Abstract While there are a number of causes of hypoglycaemia in small animals, many of these may be ruled out on the basis of clinical signs, history, age and other laboratory results. Further tests for diagnosis include insulin measurements, the glucagon tolerance test and glucose administration tests. For the diagnosis of insulinomas (β cell tumours) in dogs, serum insulin and glucose concentrations may be measured at the same time and put into the amended insulin to glucose ratio (AIGR), which is reportedly the most accurate method of diagnosis. The ratio provides an indication of whether or not the serum concentration of insulin is appropriate for the concentration of glucose. The value of this ratio in cats is not known because there are so few reports of insulinomas in this species. In cats it may be better to simply compare insulin and glucose levels to see if they are appropriate. The occasional false-positive AIGR has been reported in dogs with other tumours and severe sepsis, but with these conditions the insulin is usually also low. Insulin to glucose and glucose to insulin ratios may also be calculated but are considered less useful than the AIGR. The glucagon tolerance test is considered less accurate than the AIGR but may be used instead of, or in addition to, the AIGR if results of the AIGR are equivocal.