Doctors, hospitals and insurers are bracing for possible disruptions on Oct. 1 when the U.S. health-care system switches to a massive new set of codes for describing illnesses and injuries.

Under the new system, cardiologists will have not one but 845 codes for angioplasty. Dermatologists will need to specify which of eight kinds of acne a patient has. Gastroenterologists who don’t know what’s causing a patient’s stomachache will be asked to specify where the pain is and what other symptoms are present—gas? eructation (belching)?—since there is a separate code for each.

In all, the number of diagnostic codes doctors must use to get paid is expanding from 14,000 to 70,000 in the latest version of the International Classification of Diseases, or ICD-10. A separate set of ICD-10 procedure codes for hospitals is also expanding, from 4,000 to 72,000.

Hospitals and physician practices have spent billions of dollars on training programs, boot camps, apps, flashcards and practice drills to prepare for the conversion, which has been postponed three times since the original date in 2011.

Some coding experts warn that claims denials could double as providers and payers get used to the new, more specific codes.

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Others are more sanguine. “We’re hoping it will be like Y2K,” when the switch to 2000 dates was expected to crash computers world-wide, says Robert Wergin, president of the American Academy of Family Physicians. “Everybody will worry, and the claims will go through fine.”

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