Which ICD‐9 codes were assigned for malignant mesothelioma in the mortality data in the United States before the ICD‐10 was introduced?

Background Malignant mesothelioma (MM) is rare and fatal; survival in most cases is only about one year. Mortality rate is, therefore, a good proxy measure of incidence rate. However, the specific International Classification of Diseases (ICD) code for MM was not available until the Tenth Revision ICD (ICD‐10). Little is known on which Ninth Revision ICD (ICD‐9) codes were assigned for MM in the ICD‐9 era. Methods We used a 1996 double‐coded mortality file compiled by the National Center for Health Statistics to calculate the detection rate (DR) and confirmation rate (CR) of selected ICD‐9 codes. Results Of 2386 decedents whose underlying cause of death was MM (ICD‐10 code C45), the DR (deaths) of corresponding ICD‐9 code was 57% (1365) for code 199 “malignant neoplasm without specification of site;” 19% (448) for code 162.9 “malignant neoplasm of trachea, bronchus, and lung, unspecified;” 13% (310) for code 163 “malignant neoplasm of pleura;” and 11% (271) for other codes. The CR (deaths) for the aforementioned three ICD‐9 codes were 4.0% (1365/33,942), 0.3% (448/150,342), and 70.8% (310/438), respectively. Conclusions The three ICD‐9 codes (199, 162.9, and 163) were the most commonly used codes for MM and composed nine‐tenths of all MM deaths in the years before the ICD‐10 was introduced. Using only ICD‐9 code 163, the code most often used as the surrogate measure of MM in mortality studies in the ICD‐9 era, capture may have been only 13% of all MM deaths in the US, and the estimated number of MM deaths missed in 1996 would be 2086.