Background  Loss of function FLG alleles were first identified as causative of ichthyosis vulgaris (IV) and were subsequently found to be major predisposing factors for atopic dermatitis (AD) and atopic disorders.

Objectives  To identify independent factors associated with the clinical IV phenotype in adult caucasian patients with AD and to assess the performance of a global clinical severity score of IV in predicting common FLG null mutations.

Patients and methods  This was a prospective study conducted from January 2007 to June 2008. Adult patients attending the department of dermatology with a diagnosis of AD with or without IV were eligible to participate. For each patient, five clinical signs of IV were scored from 0 to 3 – diffuse xerosis, hyperlinearity of palms, scales on legs, scalp desquamation and keratosis pilaris – and a global IV clinical severity score was derived (0–15). Age of onset of AD, SCORAD (SCORing of Atopic Dermatitis), family and personal history for other signs of atopy, and total immunoglobulin E were recorded. Genotyping was performed for R501X and 2282del4. Univariate and multivariate analysis for factors associated with AD or AD + IV were conducted.

Results  In univariate analysis, family history of atopy, global clinical severity scoring and 2282del4 FLG mutation were positively correlated with the AD + IV phenotype. Using multivariate analysis, SCORAD for AD (OR 0·94, = 0·01) and global clinical severity scoring for AD + IV (OR 2·62, < 0·0001) were found to be independent factors.

Conclusions  The 2282del4 FLG mutation was confirmed as a good marker of early-onset disease. Moreover, our global clinical severity score yielded a good negative predictive value of common caucasian null FLG mutations.