Absorption kinetics of rectal formalin instillation

Formalin instillation has become an accepted treatment of radiation-induced hemorrhagic cystitis and proctitis since the initial report by Brown in 1969 (Med. J. Aust. 1:23, 1969). Although its use is widespread, no studies have been performed to determine the safest, volume or duration of formalin exposure. The purpose of our study was to determine the optimum technique for instillation and the safety margin regarding the maximum time that formalin can be in contact with the rectal mucosa without causing serum toxicity. In a pilot canine study, 4% neutral buffered formalin was instilled into the rectum in 30 ml aliquots for 60 seconds each after which each aliquot was withdrawn; a total volume of 400 ml was used. Our subsequent experiment involved rectal instillation of a single formalin bolus of 100 ml for 1 hour without removal during this time. Formalin metabolites were measured in the blood and urine to assess toxicity. Results indicate that with the latter technique serum formic acid reaches toxic levels within 15 minutes of instillation and may stay elevated for several hours. Metabolites in the urine similarly increase within 15 minutes, lagging only shortly behind the rise in serum levels. Performing formalin instillation in a series of 30 ml aliquots appears to be a safer treatment, as toxic serum levels were not reached and their slight rise above baseline returned to normal within 3 hours.