published in the Federal Register. Check the current PTAB guidance before you budget for an IPR; ex parte reexamination is the fallback that does not depend on institution discretion.

Europe: EPO opposition, the UPC and national courts

The EPO opposition is the cheapest central attack on a European patent. Anyone can file within 9 months of the mention of grant (Art. 99 EPC), and the opposition fee is €880 (Rules relating to Fees, from 1 April 2026). The grounds are those in Art. 100: lack of novelty or inventive step or another patentability requirement, insufficient disclosure, and added matter. The result applies in every state where the patent has effect, including a patent with unitary effect.

After the 9 months, the route is a revocation action. At the Unified Patent Court a standalone revocation action goes to the central division, unless an infringement action between the same parties is already pending at a local or regional division (UPCA Art. 33(4)). A defendant sued for infringement can counterclaim for revocation (Art. 32(1)(e)). The UPC may revoke only on the grounds in Art. 138(1) and 139(2) EPC, and a revoked patent is treated as never having had effect (Art. 65). If the patent was opted out during the transitional period, or you only care about one country, the national courts apply the same Art. 138 grounds. Background on the UPC is in the Unitary Patent guide.

In India, a patent can be opposed before grant (section 25(1)), after grant within one year of publication of the grant (section 25(2)), and revoked by the High Court (section 64).

Getting the prior art date right

• Find the effective date for each claim. A claim is only entitled to a priority or parent date if that earlier application supports it. Claims added later in prosecution often are not, and then documents published in between become prior art. At the EPO those intermediate documents are the "P" category (Guidelines B-X 9.2.4).
• Check which US law applies. AIA 35 U.S.C. 102 applies to claims with an effective filing date on or after March 16, 2013; older patents are judged under pre-AIA 102, with different categories of prior art (MPEP 2159).
• Look for the patentee's own disclosures. In the US, the inventor's disclosures in the year before filing are excepted. At the EPO they are not, so a pre-filing paper or product launch by the patentee can invalidate the European counterpart. The patentability search guide compares the two rules.
• Prove the date. For web pages, archived snapshots, server records and dated citations by others. For papers, the conference date, library receipt stamps or indexing dates. The MPEP asks whether the document was accessible to persons interested in the art (MPEP 2128); the EPO first checks the date is correct and then whether the content was public on it (G-IV 7.5.1).

How to search

1. Read the file history. The examiner's search strategy, the cited references, the search reports of the other family members and the applicant's arguments tell you what was already considered and which feature got the claim allowed. Your search should target that feature.
2. Construe the claims. Write down how each disputed term should be read. A broad reading makes the claim easier to invalidate and harder to escape, so be consistent with the position you take on infringement.
3. Search where the examiner did not. Non-English patent documents, neighboring classification codes, older art from before the field had its current vocabulary, and non-patent literature.
4. Search standards. For telecoms, video coding and similar fields, standards and standard preparatory documents are often the best art. The EPO treats final standards as prior art, and preparatory documents too if they were public without a confidentiality bar (G-IV 7.6). Cite each by version and date.
5. Look for products. Old products, manuals, catalogs and trade show material can show public use or sale. Remember these cannot ground an IPR, but work in district court, PGR, EPO opposition and the UPC.
6. Rank and chart. Pick the best single reference for each independent claim, then the best combinations, then chart them.

Claim charts

A claim chart splits a claim into elements and puts the prior art next to each one, with a pinpoint citation. It is the working product of an invalidity search, and it is what a court, the PTAB or an EPO opposition division will read. The example below is illustrative; the claim and documents are hypothetical.

Here D1 anticipates elements [a] and [b] but not [c], so the chart supports an obviousness argument over D1 and D2, with a stated reason to combine. If a single document had all three, it would be an anticipation chart, and the strongest one you can have. Good charts quote the reference rather than paraphrase it, use the claim's own element numbering, and flag any element where the mapping depends on a particular claim construction.

More patent guides

• Patent drawing requirements: USPTO, EPO and India

  37 CFR 1.84, the EPO rules that replaced Rule 46 EPC, and Indian rule 15, compared side by side.

• How to write patent claims: USPTO, EPO and India

  Claim structure and worked examples, then the claim rules and fees at the USPTO, the EPO and the Indian Patent Office, side by side.

• Types of patent searches, and which one you need

  Patentability, freedom-to-operate, invalidity and landscape searches compared by purpose, timing, scope and deliverable.

• Patentability search: how to check novelty before you file

  What counts as prior art under 35 U.S.C. 102 and Art. 54 EPC, the grace period gap, search steps and how to read results.

• Freedom-to-operate search in the US and Europe

  In-force claims only, legal status checks at the USPTO, EPO and UPC, FTO opinions after Halo, and design-arounds.

• Patent landscape analysis, step by step

  Building a CPC and keyword dataset, cleaning families and names, the metrics that matter and the 18-month publication lag.

• Google Patents search guide

  Search operators and fields, Boolean and proximity syntax, non-patent literature, and where Google Patents falls short of the official databases.

• The Cooperative Patent Classification (CPC), explained

  How CPC symbols are built, CPC against IPC, the Y section, how to find the right class and how to search it in each database.

• Patent search Boolean operators and search strings

  Boolean, proximity and truncation syntax compared across Patent Public Search, Espacenet, Google Patents and The Lens, with semantic search for contrast.

• The PCT application: timeline, fees and national phase

  The international phase month by month, receiving Office and ISA fees in CHF, USD and EUR, national phase deadlines by office, and a deadline calculator.

• How much does a patent cost?

  USPTO fees by entity size from filing to the last maintenance fee, what drives professional fees, EPO fees, and an estimator.

• How long does a patent last?

  US utility and design terms, patent term adjustment and extension, terminal disclaimers, maintenance fee deadlines, Europe and India, with an expiry estimator.

• Can AI write a patent application?

  What AI drafting does well and badly, the USPTO and EPO positions on AI tools, AI inventorship after 2025, and confidentiality.

All guides, by patent office