Urine pH and storage temperature for measurement of urine β₂MG

Abstract

It is known that β₂MG becomes a falsely low value due to acidic protease when urine pH is below 5.5. Therefore, the submission conditions for outsourced testing also state that “submit at pH 5.5 to 7.5,” but no method for pH adjustment is provided. Therefore, we aimed to clarify the relationship between urine β₂MG and pH and evaluate the appropriate submission method. The subjects were 26 outpatient specimens. Conditions for pH adjustment and differences in preservation methods (room temperature, refrigeration, freezing) were set, and compared the rate of decrease in urine β₂MG levels due to differences in the elapsed time until measurement. For samples with pH between 6.0 and 7.5, the rate of decrease in β₂MG after 24 hours was small. For samples with a pH of 4.7 to 5.5, the rate of decrease in β₂MG after 24 hours was greater in the order of room temperature storage, refrigerated storage, and frozen storage. These results indicate that urine β₂MG with a pH of 6.0 or higher is not easily affected by acid proteases, while urine β₂MG with a pH of 5.5 or lower is susceptible. In addition, in acidic urine, β₂MG decomposition was fastest when stored at room temperature, and it was thought that storage under freezing and then refrigerated storage would be less susceptible to the effects of acid proteases. Therefore, it was suggested that measurements should be taken promptly or urine pH should be confirmed immediately, and specimens should be processed and submitted for testing approp.