A blood sample for procainamide should be centrifuged and the serum separated from the red blood cells promptly. If left in contact with red blood cells the procainamide diffuses into red blood cells where it is metabolized to N-acetylprocainamide (NAPA) and other metabolites.
Consequences of keeping serum in contract with red blood cells:
(1) lower serum levels of procainamide
(2) higher serum levels of NAPA
(3) lower values for procainamide plus NAPA (due to generation of other metabolites)
The impact on serum procainamide levels is affected by:
(1) the length of time that the serum is left in contact with the red blood cells
(2) the temperature of the specimen (faster at 25°C compared to 5°C)
(3) the patient's acetylator status (rapid vs slow)
A classic situation for misleading results would be whole blood tube drawn in a physician's office and not processed for a day or two during the summer.
