Sensitization testing establishes the potential of the test article to elicit an allergenic response. In particular, the testing is aimed at a delayed-type hypersensitivity response. This is an immune response that takes a couple of days to develop. Poison ivy is a good example of a delayed-type hypersensitivity response.
Two primary test methods are used for medical devices to satisfy the sensitization testing requirement. The first is the guinea pig maximization test (GPMT); the second is the closed patch test, also known as the Buehler. Due to the use of an adjuvant to stimulate the immune system, the GPMT has been considered the more sensitive test. The GPMT has intradermal injections in the beginning of the study and topical applications at the conclusion. The Buehler consists of all topical applications. Due to the intradermal injections in the GPMT, the requirements specify that the test article be a liquid, suspendable powder, or extract. The Buehler is better performed with surface contact devices as they are used, if possible, and chemicals.
The third commonly used sensitization test method is called the mouse local lymph node assay (LLNA). The LLNA is an acceptable test method, but device manufacturers have been using the guinea pig tests for decades and are satisfied with the results obtained using those methods. The LLNA, like any test, has its pros and cons. It is shorter in duration, uses animals of a lower phylum, and requires far less test material; those factors can be appealing. On the other hand, it generates radioactive waste and is not accurate in distinguishing a sensitization response from an irritation response, which increases the possibility of a false positive. In addition, because the animals are sacrificed to harvest the lymph nodes, there is no opportunity to rechallenge if a question arises.
When proceeding with any sensitization study, it is necessary to know the irritancy potential of the test article. As already mentioned, the LLNA may confuse irritancy with sensitization. In the two guinea pig tests, the animals are scored for sensitization by evaluating erythema and edema (redness and swelling). Coincidentally, redness and swelling are also indicators of irritation. So it may appear that the GPMT and the Buehler could give a false positive to an irritating test article. But these studies have safeguards to prevent this. They are designed with primary irritation animals that receive varying concentrations of the test article all on one day at the same time. Because the animals have not been exposed to the test article before this point, any redness or swelling is considered an irritation response and not an allergenic response. These results are then considered when preparing the dosing solutions for the main part of the study. At the challenge dose, which is the final determining dosing, the animals are dosed at the highest concentration determined to not produce any irritation; any observation noted will be considered an allergenic response.