ACTH stability depends on different preanalytical procedures

It is commonly believed that ACTH in blood is highly unstable due to proteolytic degradation. Therefore it is recommended to collect blood samples on ice, to keep them cooled and to process samples immediately without further delay. In clinical practice, this might provide logistical problems, especially if the endocrine laboratory is not in close proximity to the hospital. To challenge the need for such procedures, we studied the impact of different preanalytical procedures on the results of ACTH determinations in plasma samples. Two blood samples (EDTA) were taken from each of 10 healthy volunteers at 8 a.m. One sample was centrifuged immediately after drawing and then aliquoted for storage at 24°C, 4°C or -20°C for 1, 2, 4, 24 or 48 hours before being transferred to –80°C until assay. The second sample was left uncentrifuged at either 24°C or 4°C for 1, 2, 4, 24, or 48 hours before centrifugation and transfer to –80°C until assay. All samples were analysed in one run using an automated chemiluminescence assay. Results were compared to the concentration obtained from an aliquot stored under “standard conditions“ (immediate centrifugation and freezing at –80°C until analysis).

Results: Independently from the storage temperature before freezing at -80°C, ACTH concentration decreased significantly already after 4 hours if the samples were not centrifuged immediately (P=0.036, at 4°C; P=0.0094 at 24°C). After immediate centrifugation, however, ACTH concentrations remained stable for at least 4 hours (P=0.74 at 4°C, P=0.11 at 24°C). In these samples, a decrease in measured ACTH concentration only occurred after 24 hours if stored at room temperature (P=0.0039) and after 48 hours if stored refrigerated at 4°C (P=0.006).

Conclusion: Sample temperature (4°C versus 24°C) has less influence on measured ACTH concentrations than the time to centrifugation. Transport of samples at ambient temperature and storage before analysis at 4°C is sufficient for up to 24 hours as long as centrifugation within 4 hours after collection can be ensured.