When form yields to substance: Diagnostic method patentability after Hirotsu Bio Science Inc. v. Assistant Controller of Patents

The Delhi High Court’s decision in Hirotsu Bio Science Inc. v. Assistant Controller of Patents and Designs (C.A.(COMM.IPD-PAT) 45/2023, decided on January 17, 2026) provides important clarity on the scope of Section 3(i) of the Patents Act, 1970. At its heart lies a familiar question in patent prosecution: can careful claim drafting shield an invention from the statutory exclusion applicable to diagnostic methods? 

The Court’s position is clear. The way a claim is worded cannot disguise the true nature of the invention. If it operates as a diagnostic process in substance, it will fall within the exclusion. The decision, therefore, aligns with the trajectory seen in recent High Court rulings on Section 3(i).

The patent application concerned a method for detecting cancer using the chemotaxis behaviour of the nematode Caenorhabditis elegans in response to odour signatures present in biological samples such as urine and tissue. The invention was described as achieving high levels of sensitivity and specificity and had already been commercialised in another jurisdiction. 

During examination, the Controller rejected the application under Section 3(i), concluding that the claimed process amounted to a diagnostic method. The applicant appealed to the Delhi High Court, contending that the invention fell outside the statutory exclusion. 

The appeal was dismissed. In doing so, the Court clarified the analytical framework applicable to diagnostic method claims in India.

Section 3(i) excludes from patentability “any process for the medicinal, surgical, curative, prophylactic, diagnostic, therapeutic or other treatment of human beings or animals.” 

Notably, the provision does not contain language limiting the exclusion to processes practised on the human or animal body (in vivo). The absence of such a qualification formed an important part of the Court’s reasoning. 

The central issue, therefore, was not whether the invention was laboratory-based, but whether the claimed process, in substance, identified the presence or absence of disease. That question inevitably brought into focus the appellant’s attempt to draw a distinction between “detection” and “diagnosis.”

The appellant argued that the invention merely detected cancer risk and did not amount to a diagnosis. According to this submission, a diagnosis required clinical interpretation by a medical professional and could not be equated with automated laboratory detection. 

The Court did not accept this distinction. Drawing upon earlier reasoning, including Chinese University of Hong Kong v. Assistant Controller of Patents (2023 SCC OnLine Mad 6372), it observed that a process capable of identifying the presence or absence of a disease may fall within the diagnostic exclusion, even if further confirmatory testing is contemplated. The statutory text does not confine “diagnostic” to definitive or physician-mediated determinations. 

The Court also noted that restricting Section 3(i) to methods performed by medical practitioners would create an artificial distinction. Fully automated diagnostic processes would then become patentable merely because human judgment was absent, a result not supported by statutory language. 

Equally significant was the Court’s approach to claim construction. Although the claims were drafted in guarded terms, the specification repeatedly described the invention as a cancer diagnosis system and emphasised its clinical accuracy. The Court reiterated that while claims delimit the legal scope of protection, the specification may be examined to ascertain the true nature of the invention. 

As recognised in Chinese University of Hong Kong, the embodiments disclosed in the complete specification can be scrutinised to determine whether the process is per se diagnostic. Where the disclosure reveals that the invention substantively identifies a disease, moderated claim language cannot alter that character. The emphasis, therefore, was on substance over form.

During the prosecution stage, the Controller relied on the structured four-step diagnostic framework, originally articulated by the European Enlarged Board of Appeal in G 1/04 and later discussed in Indian jurisprudence (The Chinese University of Hong Kong), to assess whether the claimed process constituted a diagnostic method. This framework considers whether the invention involves (i) data collection, (ii) comparison with reference values, (iii) identification of deviation, and (iv) attribution of that deviation to a clinical condition.  

The Court did not formally adopt the four-step test as a standalone doctrinal standard. However, in examining the claimed process and the disclosures in the specification, its reasoning followed a similar functional approach.

The Court noted that the method involved exposing biological samples to nematodes, measuring chemotactic responses, evaluating those responses against reference indicators, and correlating the results with the presence or absence of cancer. On that basis, it concluded that the invention performed a diagnostic function in substance. 

While the judgment stops short of formally adopting the four-step framework as binding law, it reinforces the relevance of a structured, functional analysis in determining whether a claimed process identifies disease. 

A further argument advanced by the appellant was that Section 3(i) should be confined to in vivo methods. Since the claimed process operated on biological samples outside the human body, it was said to fall outside the exclusion. 

The Court declined to read such a limitation into the statute. It observed that Section 3(i) contains no language restricting its scope to processes practised on the body. This textual choice distinguishes the Indian provision from certain foreign regimes that contain express qualifications. 

In reaching this conclusion, the Court drew support from earlier decisions, including Natera Inc. & Anr. v. Assistant Controller of Patents and Designs (2025:DHC:8937) and Sequenom Inc. & Anr. v. Controller of Patents (2025:DHC:8926), where it had been clarified that diagnostic processes do not escape the Section 3(i) bar merely because they are performed in  vitro. 

Accordingly, the Court held that in vitro processes may also fall within the diagnostic exclusion where they substantively identify disease. The location of the process, inside or outside the body, was not determinative.

The significance of Hirotsu lies less in novelty and more in consolidation. It forms part of a series of High Court decisions, including Chinese University of Hong Kong, Sequenom Inc. and Natera, that have progressively clarified the scope of Section 3(i). 

Looking across these decisions, a consistent approach begins to emerge. Courts have repeatedly indicated that: 

• Screening methods that identify a disease may fall within the exclusion, even if further confirmation is contemplated. 

• The distinction between detection and diagnosis is unlikely to succeed where the process ultimately identifies pathology. 

• The involvement of automation or artificial intelligence does not change the analysis. 

• The specification is often decisive in understanding what the invention truly does. 

Seen together, these rulings point towards a stable judicial understanding of Section 3(i) as a substantive boundary, not something that can be avoided through careful drafting alone.

The judgment in Hirotsu reinforces that Section 3(i) operates as a structural exclusion within the Indian patent framework. It is not merely a technical hurdle that can be avoided through careful phrasing/ re-phrasing of claims. 

The decision indicates that: 

• The specification may be examined to determine the true character of the invention.

• In vitro implementation does not, by itself, secure patentability. 

• Processes that substantively identify disease are likely to be treated as diagnostic. 

More broadly, the ruling reflects the statutory design of Section 3(i), which has consistently been interpreted to limit patent protection in the diagnostic sphere. Any shift in this position would more appropriately lie in legislative reform than judicial reinterpretation. 

By affirming that substance prevails over form in assessing diagnostic processes, the Delhi High Court has contributed to greater doctrinal certainty. For practitioners and innovators, the decision underscores a clear principle: in India, methods that functionally identify disease, whether performed on the body or in the laboratory, remain outside the scope of patentable subject matter.

About the author: Dr Lunalisa Potsangbam is a Partner at MS Law Partners.

Disclaimer: The opinions expressed in this article are those of the author(s). The opinions presented do not necessarily reflect the views of Bar & Bench.

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