Effect of Shipping Temperature on Canine N-Terminal Prohormone Atrial Natriuretic Peptide & N-Terminal Prohormone Brain Natriuretic Peptide
The current tests for canine N-terminal prohormone atrial natriuretic peptide (NT-proANP) and N-terminal prohormone brain natriuretic peptide (NT-proBNP) are ELISA assays that are run in a reference lab setting rather than at the petside. This means that samples have to be drawn from the patient and shipped to a facility for testing. Unfortunately both peptides are degraded by a range of peptidases and the half-life of both peptides is a matter of hours. This means that in order for the sample to reach the reference lab in the same state as when it was drawn some thought has to be given as to the conditions the sample is shipped under. One approach would be to add protease inhibitors to the sample to prevent, or at least slow, degradation. In the human field a number of protease inhibitors have been shown to be useful including EDTA, thiorphan, and aprotinin. EDTA can be easily added to a sample by collecting blood into an EDTA collection tube but this limits the sample type to plasma only. Requiring the routine addition of inhibitors adds complexity and cost to the process and is unlikely to enjoy complete compliance from the community.
Another option is to control the temperature of the sample during transit since reducing the temperature decreases peptidase activity and thus slows sample degradation. This approach has been taken by Veterinary Diagnostics Institute (VDxI) (Irvine, CA) who provide a shipping kit containing cold packs to ensure that the plasma sample is kept between -20 and 4°C in transit.
The aim of this study was to evaluate the effect of temperatures between -60 and 37°C on both peptides. Samples were drawn from healthy animals and from those with cardiac disease. Samples were then kept at -60, 4, 25 and 37°C for 3-5 hours and 23-25 hours before being assayed according to the kit instructions.
NT-proANP shows little or no change after 3-5 hours at any temperature analyzed. After 23-25 hours all the samples at 37°C show a minimum decrease of 25%, however no significant decrease is seen in the aliquots at -60, 4 and 25°C.
NT-proBNP shows degradation in the first 3-5 hours at 25 and 37°C with some samples, but this was not the case with all samples. At the later timepoint all the 37°C sample aliquots were at least 50% lower than the aliquots at -60°C. At 25°C 2/3 of the samples also had a 50% drop compared to the -60°C aliquot. There was no significant difference between the -60°C and the 4°C aliquot.
This data suggests that in order to be confident that the values obtained from a sample are accurate, plasma samples should be shipped using cold packs to ensure that the sample is at or below 4°C. However, it is not necessary to use dry ice as there doesn't appear to be a difference between samples shipped at 4°C and those at -60°C over a 23-25 hour time period.