The lack of a universal classification system for potency of topical corticosteroid means the categories used by different countries and jurisdictions are inconsistent and potentially misleading, a review has found.

With over 200 different topical steroid formulations worldwide there is a need for a simple, robust and user-friendly system to classify them according to their potency in real-world clinical practice, according to a new paper published in JAMA Dermatology.

The authors say that despite their widespread use in dermatology, there has been little discussion or investigation around the use of topical steroids, with the last update to US guidelines being more than a quarter of a century ago.

After reviewing the different classification systems used for potency of topical steroids, they found major discrepancies between three major systems because they ranked potency in different ways.

The Anatomical Therapeutic Chemical (ATC) classification system of the World Health Organization (WHO), for example, graded potency based only on the pharmacological action of the primary ingredient. The four categories (mild, moderate, potent, very potent) did not take into account factors such as formulation and concentration, which would have a major impact on efficacy and toxicity in clinical use, they noted.

It was therefore not surprising that there was often little agreement in potency categories used in other classification systems based on vasoconstrictor assays, they noted, such the British National Formulary (BNF) criteria recently used in a Cochrane Review of topical steroids for eczema.

The BNF categories were more useful for clinical practice because the pharmacodynamic measures reflected factors such as vehicle used and the concentration of the steroid molecule in the product.

In their comparison of the two system they found only weak agreement between potency categories, with only two thirds (64%) being concordant one in five of the ATC categories being higher in potency than those used in the Cochrane review.

The researchers also reviewed a seven category potency classification system from US lists of topical steroids, also using results based on vasoconstrictor assays.  This would better reflect the continuous nature of topical steroid potency than a four-category system but may be difficult to interpret because the uncertain clinical significance of difference  in category levels, they said.

Again there was only modest (54%) concordance with the ATC-based WHO potency categories, they found, and 38% of the US potency classifications were lower than those assigned by the ATC.

“For example, in the US classification, betamethasone dipropionate, 0.05%, is classified as potent as an ointment and upper midstrength as a cream; betamethasone valerate, 0.05%, is classified as midstrength as an ointment and lower midstrength as a cream; the ATC system classifies these formulations together as potent,” they wrote.

Variation across classifications could also limit the comparability of findings across studies, they said. For example, fluocinolone acetonide 0.01% cream was categorised as ‘mild’ in the US system, ‘moderate’ in the Cochrane review classification, and ‘potent’ in the ATC system.

In an accompanying editorial, dermatologist Dr Joerg Albrecht of Rush Medical College, Chicago, said the current potency assignment of topical steroids was unsatisfactory and they suggested the comprehensive US-based list be and modified used as the basis of a more universal system.

“The four steroid strengths in the collapsed version of the 7-category US system that is used for the comparison likely better reflect what dermatologists do, choosing only a few different formulations to cover the potencies that are commonly needed in clinical practice,” he wrote.

“Simplifying the number of available topical steroids and the steroid classification systems to better reflect clinical practice may help in both research and clinical care settings,” he added.

In the meantime, until a single classification system is universally accepted, authors of forthcoming research studies “need to explicitly state how they categorised topical steroid potency” to avoid problems with replication, Dr Albrecht advised.