In cats, stress hyperglycaemia is a major factor confusing diagnosis and monitoring of diabetes, and struggling can increase blood glucose by as much as 180 mg/dL (10 mmol/L). Cats without clinical signs of diabetes, but blood glucose concentrations above the upper cutpoint for normal, could have prediabetes or subclinical diabetes, or a hyperglycaemic response to stress or eating. Where other risk factors are present, such as senior age and obesity, it would be prudent to determine if the hyperglycaemia is persistent and consistent with abnormal glucose homeostasis, or transient and consistent with stress-induced hyperglycaemia or eating. Reversing the reversible risk factors in a prediabetic or subclinical diabetic cat can prevent overt diabetes developing. Hence the importance of differentiating whether the hyperglycaemia is from stress or disordered glucose metabolism.
It is recommended that blood glucose is measured on entry to the consultation room from a blood sample from the ear or footpad and measured with a glucose meter calibrated for feline blood - this is called a screening blood glucose. However, on average, this blood glucose concentration is higher (70 mg/dL or 4 mmol/L higher) than if measured after overnight hospitalization and withholding food. If cats are fed twice daily with a moderate or low carbohydrate diet, it is unlikely that the increase in blood glucose is mainly due to the postprandial surge in blood glucose after eating. In most cats, the increase in blood glucose measured in the consulting room is associated with stress of travel, and may also occur if there is struggling or resistance associated with blood collection. However, in individual cats, whether stress does or does not increase blood glucose, and the magnitude of the increase is difficult to predict. In a study of stress hyperglycaemia associated with a 5 min spray bath, struggling was the only behaviour associated with increased blood glucose concentration. In that study, glucose concentration increased on average by 74 mg/dL (4.1 mmol/L) within 10 minutes, and as much as 194 mg/dL (10.8 mmol/L) in some cats. The increase was associated with increased plasma lactate and norepinephrine concentrations, and resolved within 90 minutes in most cats. However, in 8/20 cats, glucose concentrations were still above baseline (>83 mg/dL; 4.6 mmol/L - 258 mg/dL; 14.3 mmol/L) at 90 mins. Based on continuing research in our laboratory using a continuous glucose monitor to identify when stress hyperglycaemia occurs, it is clear that travel to the clinic in some cats results in marked hyperglycaemia; a one hour car trip can increase blood glucose by 180 mg/dL (10 mmol/L). However, not all cats have higher values for screening compared to blood glucose measured after overnight hospitalisation and withholding food, indicating that not all cats develop stress hyperglycaemia associated with a veterinary clinic visit, and further research is required to understand the triggers for stress hyperglycaemia.
It is important to recognise that the increase in blood glucose observed in cats in the consulting room may be due to stress hyperglycaemia associated with travel to the clinic, or may be due to abnormal glucose metabolism and represent prediabetes or early subclinical diabetes. Abnormal glucose metabolism should be suspected as the possible cause if the cat has multiple risk factors for diabetes such as senior age, obesity, predisposed breed (European Burmese, Russian Blue). Steroid administration increases the risk of diabetes markedly if superimposed on other risk factors.
If blood glucose during the screening test is higher 116 mg/dL; 6.5 mmol/L, we recommend the cat is retested 3–4 hours later to determine if the hyperglycaemia has normalized and was likely associated with stress or eating. If hyperglycaemia is persistent, the cat should be retested at home or after overnight fasting and hospitalisation, ideally with a glucose tolerance test. In the author’s experience, 3–4 hours is sufficient for stress hyperglycaemia to resolve in the majority of cats if they are quietly housed, and the second sample from the ear or pad is collected with the cat in the cage or carry basket. There is lack of definitive research in this area, but stress hyperglycaemia is likely affected by the nature and duration of the stressor(s) and the individual cat.
Senior cats with an initial screening blood glucose >189 mg/dL (10.5 mmol/L) should have fasting blood glucose and glucose tolerance measured after an 18–24 hour fast and overnight hospitalisation. Based on data in humans, senior cats with persistent mild hyperglycaemia or glucose intolerance should be considered pre-diabetic and at risk of developing diabetes, especially if other risk factors are present. Cats in diabetic remission with persistent fasting blood glucose between 135 mg/dL (7.5 mmol/L) and 151 mg/dL (8.4 mmol/L) have an estimated 14 times higher odds of development of diabetes compared to those whose glucose is less than 135 mg/dl (7.5 mmol/L). Mild persistently increased fasting blood glucose is therefore likely to be an indicator or risk factor for diabetes in other groups of predisposed cats, such as senior obese or Burmese cats.
It is strongly recommended that for diabetic cats, assessment of glycaemic control is performed using home glucose monitoring to avoid stress hyperglycaemia. Spot checks in hospital should not be used to adjust insulin dose. If home glucose monitoring is not possible, admit the cat to the clinic in the early morning, so that the nadir glucose concentration would be expected to occur after resolution of stress hyperglycaemia, as this is the most critical measurement. If the nadir glucose concentration is affected by stress hyperglycaemia, it can result in the clinician making an inappropriate increase in insulin dose.
In summary, based on the increased incidence of diabetes in cats 8 years of age or older, we recommend all senior cats have a screening blood glucose measured at each health check using a screening blood sample obtained from a pad or ear sample prior to physical examination and measured with a glucose meter calibrated for feline blood. This is especially important in cats with one or more additional risk factors for diabetes such as obesity, Burmese breed, male sex or glucocorticoid therapy. Measuring blood glucose in capillary blood from the ear or paw is easy and rapid to perform, and is less labour intensive and better tolerated than traditional methods of venous sampling. Cats with suspected stress hyperglycaemia should be retested 3–4 hours later using an ear or pad sample, minimizing stress and struggling. In cat without signs of diabetes, if blood glucose is still increased, hospitalize the cat overnight and withhold food. Retest measuring a fasting glucose and performing a glucose tolerance test. Diabetic cats should wherever possible have home glucose monitoring to adjust insulin dose and in-hospital spot tests should not be used.
References
1. Reeve-Johnson MK, Rand J, Vankan D, Anderson S, Marshall R, Morton JM. Diagnosis of prediabetes in cats: glucose concentration cut points for impaired fasting glucose and impaired glucose tolerance. Domest Anim Endocrinol. 2016; 57:55–62.
2. Reeve-Johnson MK, Rand JL, Vankan D, Anderson ST, Marshall R, Morton JM. Cutpoints for screening blood glucose concentrations in healthy senior cats. J Feline Med Surg. 2017.