A good scar assessment scale should meet basic clinical requirements. The most relevant requirements are listed here.
A scar assessment scale has a good internal consistency if all items concern the same topic, in this case ‘scar quality’. All items should correlate substantially to each other as they all should correlate to ‘scar quality’. On the other hand, there should not be too much overlap and correlation between the items. Then, there may be redundancy of items.
A scale has a good reliability if the same ratings are obtained on repeated assessments. Reliability is also referred to as ‘agreement’. The reliability can be tested between assessments of different observers performed at the same time (inter-individual reliability) or by one observer performed at the different timepoints assuming that no change in scar quality has occurred (intra-individual reliability). A high reliability is necessary to come to the maximum possible validity.
A scale has a good validity if it measures what it was intended to measure. Validity is also referred to as ‘accuracy’. Only few scar assessment scales have been subjected to validity tests with varying results. This may be partially explained by the lack of the ability to test it against the actual (‘true’) value or even an approximation of this value (the ‘gold standard’). It clearly complicates the validation of new scales and forces one to focus on the ‘content’ and ‘concurrent’ validity of new scales. The ‘content validity’ of a scar scale concerns the extent to which the scar features are comprehensively sampled by items of the scale. The ‘concurrent validity’ concerns the extent to which ratings of the scale relate to what is considered as the best criterion (‘gold standard’) that is available.
Besides being reliable and valid, an instrument should be feasible and able to identify a change of scar characteristics that refers to responsiveness of a scale. This, and the minimal clinically important difference (MCID) are requirements that are often considered as relevant but sparsely tested for any of the scar assessment scales so far.