The ability to predict impairment outcomes in large databases using a simplified technique allows researchers to focus attention on preventing costly impairing injuries. The dilemma that exists for researchers is to determine which method is the most reliable and valid. This study examines available methods to predict impairment and explores the differences between the IIS and pFCI applied to real world crash injury data. Occupant injury data from the UK Co-operative Crash Injury Study (CCIS) database have been coded using AIS 1990 and AIS 2005. The data have subsequently been recoded using the associated impairment scales namely the Injury Impairment Scale (IIS) and the predicted Functional Capacity Index (pFCI) to determine the predicted impairment levels of injuries at one year post crash. Comparisons between the levels of impairment were made and any differences further explored. Injury data for the period February 2006 to September 2008 from the CCIS database were used in the analysis which involved a dataset of 2,437 occcupants who sustained over 8000 injuries.

This study found some differences between the impairment scales for injuries coded to the AIS 1990 and AIS 2005 coding dictionaries. The pFCI predicts 31.5% of injuries to be impairing in AIS 2005, less than the IIS (38.5%) using AIS 1990. Using CCIS data the pFCI predicted that only 6% of the occupants with a coded injury would have an impairing injury compared to 24% of occupants using the IIS. The main body regions identified as having the major differences between the two impairment scales for car occupants were the head and spine. Follow up data were then used for a small number of cases (n=31, lower extremity and whiplash injuries) to examine any differences in predicted impairment versus perceived impairment. These data were selected from a previous study conducted between 2003 and 2006 and identified the discrepancy between predicted impairment and actual perceived impairment as defined by the participant. Overall the work highlights the variaton between the pFCI and IIS and emphasises the importance and need for a single validated impairment scale that can be universally applied. This would allow emphasis to be directed towards preventing injuries that are associated with the most significant impairment outcomes.