Patch Testing: Sensitization vs. Irritation

In the personal care marketplace, we often hear of products advertised “safe for sensitive skin,” usually interpreted as less likely to irritate the skin. This may be both a cause and a result of some confusion regarding the terms “sensitization” and “irritation” in common language – confusion that sometimes crops up in even professional discussions of product testing and marketing claims. It doesn’t help that both sensitization and irritation are termed “contact dermatitis” and that they share symptoms in common. Physiologically, however, sensitization and irritation are very different.

Understanding Irritation

“Irritation” describes temporary damage to epidermal cells caused by direct contact with a substance, including some ingredients in cosmetics and personal care products. Irritation – also known as irritant contact dermatitis – may be characterized by temporary, redness, dryness with itching, flaking, peeling, rash, or some combination of these symptoms at the site of exposure.

For one example, excessive concentrations of certain surfactants can cause these symptoms at the site of exposure. Some skin may be “more sensitive” to a substance, some less so. After exposure to the same irritant, symptoms may vary in type, severity, or duration from one person to another or from one exposure site to another. But contact with the same substance is likely to produce irritation in everyone. And once contact with the irritant stops, the symptoms will clear, the discomfort will pass, and the site will heal without lasting physiological, immunological change.

Understanding Sensitization

“Sensitization,” on the other hand, describes an inflammatory immune response that manifests only in some people and then frequently after repeated exposure. Sensitization – also known as allergic contact dermatitis – usually results in a red, itchy, bumpy rash and may be mistaken for irritation. The difference is in the physiology behind the symptoms.

In short, sensitization occurs during exposure to an irritant, and then it goes away. Sensitization is caused not by the product, but by the body’s immune system. Allergic contact dermatitis is generated in two phases: induction and elicitation. Induction phase is the formation of specialized proteins in the immune system after exposure to the product. Elicitation is the reaction of these new proteins to subsequent exposure, commonly known as an allergic response. Symptoms may not be limited to the exposure site. Not everyone will become sensitized to the same substance; those who do become sensitized may also be sensitized to chemically similar substances (think poison ivy and poison oak).

How Patch Testing Helps to Understand Sensitization and Irritation

Patch testing exposes a small area of the skin to a product to predict how the skin will respond. The patch test may be part of a clinical trial to assess product safety or define claims prior to a product introduction. In some cases, product instructions may advise the individual consumer to conduct patch testing prior to product use.

There are a number of standard patch test protocols to choose from. You may choose to employ a combination of methods or develop a custom trial to ensure a complete safety analysis. Primary skin Irritation trials, such as 48-hour patch testing, assess subjects’ responses to a single product exposure, measured at intervals during and after exposure. Cumulative irritation trials measure the skin’s response to prolonged repeated exposure. Repeat Insult Patch Tests (RIPT’s) assess sensitization potential by measuring subjects’ allergic reactions following repeated exposure over three to four weeks.

Unexpected skin reactions significantly influence consumer perception of your product, and likewise can significantly influence the value of your brand as well as your product liability risk. FDA requires that manufacturers substantiate product and ingredient safety of cosmetics, drugs, and devices before going to market. Patch testing is a simple and effective way to evaluate your product’s potential to cause irritation or sensitization.

To learn more about sensitization and irritation, and to discuss how patch testing clinical trials at CPT℠ can help support your marketing programs and consumer health interests, contact us at (973) 609-5747.

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