Disparate SPF Testing Methodologies Generate Similar SPFs. II. Analysis of P2 Standard Control SPF Data

In the original scientific publication evaluating sunscreen methodologies, Garzarella and Caswell showed there to be no clinically significant or statistically significant difference in the average Sun Protection Factor (SPF) of a sunscreen formulation between any of three methodologies, Food and Drug Administration (FDA) Final Monograph, Australia/New Zealand, and European Cosmetics Association (COLIPA) International, suggesting that any differences in methodology were insignificant in the resulting SPF determined. These three major older methodologies have coalesced into two methodologies, 2011 FDA-Final Rule and ISO 24444, so that current sunscreen SPF testing is mostly 2011 FDA-Final Rule and ISO 24444. Another approach to evaluating the impact of methodological differences in sunscreen testing is to compare data on a control standard or reference sunscreen. If the difference between the two SPF values of P2 is statistically significant for the two different methodologies, then this would present evidence for a clinically significant difference in the SPF value between the two methodologies. For 2011 FDA-Final Rule, the expected SPF of P2 is 16.3 ± 3.43; for ISO 24444, the expected SPF of P2 is 16.1 ± 2.42. Using least squares average and standard error on 952 observations, the 2011 FDA-Final Rule SPF of P2 is 15.4 ± 0.12; using least squares average and standard error on 1,551 observations, the ISO 24444 SPF of P2 is 15.6 ± 0.10. The data described herein indicate no clinically significant nor statistically significant difference between the SPF average of P2 using the 2011 FDA-Final Rule methodology versus that using ISO 24444 methodology. Further statistical analysis indicates that the average SPF of P2 is independent of solar simulator type, time of year (month), age of subject, gender of subject, or Fitzpatrick Skin Phototype of subject. A statistically significant negative correlation was found between a subject's SPF of P2 and the subject's unprotected minimal erythemal dose. The implications of this relationship on SPF testing are explored.