Preservation of Blood Ammonia by Rapid Freezing--a Method for Delayed Determinations.

A method is described for preservation of blood for delayed determinations of ammonia by rapid freezing of the samples in a dry ice and acetone solution followed by storage of the specimens in the frozen state. The method was evaluated by first establishing the reproducibility of the chemical method used for blood ammonia analysis in 714 consecutive samples of dog blood which were analyzed in duplicate, and then determining whether freezing caused greater changes in ammonia concentration than one would expect knowing the reproducibility of the chemical method. Studies in 200 samples of dog blood with ammonia levels of 16 to 2904 g./100 ml., each of which was analyzed immediately and after 24 hr. of freezing, showed that freezing and storage for 24 hr. did not significantly affect the blood ammonia content. Studies of 22 samples of frozen dog blood with ammonia levels of 11 to 2624 /Lg./100 ml., parts of each of which were analyzed daily for 5 consecutive days, showed that storage in the frozen state for up to 72 hr. had no significant effect on the ammonia concentration, but that storage for 96 hr. or longer resulted in increases in blood ammonia beyond acceptable limits. It is concluded that accurate delayed measurements of blood ammonia may be accomplished by rapid freezing of the blood immediately after shedding, and storage of the frozen samples for up to 72 hr.