From our results, self-standing Transpore, quartz plate and
Vitro-Skin are preferable for SPF tests due to a better correlation
between in vitro and in vivo measurements. To recover the linear
regression slope, an initial calibration must be performed for
different sunscreen products labelled with different SPF values.
Calibration should be performed for each instrumentation used,
each method adopted (substrates, amount of sunscreen applied
etc.), each type of sunscreen formula (lait, gel, etc.). Finally, the
results of our measurements demonstrate that, while a correlation
between in vitro and in vivo SPF results can be established, this
correlation is never exactly 1:1; the linear regression slope must be
always used to correct the data in order to use them for labelling.
Therefore in vitro SPF testing, regardless of substrate, is not a viable
substitute for in vivo SPF testing, and should not be used as such by
any regulatory bodies for ‘‘quality control’’ of sunscreens. Further
studies are needed in order to achieve a 1:1 correlation.