An anti-passback algorithm for an access control system is described. The anti-passback algorithm prevents the use of valid credentials to gain access to an access-controlled area by more than one person within a given period of time. The algorithm is capable of distinguishing between credentials intentionally presented to the access control system and credentials that are unintentionally read by the access control system. Certain variables may be set by the access control system manufacturer or a trusted individual to adapt the algorithm for applications.