A patch test ensures your client does not react negatively to any product being used on or applied to their hair/scalp.
Reactions to products can range from very little to severe so it is always better to be safe than sorry!
To carry out a patch test –
Place a small amount of the product/products to be used on a sensitive area of skin.
Put a band-aid over this area and send your client home for the night, you will inform them to remove the band-aid the next day and to inform you of the results e.g. irritation and so on.
If any reaction occurs such as redness or irritation then do not use this particular product on that client and find an alternative.