Performance assessment of the Double Plate method (ISO23675) in ALT-SPF Consortium: A highly reproducible and accurate in vitro method to determine SPF.

Objective: To perform a comprehensive statistical characterization of the Double Plate method (ISO 23675) as a potential in vitro alternative to the in vivo reference method (ISO 24444:2019) for determining the sun protection factor (SPF).

Methods: Five qualified laboratories were involved in testing the Double Plate method, as part of a large-scale ring test conducted by the International Consortium 'ALT-SPF'. About 32 sunscreen products, reflecting diverse global formulations, were tested using both the in vivo reference method and the Double Plate method. This method employs spectrophotometric measurements of sunscreen applied to UV-transparent plates with standardized robotic application to minimize the variability. A mathematical adjustment was introduced to address the initial bias compared to the reference method and further assessed in a follow-up ring test with 12 products, including ISO standards.

Results: The Double Plate method exhibited very high intra- and inter-laboratory reproducibility throughout the study, consistently meeting the precision criteria set by ISO/TC217/WG7. Following the mathematical adjustment, the accuracy significantly improved, demonstrating a close alignment with the reference method.

Conclusion: The Double Plate method demonstrates excellent reproducibility and, following optimization, high accuracy for the SPF determination. These findings endorse the Double Plate method as a robust and ethical in vitro alternative to the in vivo reference method for SPF testing. This paved the way for its publication as a new ISO standard (ISO 23675), in December 2024, supporting consumer safety and informed product choices.