In vitro and in vivo sPF The in vitro SPF values for the formulations tested were not significantly different. This may be because of the limitations of the spectrophotometric method in assessing the interactions of the formulations with the skin. This result is based on the total concentration of the ultraviolet filter in the formulations. The in vitro SPF test is rapid and cost-effective, and can estimate the SPF of formulations prior to in vivo SPF tests in humans.