Instability of corticotropin during long-term storage - myth or reality?

Objectives: Corticotropin is notorious for its instability. Whereas several studies have investigated its short-term stability in plasma following venous blood sampling, studies on long-term stability are lacking. Here we investigated the long-term storage stability of corticotropin in ethylenediaminetetraacetic acid containing plasma. Methods: Specimens from healthy volunteers (neat, spiked) were stored in polypropylene microcentrifuge tubes with socket screw-caps at -20 degrees C and -70 degrees C for up to one and a half years. Corticotropin in plasma was measured using an Abbott research only immunoassay. Separately, specimens from patients were collected during diagnostic routine testing and stored in polystyrene tubes with push-caps at -20 degrees C for up to 6 years. In these samples corticotropin hormone was measured using the Diasorin corticotropin immunoassay. Results: Storage of specimens at -20 degrees C or -70 degrees C for up to one and a half years showed minimal changes (<11%) in corticotropin levels, while storage of patient samples at -20 degrees C for up to 6 years showed a significant (54%) reduction in corticotropin levels. Conclusions: Corticotropin levels are stable in plasma when stored at -20 degrees C for one and a half years using the Abbott research only assay, but with longer storage time a significant reduction in corticotropin levels can be expected. Once specimens are stored for future corticotropin measurements, one should consider storage time, storage temperature and assay differences.

Automated summary

This study investigated the long-term storage stability of corticotropin in plasma. The results showed that corticotropin levels remained stable when stored at -20 degrees C for one and a half years, but a significant reduction was observed in patient samples stored for up to 6 years at the same temperature.