Long-term stability of 6-hydroxymelatonin sulfate in 24-h urine samples stored at-20 degrees C

The purpose of the present feasibility study was to determine whether the c oncentration of 6-hydroxymelatonin sulfate (6-OHMS) remains stable in urine samples stored over at least 15 yr. To test this, 117 twenty-four-hour uri ne samples were analyzed, which were obtained from healthy children ages 8 to 9 yr within the periods of 1985-1987,1991-1993, and 1997-1999. 6-OHMS co ncentrations were determined by enzyme-linked immunosorbent assay. The stat istical analyses clearly indicate that the concentration of 6-OHMS remains stable for at least 15 yr if the urine is stored at -20 degreesC.