Struck by a Duck: What bizarre billing codes reveal about how countries live - and get hurt

NEW YORK and COPENHAGEN -  April 1, 2026 – Corti, the frontier lab for clinical-grade AI, today published Struck by a Duck, a data report analyzing medical billing codes across the United States, United Kingdom, Germany, and the Nordics, highlighting a system in which errors cost the US healthcare sector an estimated $36 billion annually.

Among tens of thousands of billing codes, there is one for being struck by a duck (W61.62XA) and one for injuries sustained while knitting (Y93.D1). There is one for walking into a lamppost (W22.0). There is even a code for "problems in relationship with in-laws" (Z63.1) - they are all part of a system designed to capture every way a human body can be injured. Yet despite that level of specificity, it often fails to capture what actually happened.

Medical coding determines how hospitals are paid, how insurance claims are processed, and how health systems track disease. An estimated 12% of claims are inaccurate. 68% of hospital claim denials stem from coding errors, and 65% of denied claims are never resubmitted. The cost is not only financial: when codes are wrong or missing, patients fall out of follow-up, public health trends go undetected, and interventions that depend on accurate data never get designed.

Among the report findings:

Spring, everywhere: With Easter approaching and lawnmowers coming out of storage, the data has become predictable. Codes W27, W28, and W29 - hand tools, powered lawnmowers, and household machinery - are among the most seasonally consistent in the entire system. Lawnmowers alone account for approximately 26,679 American hospital admissions a year. In England, 8,500 DIY and gardening injuries occur annually, 56% between April and September, 90% of them male.

United States: American drivers filed 1.7 million animal collision insurance claims last year, 1.1 million involving deer, at a total cost of $10 billion annually. West Virginia has the highest risk at 1 in 40. On July 4th and 5th alone, more than 45,000 Americans visit emergency rooms, and the fireworks code (W39) surges from 196th to 16th most common injury cause over a single weekend. The US coding system contains an entire chapter of activity codes - injuries while knitting (Y93.D1), riding roller coasters (Y93.I1), playing drums (Y93.J2) - that does not exist in any other country's system. Rock and roll has a billing code, but only in America.

United Kingdom: Code W22.0 - striking against a stationary object - is the clinical record of what happens when 29% of the British population has walked into a lamppost at least once. Manchester tops the regional rankings for this. Code X10, contact with hot drinks, captures the spillage from sixty billion cups of tea a year. England also recorded 339,916 alcohol-specific hospital admissions in 2023–24, the highest since recording began, with Southampton's rate nearly ten times Rutland's - an economic and social map of England drawn in billing codes. Z63.1, problems in relationship with in-laws, remains a billable NHS diagnosis. No revision has ever removed it.

Germany: Berlin's forests cover 20% of the city, creating habitat for wild boar that routinely wander into parks, gardens, and beaches. In 2020, one made international headlines after grabbing a sunbather's laptop bag at Teufelssee lake. The German system codes such encounters under W55.4 (contact with pig) and W55.8 (contact with other mammals). Munich's Oktoberfest drew 7.2 million visitors in 2023, with more than 8,000 requiring medical attention - severe hand injuries spike 66% during the festival, driven by beer stein lacerations. A mobile CT scanner has been deployed on the grounds every year since 2022. German car insurance includes a dedicated "marten damage" clause for animals chewing through car wiring.

Denmark and the Nordics: Sweden has approximately one moose for every 40 people, producing around 5,000 vehicle collisions annually. In Denmark, roughly 17,500 cyclists a year report injuries, 70% of them in single-rider incidents - no car, no pedestrian, just potholes, ice, or a Saturday night. There is no maximum blood alcohol limit for cyclists, and only 15% wear helmets. The coding system (V10–V19) does not distinguish between these incidents and multi-vehicle road collisions. Finland has approximately 2 million saunas for 5.2 million people - more saunas than cars. Sauna-related burns generate roughly one hospital admission per day, coded under T67, the same classification a hospital in Phoenix would use for heatstroke.

What connects all four countries: Children continue to put things in their ears and noses (T16, T17) at rates that have not declined over any measured period. Children aged 1–4 favour noses; those aged 5–9 prefer ears. The research paper that aptly established this is titled: Will Children Ever Learn?

The Struck by A Duck report accompanies the launch of Symphony for Medical Coding, Corti's new agentic model, built on Code Like Humans - a multi-agent framework trained on 1.8 million patient encounters. Rather than treating coding as a pattern-matching task, it mirrors the reasoning process of expert human coders. It operates natively across US (ICD-10-CM), German (ICD-10-GM), and WHO ICD-10 systems without retraining, links every assigned code to the clinical evidence that justifies it, and outperforms systems from OpenAI, Anthropic, Amazon, Oracle, and Google by up to 23% in clinical accuracy benchmarks.

"The codes are not the problem. The codes are, in their strange and specific way, trying to tell the truth," said Joakim Edin, AI researcher at Corti and key author of Code Like Humans. "Coding has been treated as a labelling task for forty years - but it isn't. It's a reasoning task. That is why the accuracy gap has been so persistent, and that is what Symphony for Medical Coding is built to close."

The full report is available at corti.ai/struck-by-a-duck.