Characterization of LED-based hybrid diffuse reflectance spectroscopy method for determination of SPF and UVA-PF in blinded multi-centre study (ALT-SPF).

Objective: The consortium ALT-SPF performed an international round robin test to characterize non-invasive methods as alternatives to the erythema-based testing of sun protection factor (SPF) according to ISO 24444:2019.

Methods: Hybrid diffuse reflectance spectroscopy (HDRS) based on a multi-lambda LED light source uses in vivo reflectance spectra on skin to determine sunscreens in vivo absorbance spectra, which are fused with respective in vitro absorbance spectra measured as thin films transmission as described in ISO 24443:2019. As a part of the ALT-SPF consortium initiative, a blinded study on 64 samples was performed in four European laboratories. After further improvements of the method, a blinded re-evaluation based on 16 samples was performed. Five statistical acceptance criteria for new methods were assessed by an independent statistical institute to compare the obtained results to the reference methods for SPF and UVA-PF.

Results: The initial ALT-SPF study 1 showed that the bias criterion was acceptable, while the reproducibility and interlaboratory variability needed further improvement. The re-evaluation study 2 showed that the reproducibility and interlaboratory variability could be considerably improved. Using only n = 10 volunteers and a bias correction based on the initial ALT-SPF study 1 data, the SPF results of the re-evaluation study 2 were close to the acceptance criteria of the ALT-SPF study with criterion 1 (reproducibility) and only 11% over the limit defined by the performance of the gold standard. The UVA-PF results were within the acceptance limits for the acceptance criteria, except criterion 3 being in the 'almost met' range. The re-evaluation study indicates that the method has a comparable precision to the gold standard methods ISO 24444 and ISO 24443.

Conclusion: This study showed that the LED-HDRS method is capable of providing reasonable non-invasive SPF and UVA-PF results and that the performance shows close alignment to the reference method. Thus, it can be proposed as an alternative method to the current standards ISO 24444 and ISO 24443.