The human repeated insult patch test in the 21st century: a commentary.

The human repeated insult patch test (HRIPT) is over half a century old, but is still used in several countries as a confirmatory test in the safety evaluation of skin sensitizers. This is despite the criticism it receives from an ethical perspective and regarding the scientific validity of such testing. In this commentary, the HRIPT is reviewed, with emphasis on ethical aspects and where the test can, and cannot, contribute in a scientifically meaningful manner to safety evaluation. It is concluded that where there is a specific rationale for testing, for example, to substantiate a no-effect level for a sensitizing chemical or to ensure that matrix effects are not making an unexpected contribution to sensitizing potency, then rigorous independent review may confirm that an HRIPT is ethical and scientifically justifiable. The possibility that sensitization may be induced in volunteers dictates that HRIPTs should be conducted rarely and in cases where the benefits overwhelmingly outweigh the risk. However, for the very large majority of HRIPTs conducted concerning the risk of skin sensitization, there is neither scientific justification nor any other merit.