Post coital interval is not usually determined from:-

POSTCOITAL INTERVAL (PCI) : It is the time lapsed since last act of sexual intercourse (time interval between the deposition and collection). PCI is determined from the persistence of the seminal materials--spermatozoa, P30, SAP, PGM and Pep A depending upon their deferential stability. Thus, a significant level of P30 tends to be lost within 24 hr of deposition in the vaginal vault. SAP (serum alkaline phosphatase) is normally lost 48 hours postcoitus, PGM drops below threshold by 6 hours, Pep A is not usually recovered after 3 hours. Spermatozoa do not normally persist after 72 hours.