The classic patch test (apply to inside wrist for 24 hours) underestimates face reactions because wrist skin is far less sensitive than face. Behind-the-ear or jawline patch testing predicts much more accurately.
Why wrist underpredicts
Wrist skin is thicker, less vascular, less reactive. Many products that test fine on wrist trigger redness or breakouts when applied to face. The wrist test usually catches only severe allergic reactions, not the common irritation patterns.
Better test sites and timing
Apply behind the ear or on jawline. Wait 48 hours minimum, ideally 72 hours. Apply twice to test for delayed reactions. Mild itch or redness at the site means the product won't suit your face either.
Spend the extra two days. Discovering a reaction on a small jawline patch is much cheaper than discovering one across your whole face.