Clinical measures of disease severity and outcome in psoriasis: a critical appraisal of their quality

In clinical trials, a wide range of outcome measures has been used to evaluate the severity of psoriasis and its response to treatment. Despite their widespread use, many measures have received little attention with regards to their reliability and validity. Selecting an appropriately developed measurement tool is therefore of critical importance. We conducted a literature survey to examine the status of clinical outcome measures used in psoriasis research. The measures most commonly used were individual sign scores, e.g. for erythema, plaque thickness or scaling, and pooled indices, e.g. the Psoriasis Area and Severity Index. None of these, however, systematically fulfilled all the requirements of a validated instrument for disease assessment. Ideally, a core set of reliable and validated outcome measures for use in all psoriasis clinical trials is needed. Objective instrumental methods should minimise observer variation, but unless a simple non-invasive method can be developed, the uptake of such technology will probably be limited by cost and lack of practicality. Moreover, the translation of instrumental readings into clinically relevant measures is always a major problem, and for none of the methods has there been a robust mapping of instrumental readings on to a clinically meaningful scale. Further research is needed to determine the most appropriate and sensitive parameters to use as surrogate measures for capturing the distress which psoriatic patients feel but which is not measured with sufficient sensitivity or precision with current quality of life or distress questionnaires.