AHRQ - Emergency Severity Index, Version 4

The Emergency Severity Index (ESI) is a five-level emergency department (ED) triage algorithm that provides clinically relevant stratification of patients into five groups from 1 (most urgent) to 5 (least urgent) on the basis of acuity and resource needs.
The Agency for Healthcare Research and Quality (AHRQ) funded initial work on the ESI.

www.ahrq.gov/research/esi/esi1.htm
www.ahrq.gov/research/esi/index.html#copies