[Musings] Sunscreen Testing: Research Showing Different Results Between in vivo vs. in vitro

 

 

Link to doi: 10.1111/ics.12377 on PUBMED here

Skin isn't paper. It isn't a potato. It isn't a sticker. It isn't plastic. There's...a lot going on with your largest organ - melanin, for one. There are also sunscreens that work with human sebum to increase the sunscreen's protective abilities, like Allie's ADVAN and Anessa's Auto Booster.

The back of Anessa's Day Serum Trial Set. Left is for their Day Serum, Right if for the Perfect UV Milk.The blue box also talks about their Auto Booster Tech

Obvious, right? You'd think fearmongerers and even some UV sticker inventors would have known too 🤷🏻‍♀️Warning: responses even more underwhelming than what you would hope for

I came across the 2016 research pictured above (and summarizing below) randomly and it's always nice to see a chart so plainly lay it out for you. If you want to just skip below to the chart, I get it, but I think knowing a bit about the work put into the research makes it much more interesting.

I want to emphasize that this was done in 2016, which at the time, ISO 24444:2010 was the in vivo standard, it is currently ISO 24444:2019. If you're interested, these are the changes from 2010 to 2019:

Purpose 

There are two ways to test sunscreens - during the R&D process or for rating: in vivo (on human skin and therefore human participants) and in vitro (not human, usually a tape or plate). 

in vitro data does not support "reproducibility and correlation to in vivo results" so in vivo is still the gold standard, but it's expensive and there's the humane aspect to consider because you're literally burning humans, especially as sunscreens become more complex (eg: boosters, film-forming ingredients). The researchers' stated goal was that they wanted to find if a "simple and cheap in vitro method could be optimized in order to provide both statistically repeatable and predictive SPF measurement".

The Test

They tried two different in vitro methods: 

• Method A: Transpore tape (surgical 3M tape used in the Diffey-Robson's method)
• Method B: sand blasted PMMA plates (used in the ISO 24443:2012 method for UVA photoprotection)
  

For each method and each product, they used a spreading pressure 100g and 200g to see if that would influence the consistency of results. They go into a bit more about how those tests are carried out, what affects the accuracy and reproducibility of the test, how they calibrated, etc. that I don't want to go too into because it's going deeper than my main point.

They used 75 sunscreen products of varying textures and filters ranging from low to very high SPF protection (labeled SPF 6 to 100+) that were purchased or gifted to their lab from public pharmacies "from public pharmacies in EU, Canada, US, and Australia". They list the findings of all 75 products, I've included the first 20 here:

CI% is the Criteria of Interpretation which evaluates the repeatability of data based on ISO Standard 24443 parameters which established that "CI [%] value should not be greater than 17%, otherwise the result should be considered not statistically significant." The CI% is listed for ISO Standard 24444.

Of those 75 products, they chose 11 products (remember, they're burning people so they can't test them all) and tested them in vivo (according to ISO 24444:2010 standards) on 10 human volunteers: male and female, ages 20-35, Fitzpatrick I, II, III.

They found that Method A of both pressures produced more statistically correlated results between in vivo and in vitro, but the PMMA plates (Method B) provide more repeatable data, with Method B2 ultimately being the "most reliable according to the data repeatability and accuracy."

I should also note that in their conclusion, they added

The present study was conducted entirely in our laboratory, in highly standardized operating conditions with regard to the operator, environmental conditions, the substrates used and the instruments, in order to evaluate whether the application pressure of the sample on the substrate and the intrinsic characteristics of the formulations could affect the result...it is possible for a single laboratory to optimize internal methods and protocols to achieve repeatable and predictive in vitro results, as we demonstrated in this work, while it is extremely difficult to make methods reproducible and equally reliable in different laboratories.

The Chart

 

Enlarge the chart to see the SPF differences.

Look at the SPF differences between in vivo vs the in vitro methods, particularly products on the higher in vivo SPF end.

It's important to ask what testing method was used when anyone claims that a sunscreen isn't meeting labeled protection. If they're not literally burning people according to the most up-to-date industry standard, approach with skepticism (looking at you and your myriad of issues, HKCC).