The results of our study support the reliability (unidimensionality, internal consistency) and construct validity of the SF-36 and GARS in a polytrauma population with lower extremity injury. Whereas the construct validity of the SIP-136 in this population was supported, the unidimensionality and internal consistency of the subscales are not supported in the present study.
The analysis of the GARS showed that the ADL and IADL scales can be used as separate (unidimensional) scales but the strong association between the two scales indicated that the scales do not measure different aspects of functional outcome. Other studies [7, 11, 12] also suggest one strong and reliable factor representing one underlying dimension of functional limitations.
Our study raises questions on the unidimensionality of most subscales of the SIP-136, suggesting that these subscales are not appropriate for use in a polytrauma population with injury of the lower extremity.
The internal consistency of the SIP-136 in the present study was low for most subscales. To our knowledge, little information about the internal consistency of the 12 separate scales of the SIP-136 is available in the literature. One study was found that reports sufficiently high Cronbach's α for the separate categories , while two other studies assessing the Cronbach's α of the subscales of the SIP-136 reported low Cronbach's [14, 15]. High Cronbach's α from the total SIP-136 [5, 13, 16] and the physical and psychological dimension scores are reported . However, Cronbach's α is dependent on the number of items in a questionnaire, a high α coefficient of the sum scores of the SIP-136 is therefore not surprising and not informative.
Our findings supported the construct validity of the SF-36 and GARS, these findings are comparable with the literature [3, 11, 17, 18]. In our patient group, the construct validity of the SIP-136 was supported to a lower extent. Ho et al  found an advantage in using the SF-36 above the SIP because of its more robust construct validity, while others found some evidence to support the construct validity of the SIP [13, 15, 19].
The present study gives information about internal consistency and construct validity but does not provide information about other psychometric properties such as sensitivity to change over time and test-retest reliability. Additionally, other instruments may be suitable for this study population. Based on our results, further psychometric testing of the SF-36 and GARS in this population is recommended.