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 a][ 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.