According to ISO 10993-17, Allowable Limit (AL) is the largest amount of leachables that is deemed acceptable on a daily basis, when taken into the body through exposure to a medical device. This limit is expressed in dose to the patient for each applicable exposure period, and the units used are mass per unit time, e.g. milligrammes per day. The doses represent tolerable risks for medical devices under the circumstances of intended use.
Uncertainty factor (UF) provides a margin of safety for allowable human exposure, derived from measurements regarding the reaction of the most sensitive animal species in order to estimate an NOAEL or a LOAEL for humans.
Threshold of Toxicological Concern (TTC) allows for the estimation of safe exposure levels for chemicals where toxicological data are limited or non-existent. While it is mainly used for the evaluation of impurities in pharmaceuticals and food-/feedstuff, it can also be used for safety evaluation and toxicological qualification of substances found in or intentionally added to medical devices.
Lowest-observed-adverse-effect Level (LOAEL) and No-Observed(-Adverse)-Effect Level (NOAEL) data are frequently used to determine allowable limits for exposure to the leachable compounds. In several studies on different effects, relevant NOAEL (or LOAEL) is selected to form the basis of AL-calculations.
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