What does a patch test help determine before a chemical service?

A patch test is a crucial step before proceeding with any chemical service, particularly those involving hair color or chemical relaxers. It serves to identify any potential allergic reactions a client may have to the products being used. By applying a small amount of the product to a discreet area of the skin, typically behind the ear or on the inner arm, the cosmetologist can monitor for any adverse reactions, such as redness, itching, or swelling, which could indicate an allergy. This proactive measure is essential for ensuring the safety and well-being of the client, allowing the professional to make informed decisions about proceeding with the treatment.

The other options do not directly relate to the purpose of a patch test. Hair density pertains to the thickness and number of hair strands but has no bearing on allergic reactions. A client's budget is irrelevant to how their skin reacts to a product, while the desired outcome of the service relates to client preferences and expectations, rather than safety assessments associated with possible allergies.