Originally developed to classify mortality and to provide a coding schema for listing causes of death, the ICD has expanded over the years to include classifying morbidity and many other items and procedures related to the delivery of healthcare . In the U.S., the ICD consists of two components, identified as ICD-10-CM, for clinical modifications, and ICD-10-PCS, for procedural coding system.
The NCHS, with guidance from the Centers for Medicare and Medicaid Services, is responsible for developing the ICD-10′s clinical modifications used in the U.S. . Chapter 19 of the ICD-10 provides two subcategories identified as S and T codes . The S codes are for various single body region injuries, and the T codes cover injuries to unspecified body locations, poisonings, and other external consequences .
Six T-codes identify all opioids: Opium (T40.0); Heroin (T40.1); Natural and Semisynthetic Opioids (including morphine, codeine, oxycodone, hydrocodone, hydromorphone, and oxymorphone) (T40.2); Methadone (T40.3); Synthetic Opioid Analgesics Other Than Methadone (including fentanyl, meperidine, pentazocine, propoxyphene, tapentadol, buprenorphine, and tramadol) (T40.4); and, finally, Other and Unspecified Narcotics (T40.6)  (see Fig. 2).
Until 2016, the NVSS calculated annual mortality involving prescription opioids by summing deaths identified by ICD codes T40.2, T40.3, and T40.4 . In 2016, the sum of these three codes amounted to 32,445 deaths  (see Table 1). Code T40.4 was identified as the source of the error described by the CDC analysts . This systemic error likely began in 2005–2007, when the CDC and the Drug Enforcement Administration (DEA) identified 1013 deaths caused by illicitly manufactured fentanyl (IMF) mixed with, and sold as, heroin in several mid-western and northeastern states .
In 2013, IMF reappeared, causing deaths in the northeast . The CDC was informed by Rhode Island authorities that acetyl fentanyl, a fentanyl analog up to five times as potent as fentanyl, had been identified in ten drug overdose deaths in the state between March 7 and April 11, 2013 . During and shortly after the month-long investigation in Rhode Island, four more overdose deaths occurred . Besides the Rhode Island deaths, a CDC field report at the time cited a cluster of 50 similar IMF deaths reported by authorities in Pennsylvania .
Despite having information about IMF since the 2005–2007 outbreak, the NVSS continued to code all incoming death certificate information that mentioned fentanyl as a single or contributory cause of death as the prescribed form of the drug, a serious miscalculation that would have substantial consequences on public policy, health, and safety.
Two years after discovering their error in the 2016 figures for prescription opioid overdose deaths, the CDC analysts proposed what they called a “conservative” method for re-calculating the numbers . Their conservative method simply removed all deaths coded T40.4 . This reduced the number of prescription opioid deaths in 2016 from 32,445 to 17,087—a sizable drop of 47.3% . The analysts acknowledged, however, that their conservative approach likely produced an undercount because by deleting all deaths coded T40.4, they were removing overdose deaths involving prescription fentanyl, as well as overdose deaths involving other synthetic opioids other than methadone identified by the same T40.4 code (e.g., meperidine, pentazocine, propoxyphene, tapentadol, buprenorphine, and tramadol) .
Congress addressed this issue in October 2018, when the Support for Patients and Communities Act was passed and signed into law by the president . The Act directed the CDC to modernize its system for coding causes of deaths related to drug overdoses .