Classifications in routine use: lessons from ICD-9 and ICPM in surgical practice

Classifications of diagnoses and procedures are very important for the economical as well as the quality assessment of surgical departments. They should reflect the morbidity of the patients treated and the work done. The authors investigated the fulfillment of these requirements by ICD-9 (International Classification of Diseases: 9th Revision) and OPS-301, a German adaptation of the ICPM (International Classification of Procedures in Medicine), in clinical practice.

A retrospective study was conducted using the data warehouse of the Surgical Center II at the Medical Faculty in Essen, Germany. The sample included 28,293 operations from the departments of general surgery, neurosurgery, and trauma surgery. Distribution of cases per ICD-9 and OPS-301 codes, aggregation through the digits of the codes, and concordance between the classifications were used as measurements. Median and range were calculated as distribution parameters. The concentration of cases per code was graphed using Lorenz curves. The most frequent codes of diagnoses were compared with the most frequent codes of surgical procedures concerning their medical information.

The total number of codes used from ICD-9 and OPS-301 went up to 14 percent, depending on the surgical field. The median number of cases per code was between 2 and 4. The concentration of codes was enormous: 10 percent of the codes were used for about 70 percent of the surgical procedures. The distribution after an aggregation by digit was better with OPS-301 than with ICD-9. The views with OPS-301 and ICD-9 were quite different.

Statistics based on ICD-9 or OPS-301 will not properly reflect the morbidity in different surgical departments. Neither classification adequately represents the work done by surgical staff. This is because of an uneven granularity in the classifications. The results demand a replacement of the ICD-9 by an improved terminological system in surgery. The OPS-301 should be maintained and can be used at least in the medium term.