What is the main purpose of a patch test before a chemical service?

The primary purpose of conducting a patch test before performing a chemical service is to check for allergic reactions. This test involves applying a small amount of the chemical product to a discreet area of the client's skin—usually behind the ear or on the inner arm—to observe any potential adverse reactions. Allergic responses can vary from mild irritation to more severe reactions, which could pose serious health risks. By identifying any allergies prior to the full application, cosmetologists ensure the client's safety and help to prevent potentially harmful outcomes.

The other options address different aspects of hair and service evaluation but do not relate to the necessity and function of a patch test specifically. Determining color effectiveness involves assessing how well a dye interacts with the hair, evaluating hair strength focuses on the integrity of the hair structure, and assessing client satisfaction is about the client's overall experience and outcome after the service, none of which are relevant to identifying allergic reactions to a product.