A study of longer-time stability of plasma free metanephrines

Previous stability studies of plasma free metanephrines do not extend beyond one month. For retrospective evaluation and documentation purposes, knowledge of stability for more prolonged storage periods is required.

A panel of seven plasma samples was aliquoted and stored at -20 and -80°C. Aliquots were thawed and assayed by high-performance liquid chromatography with electrochemical detection at regular intervals during three years. A final set was assayed after five years at -80°C. Results were evaluated by repeated-measures analysis of variance.

After a stable period of over one year, an upward trend for plasma free metanephrine and a downward trend for normetanephrine was observed. For both analytes, measurement results were parallel between samples during the study. Concentrations in samples stored at -20°C did not differ from those stored at -80°C.

Storage at either -20 or -80°C must be considered as safe for at least one year. The residual variation with respect to time of the concentrations of metanephrines was identical in all of the samples assayed and almost did not exceed previously determined within-run variation. This suggests that between-assay variability is the cause of the overall trends and not sample deterioration. Moreover, between-assay variability manifesting itself as drift or trend remained within the range of earlier observed between-assay variation. Despite this, the assay for plasma metanephrines remains capable of detecting catecholamine overproduction in plasma samples that have been stored for prolonged periods of time.