The importance of verifying manufacturer's claim on specimen stability: An example in serum angiotensin converting enzyme testing

Appropriate specimen handling is integral to quality and minimizing medical errors. Clinical laboratories often rely on manufacturer's claims for handling specimens, such as sample stability conditions. Serum angiotensin converting enzyme (ACE) is an example in which manufacturer claims and stability in the literature is limited. The purpose of this study was to demonstrate the importance to verify manufacturer's stability using serum ACE as an example. Serum was collected from 39 healthy volunteers and ACE activity levels measured at baseline, after 4 h, 1, 3, 7 days at room temperature, after 3, 7, and 14 days refrigerated at 4 °C, after 1, 2, 4 and 8 weeks frozen at -20 °C, and after three freeze/thaw cycles. An additional 42 discarded patient serum specimens were re-analyzed after 1 or 2 weeks frozen at -20 °C. To evaluate stability performance, percent difference was compared to the clinical acceptance criteria, which was defined as a ½ total allowable error of ±10.9 %. This study found serum ACE to be stable 4 h at room temperature, 14 days refrigerated at 4 °C, up to 1 week frozen at -20 °C, and up to three freeze/thaw cycles. The preferred storage condition for serum ACE is refrigerated at 4 °C as there was minimal change in percent bias over the 14 day period. The false increase observed in samples stored frozen longer than 1 week could impact clinical decision making. The stability findings differed from manufacturer claims, highlighting the importance of verifying stability, especially for esoteric testing such as serum ACE where specimens travel long distances in varying climates to reach centralized testing locations.