BACKGROUND: Injury severity measures are based either on the Abbreviated Injury 
Scale (AIS) or the International Classification of diseases (ICD). The latter is 
more convenient because routinely collected by clinicians for administrative 
reasons. To exploit this advantage, a proprietary program that maps ICD-9-CM 
into AIS codes has been used for many years. Recently, a program called ICDPIC 
trauma and developed in the USA has become available free of charge for 
registered STATA users. We compared the ICDPIC calculated Injury Severity Score 
(ISS) with the one from direct, prospective AIS coding by expert trauma 
registrars (dAIS).
METHODS: The administrative records of the 289 major trauma cases admitted to 
the hospital of Udine-Italy from 1 July 2004 to 30 June 2005 and enrolled in the 
Italian Trauma Registry were retrieved and ICDPIC-ISS was calculated. The 
agreement between ICDPIC-ISS and dAIS-ISS was assessed by Cohen's Kappa and 
Bland-Altman charts. We then plotted the differences between the 2 scores 
against the ratio between the number of traumatic ICD-9-CM codes and the number 
of dAIS codes for each patient (DIARATIO). We also compared the absolute 
differences in ISS among 3 groups identified by DIARATIO. The discriminative 
power for survival of both scores was finally calculated by ROC curves.
RESULTS: The scores matched in 33/272 patients (12.1%, k 0.07) and, when 
categorized, in 80/272 (22.4%, k 0.09). The Bland-Altman average difference was 
6.36 (limits: minus 22.0 to plus 34.7). ICDPIC-ISS of 75 was particularly 
unreliable. The differences increased (p < 0.01) as DIARATIO increased 
indicating incomplete administrative coding as a cause of the differences. The 
area under the curve of ICDPIC-ISS was lower (0.63 vs. 0.76, p = 0.02).
CONCLUSIONS: Despite its great potential convenience, ICPIC-ISS agreed poorly 
with its conventionally calculated counterpart. Its discriminative power for 
survival was also significantly lower. Incomplete ICD-9-CM coding was a main 
cause of these findings. Because this quality of coding is standard in Italy and 
probably in other European countries, its effects on the performances of other 
trauma scores based on ICD administrative data deserve further research. Mapping 
ICD-9-CM code 862.8 to AIS of 6 is an overestimation.