Development and testing of a hierarchical method to code the reason for admission to intensive care units: the ICNARC Coding Method

A computer‐based hierarchical method was developed to code conditions leading to admission to intensive care in the UK. The hierarchy had five tiers: surgical status, body system, anatomical site, physiological or pathological process and medical condition. The hierarchy was populated initially using the free‐text descriptions of the reason for admission from 10 806 admissions recorded as part of the Intensive Care Society's UK APACHE II study. After refinement and error‐checking, a prospective evaluation was undertaken on 22 059 admissions to 62 UK intensive care units. Individual units coded between 60 and 1610 (mean 356) admissions. All but 50 (0.2%) of the admissions could be coded and 38 units coded every admission. Fifty admissions (0.2%) could not be coded within 24 h of admission but were coded subsequently when more information became available. Of the admissions, 96.1% were coded at all levels of the hierarchy in the coding method. Six hundred and thirty‐seven of the 741 unique conditions (85.9%) were used in one of the five reasons for admission and 564 (76.1%) in the primary reason for admission. Five conditions account for 19.4% of all primary reasons for admission. This is the first method to be developed empirically for coding the reason for intensive care admission.