What purpose PDE is used in the context of cleaning validation?

PDE is used to determine the acceptable amount of a previous drug that may be allowed in the next drug as carry over cross contaminant. This acceptable level is called MACO (Maximum Allowable Carryover). The MACO is then used for the calculation of the maximum safe surface residue or rinse concentration (the operational cleaning limit), which takes into account the shared equipment surface areas or volumes of the rinse of equipment between products A and B. MACO = PDEPrevious x MBSnext TDDnext Where, MACO (Maximum Allowable Carryover): Acceptable transferred amount from the previous product into your next product (mg). PDE- Permitted daily exposure. MBSnext- Minimum batch size for the next product(s) (where MACO can end up) (mg) TDDnext: Standard therapeutic daily dose for the next product (mg/day). As evident from the equation, larger the PDE value, higher will be MACO. This means more amount of a relatively safe drug may be allowed as a cross-contaminant while highly toxic compound can be allowed only a minute amount in the next product.