A linear transfer function model comprising 4 parameters is used to describe the post-prandial breakfast excursions of a group of 10 type 1 diabetes patients who are treated with multiple daily insulin injections. The model is able to simulate the glucose concentration in blood and uses the information of carbohydrate content of the breakfast and the administered insulin injections as inputs. Additionally, a measurement of the actual blood glucose concentration at the time when the breakfast occurs is required. No additional information, in particular the use of a continuous glucose monitoring system is necessary. Based on a 3 day observational period, parameter intervals are calculated such that the measured glucose responses are inside the bounds given by the output of the model. The model together with the parameter intervals can be used for robust control design.