Collecting blood samples for insulin testing

This is important because of the wide spread use of EDTA as a anticoagulant in blood collection (purple top tube) in veterinary medicine. For insulin testing either a red top (no anticoagulant resulting in serum and a clot) or green top (heparin) tube should be used and then refrigerated if it is going to take more than 3 days to get results. Purple top tubes are not suitable.
DrO

J Vet Intern Med. 2019 Oct 16. doi: 10.1111/jvim.15629. [Epub ahead of print]
Immunoreactive insulin stability in horses at risk of insulin dysregulation.
Leschke DH1, Muir GS1, Hodgson JK1, Coyle M2, Horn R1, Bertin FR1.

Author information:
1. School of Veterinary Science, The University of Queensland, Gatton, Queensland, Australia.
2. School of Agriculture and Food Sciences, The University of Queensland, Gatton, Queensland, Australia.
Abstract
BACKGROUND:

Diseases associated with insulin dysregulation (ID), such as equine metabolic syndrome and pituitary pars intermedia dysfunction, are of interest to practitioners because of their association with laminitis. Accurate insulin concentration assessment is critical in diagnosing and managing these diseases.
HYPOTHESIS/OBJECTIVES:

To determine the effect of time, temperature, and collection tube type on insulin concentrations in horses at risk of ID.
ANIMALS:

Eight adult horses with body condition score >6/9.
METHODS:

In this prospective study, subjects underwent an infeed oral glucose test 2 hours before blood collection. Blood samples were divided into ethylenediaminetetraacetic acid, heparinized, or serum tubes and stored at 4 or 20°C. Tubes were centrifuged and analyzed for insulin by a chemiluminescent assay over 8 days. Changes in insulin concentrations were compared with a linear mixed effects model.
RESULTS:

An overall effect of time, tube type and temperature was identified (P = .01, P = 0.001, and P = 0.001, respectively). Serum and heparinized samples had similar concentrations for 3 days at 20°C and 8 days at 4°C; however, after 3 days at 20°C, heparinized samples had significantly higher insulin concentrations (P = .004, P = .03, and P = .03 on consecutive days). Ethylenediaminetetraacetic acid samples had significantly lower insulin concentrations regardless of time and temperature (P = .001 for all comparisons).
CONCLUSIONS AND CLINICAL IMPORTANCE:

These results suggest an ideal protocol to determine insulin concentrations involves using serum or heparinized samples with analysis occurring within 3 days at 20°C or 8 days at 4°C.