Sensitive skin involves a lower threshold for sensory reactions to a number of often non-specific, different factors. The concept is poorly defined, but the symptom is very common and hence of relevance to skin testing. A number of tests have been used to study it, and because of the absence of inflammation in this condition, the core test is the sensory test. This test exposes a panel of test persons to potentially stinging substances, usually lactic acid, in a standardised setting; the response is assessed within minutes of application at predetermined times, and the subjective response is graded on a simple scale. The sensitivity of the test person is inversely correlated with the time to reaction and the concentration of the test substance, that is, reactions are seen sooner and to weaker concentrations in the sensitive. Similarly, an early strong reaction is seen even in less sensitive individuals if a stronger irritant is used.