Glycated lysine increases in serum samples stored at -20 degrees C

The glycated lysine in serum protein was determined by HPLC measurement of furosine in hydrolyzed serum, with synthetic N alpha-Boc-N epsilon-(1-deoxy-D-fructosyl)-L-lysine as the standard. The glycated lysine value increased with time of storage when the serum was kept at -20 degrees C, possibly because of an ongoing reaction between glucose and the epsilon-amino groups of lysine in the frozen serum. The problem was eliminated by using storage at -70 degrees C or by acidifying the serum before storing it at -20 degrees C.