Epidermolysis bullosa (EB) is a group of rare and currently incurable genetic blistering disorders. As more pathogenic‐driven therapies are being developed, there is an important need for EB‐specific validated outcomes measures designed for use in clinical trials.

To test the reliability and construct validity of an instrument for scoring clinical outcomes of research for EB (iscorEB), a new combined clinician‐ and patient‐reported outcomes tool.

We conducted an observational study consisting of independent 1‐day assessments (six assessors) at two academic hospitals. The assessments consisted of iscorEB clinician (iscorEB‐c), Birmingham Epidermolysis Bullosa Severity (BEBS) and global severity assessment for physicians; and iscorEB patient (iscorEB‐p), Quality of Life evaluation in Epidermolysis Bullosa and Children's Dermatology Life Quality Index for patients. Construct validity and intraclass correlation coefficients (ICCs) for interobserver, intraobserver and test–retest reliability were calculated.

Overall, 31 patients with a mean age of 19·5 years (1·8–45·2) were included. Disease severity was mild in 42% of cases, moderate in 29% and severe in 29%. The interobserver ICC was 0·96 for both the clinician‐reported section of iscorEB‐c and BEBS. The ICC for intraobserver reliability was 0·91 and 0·70 for the skin and mucosal domains of iscorEB‐c, respectively. Cronbach's alpha for iscorEB‐c was 0·89. The test–retest reliability of iscorEB‐p was 0·97 and Cronbach's alpha was 0·84. The clinical score differentiated between subjects with mild, moderate and severe disease, and both clinical and patient subscores discriminated between recessive dystrophic EB and other EB subtypes.

iscorEB has robust reliability and construct validity, including strong ability to distinguish EB types and severities. Further studies are planned to test its responsiveness to change.