Each blood sample taken at a particular private clinic is given an 8-digit reference number. Example: 12404360 This was the reference number of the 36th sample taken on 4th September 1993 (124 means the 124th month of operation of the clinic, since opening in June 1983 month 001). The final digit is a security check digit, and has the value that makes the sum of the eight digits a multiple of ten.