Noncardiac factors impact results
NT-proBNP is a biomarker that's increased by progressive cardiac disease. In people, hydration status can have an impact on
measured values. One possible cause of increased concentrations could be physical activity.
Researchers from the University of Pennsylvania looked at this issue in 14 healthy dogs older than 1 year.4 The dogs were subjected to five minutes of submaximal exercise. NT-proBNP concentrations before, immediately after and one
hour after exercise were evaluated. The normal reference range for the assay was < 900 pmol/L. There was no statistical difference
between any of the time points. A single dog had its concentration go from 790 to 1,054 pmol/L immediately after exercise.
This study demonstrates that in healthy dogs, submaximal exercise won't impact the NT-proBNP significantly, and only in rare
cases will this result in a false positive result. But it's also unlikely that veterinarians would be sampling dogs immediately
after exercise, so the chance of a false positive result seems slim. Of course, this study doesn't address what submaximal
exercise does to dogs with underlying cardiac disease.
Other factors also may impact measured NT-proBNP results, such as how the sample is shipped to the laboratory for testing.
A group of researchers from the California Animal Hospital Veterinary Specialty Group, Los Angeles, addressed this issue.5 Blood samples were drawn from 18 cats with cardiac disease and six control cats and placed in plastic EDTA tubes. The samples
were centrifuged, and plasma was collected. Some of the plasma was placed in tubes containing a protease inhibitor and stored
at 39.2 F (4 C), some was frozen at -4 F (-20 C) and some was frozen at -112 F (-80 C). The samples were then shipped to the
laboratory for assay.
Both frozen methods resulted in similar results; however, the protease inhibitor method resulted in significantly higher values.
This study is important in that it shows consistency in how a sample is shipped to the laboratory for assay of NT-proBNP is
vital. Results between freezing and protease inhibitor tubes aren't comparable and could result in misinterpretation of the
results. Given that it's easier to keep a sample cooled rather than frozen, preference should be given to shipping samples
for assay using the suggested protease inhibitor tubes. This should result in more consistent and accurate results.