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Drink Driving in the UK

Police Procedure: Back Calculations

Alcohol Back Calculations in Drink Driving Cases

What is a back calculation?

A back calculation is a process which is carried out by a forensic toxicologist in order to determine what a person's alcohol content was at an earlier point in time.

Back calculations can be carried out in cases where the analysis of evidential blood, breath or urine specimens indicate a level of alcohol in the specimen provided which is below the maximum legal prescribed limit but investigators have reason to suspect that at the time of the alleged incident the alcohol in a persons system would have exceeded the legal limit.

Back calculations are usually only used in connection with investigations into serious cases where a road traffic collision results in serious injury or fatality.

When a back calculation is being used in an investigation the police are required to fill out a MGDD/D form, this form is used to provide forensic toxicologists with details that will aid them in performing the back calculation. The details include a persons:

  • sex
  • age
  • height
  • weight
  • build

Other information to be included in the form includes:

  • Time of incident
  • Time any samples were taken (breath, blood or urine)
  • Details of food consumption
  • Details of any medical conditions
  • Details of any medication taken
  • Details of alcohol consumed including quantities and times

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