--- title: "OpenEvidence's prior-auth letters no better than ChatGPT's in Mayo test; $15 billion buys its distribution" description: "A Mayo Clinic head-to-head and a Nature Medicine study put the quality of OpenEvidence's answers on the record in one week: good enough to use, no longer uniquely good. At $15 billion, investors are paying for distribution, and distribution now has company." publisher: "The Inference" section: "Health" published: 2026-10-24T17:12:02.212Z modified: 2026-10-24T17:12:02.212Z canonical: https://theinference.org/article/openevidence-s-prior-auth-letters-no-better-than-chatgpt-s-in-mayo-test-15-billion-buys-its-distribution language: en keywords: "OpenEvidence, AI In Healthcare, Venture Capital, Doximity, Prior Authorization, Medical AI" --- # OpenEvidence's prior-auth letters no better than ChatGPT's in Mayo test; $15 billion buys its distribution > A Mayo Clinic head-to-head and a Nature Medicine study put the quality of OpenEvidence's answers on the record in one week: good enough to use, no longer uniquely good. At $15 billion, investors are paying for distribution, and distribution now has company. The most valuable document in American medicine is short, unglamorous, and decides whether an insurer pays for a drug: the prior authorization letter. A Mayo Clinic-led team [tested who drafts it better](https://www.ajmc.com/view/prior-authorizations-trial-gaps-hinder-glomerular-disease-drug-access), [OpenEvidence](https://theinference.org/markets/companies/openevidence) or ChatGPT-5, across 28 nephrology scenarios at ASN Kidney Week in Denver this week [1]. OpenEvidence, the AI answer engine 65% of US physicians use, put false statements in 7.1% of its letters against 3.6% for ChatGPT-5, and its clinical reasoning was rated strong in 78.6% of letters against 89.3% [1]. The ICD-10 coding was correct in 67.9% of OpenEvidence letters against 78.6% for ChatGPT-5, and the Mayo authors tied that gap to length: OpenEvidence wrote longer letters that often blew past the 350-word target, and more words mean more chances to be wrong [1]. *Chart: **OpenEvidence trailed ChatGPT-5 on every letter-quality measure, none by a significant margin** Column chart comparing OpenEvidence and ChatGPT-5 across three prior authorization letter measures in the Mayo Clinic test: false statements, strong clinical reasoning and correct ICD-10 coding.* *Share of 28 nephrology prior authorization letters meeting each measure, Mayo Clinic-led test presented at ASN Kidney Week 2026. No gap reached statistical significance. [1]* No gap reached statistical significance, and the authors' conclusion was measured: these are drafts for a clinician to review, not finished letters. Both tools hallucinated [1]. ## The stakes: a denial that is winnable and abandoned Medicare Advantage insurers made [52.8 million prior authorization determinations in 2024](https://www.kff.org/medicare/medicare-advantage-insurers-made-nearly-53-million-prior-authorization-determinations-in-2024/), denied 4.1 million (7.7%), and only 11.5% of denials were appealed [2][10]. When clinicians did appeal, 80.7% were fully or partially overturned [2][10]. The denial is routinely winnable and routinely abandoned, which is precisely the workflow AI letter-writing is being sold into [2]. ## The valuation buys distribution, and the accuracy record now says so OpenEvidence reached $15 billion in September after a $250 million round co-led by Andreessen Horowitz and Byers Capital, its third markup this year, on a projected $400-500 million in 2026 advertising revenue from pharmaceutical companies [3]. What the money buys is distribution: [952,000 clinicians had submitted 271 million queries](https://www.beckershospitalreview.com/healthcare-information-technology/ai/openevidence-drew-271m-queries-from-952000-clinicians-in-2-5-years/) by June 2026, monthly actives hit 703,265, and the platform handled 42 million prompts in October [3][11]. What it does not buy, on the published record, is the best model. In a [June Nature Medicine study](https://www.neoteo.com/en/openevidence-has-no-single-medical-ai-accuracy-score) OpenEvidence scored 89.6% on 500 MedQA questions, below Gemini 3.1 Pro at 97.4% and GPT-5.2 at 94.2%; blinded clinicians rated it level with UpToDate Expert AI and Google AI Overview [4]. *Chart: **On MedQA, OpenEvidence trailed Gemini 3.1 Pro by 7.8 points** Bar chart of MedQA accuracy for OpenEvidence, Claude Opus 4.6, GPT-5.2 and Gemini 3.1 Pro on the same 500-question test.* *Accuracy on the same 500 MedQA questions, Nature Medicine study, June 2026. [4]* ## The moat is the network, and this week its edges got crowded Memorial Sloan Kettering is wiring its [OncoKB tumor-genetics database](https://magica.com/news/openevidence-msk-oncokb-precision-oncology) into OpenEvidence and Epic [7]. OneOncology, whose community oncologists treat 1.5 million patients, [embedded the tool in its workflows](https://www.ajmc.com/view/oneoncology-partners-with-openevidence-to-give-physicians-access-within-clinical-workflows) [5]. Meanwhile [Doximity](https://theinference.org/markets/companies/doximity), which reaches more than 85% of US doctors, [settled the lawsuit OpenEvidence filed against it in June 2025](https://www.fiercehealthcare.com/ai-and-machine-learning/openevidence-doximity-reach-settlement-contentious-ai-lawsuit), all claims dismissed with prejudice on October 17, each side bearing its own costs [8][9][12]. In that suit OpenEvidence alleged that Doximity staff posed as doctors and used planted prompt-injection instructions to trick the tool into revealing its confidential system prompts [8][9]. Two days after the settlement, Doximity put Wolters Kluwer's UpToDate, 13,000 expert-reviewed topics, inside its rival Ask tool, and the stock rose 7.3% [8][9][12]. The company that owns the licensed content now sits on both sides of the trade [8][9][12]. [The usage study itself](https://bioengineer.org/ai-at-the-bedside-study-tracks-how-nearly-a-million-clinicians-use-openevidence/) concedes the open question: it counts who asks, not whether the answers are right or whether patients do better. Treatment and management questions, 33.1% of physician queries, outnumber diagnostic ones at 13.9%, so the unmeasured surface is the widest one [6][13]. ## The owners of licensed content are the investable exposure The scarce assets in evidence-based medicine are licensed content and the physician workflow, and the owners of both just moved against the private mark. The conflict to watch sits between the advertising line and the answers: if pharma money inside the tool starts shaping what doctors read, the first signs would come from disclosures around the $400-500 million 2026 ad-revenue projection, whose next test is the year-end reporting season, or from a clinician or publisher challenging the ad-and-answer model in court or in a study [3]. Until then the money is on distribution, and the quality of the answers is now on the record. ## Takeaway A Mayo Clinic-led test found OpenEvidence's prior authorization letters slightly worse than ChatGPT-5's on every measure, none significantly. At $15 billion, OpenEvidence's moat is distribution, and Doximity, MSK and OneOncology are now crowding it. ## Sources 1. [AJMC, CKD Risk Stratification, Payment Models, LLMs Aim to Close Care Gaps (ASN Kidney Week 2026 coverage), October 24, 2026](https://www.ajmc.com/view/ckd-risk-stratification-payment-models-llms-aim-to-close-care-gaps) 2. [AJMC, Prior Authorizations, Trial Gaps Hinder Glomerular Disease Drug Access, October 24, 2026](https://www.ajmc.com/view/prior-authorizations-trial-gaps-hinder-glomerular-disease-drug-access) 3. [Crypto Briefing, OpenEvidence hits $15 billion valuation as AI reshapes how doctors treat patients, October 21, 2026](https://cryptobriefing.com/openevidence-15-billion-valuation-ai-doctors/) 4. [NeoTeo, OpenEvidence has no single medical AI accuracy score, October 23, 2026](https://www.neoteo.com/en/openevidence-has-no-single-medical-ai-accuracy-score) 5. [AJMC, OneOncology Partners With OpenEvidence to Give Physicians Access Within Clinical Workflows, October 24, 2026](https://www.ajmc.com/view/oneoncology-partners-with-openevidence-to-give-physicians-access-within-clinical-workflows) 6. [OpenEvidence on X, on the Harvard-led JAMA usage study, October 2026](https://x.com/OpenEvidence/status/2112248333070631125) 7. [Magica, OpenEvidence and MSK Put OncoKB Into Oncology AI, October 22, 2026](https://magica.com/news/openevidence-msk-oncokb-precision-oncology) 8. [Fierce Healthcare, OpenEvidence, Doximity reach settlement in contentious AI legal battle, October 19, 2026](https://www.fiercehealthcare.com/ai-and-machine-learning/openevidence-doximity-reach-settlement-contentious-ai-lawsuit) 9. [Fierce Healthcare, Doximity adds UpToDate to Ask in push to become doctors' go-to AI platform, October 21, 2026](https://www.fiercehealthcare.com/ai-and-machine-learning/doximity-adds-uptodate-ask-push-become-doctors-go-ai-platform) 10. [KFF, Medicare Advantage Insurers Made Nearly 53 Million Prior Authorization Determinations in 2024, January 28, 2026](https://www.kff.org/medicare/medicare-advantage-insurers-made-nearly-53-million-prior-authorization-determinations-in-2024/) 11. [Becker's Hospital Review, OpenEvidence drew 271M queries from 952,000 clinicians in 2.5 years, October 15, 2026](https://www.beckershospitalreview.com/healthcare-information-technology/ai/openevidence-drew-271m-queries-from-952000-clinicians-in-2-5-years/) 12. [StockStory via FinancialContent, Why Is Doximity (DOCS) Stock Rocketing Higher Today, October 20, 2026](https://www.financialcontent.com/article/stockstory-2026-10-20-why-is-doximity-docs-stock-rocketing-higher-today) 13. [Bioengineer.org, AI at the Bedside: Study Tracks How Nearly a Million Clinicians Use OpenEvidence, October 16, 2026](https://bioengineer.org/ai-at-the-bedside-study-tracks-how-nearly-a-million-clinicians-use-openevidence/) --- OpenEvidence's prior-auth letters no better than ChatGPT's in Mayo test; $15 billion buys its distribution — The Inference. Canonical: https://theinference.org/article/openevidence-s-prior-auth-letters-no-better-than-chatgpt-s-in-mayo-test-15-billion-buys-its-distribution