Interest of Concomitant Insulin and Glucose Measurement in Blood
By Jeremie Korchia, DVM, MS, DACVP
In endocrinology, regulation is often reciprocal (directly or indirectly), and a regulation loop aims in maintaining homeostasis. Blood glucose is one of the most important nutrients (as for example oxygen and water can be) for cell survival. Thus, blood glucose is tightly regulated by multiple hormones. Several hormones promote the increase in blood glucose by different pathways, however only insulin promotes the decrease in blood glucose, by triggering its penetration inside cells (along with potassium). Insulin secretion is in turn physiologically repressed by decreasing blood glucose concentration.
Because insulin concentration varies broadly within the same day, always adapting to the blood glucose, insulin interpretation is tightly depending on the concomitant blood glucose concentration. High or low insulin will have a different interpretation depending on if it is associated with high or low blood glucose.
Insulin is mostly measured for 3 clinical purposes: investigating an insulinoma (mostly in dogs), investigating insulin resistance (dogs, cats, horses), and monitoring insulin therapy in diabetic animals (dogs and cats).
To investigate insulinomas, the dog should be fasted in the evening (water always allowed), and the blood sampling should be made sometime in the morning. The sensitivity of this test can be increased by repeat blood sampling every 2 hours, 2-3 times while the dog remains fasted. Indeed, the diagnosis of insulinoma is based on an insulin/glucose ratio, not on insulin alone. The blood sampling can be made at any time if the dog is displaying clinical signs of hypoglycemia, after which the dog should be fed. It is best to: 1) measure glucose on site (at the veterinary practice), to make sure there is indeed a hypoglycemia; 2) to separate the serum or plasma from the red blood cells quickly (mandatorily < 1 hour), and place in a different red top tube; 3) if several tubes are sent, specify which glycemia corresponds with which tube. Finally, as glucose measurement may be inaccurate on site (especially with glucometers), having the glucose measured along with insulin is a double asset: it increases the accuracy of the screening, and also assesses the accuracy of the onsite glucose measurement (provided that the plasma/serum was separated from the red blood cells in a timely manner).
Insulin resistance is defined as the failure of increased insulin to regulate hyperglycemia, and thus is best characterized by persistent concomitant high glucose and high insulin. This can be seen in some types of diabetes mellitus, or in any disease or condition promoting insulin resistance (for example Cushing’s disease, diestrus, etc.).
Finally, creating an insulin curve in parallel to a glucose curve in treated diabetic animals can document an insulin treatment that is too short-acting, indicating that the treatment should be switched to a longer-acting insulin formulation.
In all three cases listed above, the insulin interpretation is much better with the corresponding blood glucose. It is highly recommended to consistently run both tests on the same sample so that the regulation loop can be completely assessed.
For more information about endocrinology testing, contact Dr. Jeremie Korchia, lead endocrinology scientist, at the College Station laboratory at 1.888.646.5623.