How Topical Corticosteroid Potency is Measured
Topical corticosteroid potency is primarily measured using the vasoconstrictor assay (also called the Stoughton assay), which assesses the ability of the steroid to cause skin blanching through vasoconstriction, though this method has significant limitations in predicting actual clinical efficacy. 1, 2
Primary Measurement Method: Vasoconstrictor Assay
- The vasoconstrictor assay is the most widely used method for ranking topical steroid potency and forms the basis of the 7-class classification system used by the American Academy of Dermatology 1, 3
- This assay measures the degree of skin blanching (vasoconstriction) produced when the corticosteroid is applied to the skin, with greater blanching indicating higher potency 2, 4
- The mechanism involves binding to intracellular corticosteroid receptors and regulation of gene transcription, producing vasoconstrictive effects that can be visually assessed 5
Classification System Based on Vasoconstrictor Activity
The 7-class potency ranking system ranges from ultra-high to low potency 1:
- Class 1 (Ultra-high/Superpotent): Clobetasol propionate 0.05%, halobetasol propionate 0.05% with efficacy rates of 58%-92% 5, 1
- Class 2 (High/Potent): Amcinonide 0.1%, betamethasone dipropionate 0.05%, fluocinonide 0.05% with efficacy rates of 68%-74% 5, 1
- Classes 3-4 (Medium potency): Efficacy rates of 68%-72% 5, 1
- Classes 5-7 (Low potency): Hydrocortisone 1% and 2.5% with efficacy rates of 41%-83% 5, 1
Critical Limitations of Vasoconstrictor Measurement
The vasoconstrictor assay has substantial limitations and does not reliably predict clinical efficacy. 3
- Clinical outcome rankings corresponded with vasoconstrictor rankings in only 62% (11 of 17) of topical steroid preparations studied 3
- Therapeutic index rankings (ratio of efficacy to systemic safety) did not correspond with either clinical outcome (33%) or vasoconstrictor assay (33%) rankings 3
- While some evidence suggests a recognizable correlation exists between vasoconstrictor potency and therapeutic efficacy, this relationship is inconsistent 2, 3
Alternative Measurement Methods
Beyond vasoconstriction, potency can be assessed through 3, 4:
- Clinical outcome measures: Direct assessment of therapeutic efficacy in treating specific dermatoses
- Therapeutic index: Ratio of clinical efficacy to systemic safety profile
- Cost-effectiveness analysis: Balancing efficacy, safety, and economic factors
- Cellular activity assays: Measuring effects on Langerhans cell function and immune modulation 6
Clinical Implications of Measurement Methods
- The lack of standardization in clinical trials makes comparison among preparations difficult 3
- Topical steroid rankings should not be based solely on vasoconstrictor assays, as they fail to capture the full therapeutic profile 3
- Factors affecting actual potency include vehicle formulation, integrity of the epidermal barrier, use of occlusive dressings, and anatomical site of application 2, 7
- The correlation between vasoconstrictor potency and effects on immune function (such as Langerhans cell reduction) has been demonstrated, supporting some validity of the assay 6