Gauging the appropriate interval between preprandial injections and eating, known as the “lag time,” is essential for coordinating insulin availability with glycemic excursions following meals. The timing of the injections should also be adapted to the level of premeal glycemia. Insulin lispro, insulin aspart, and insulin glulisine have rapid onset of action and, ideally, should be given within 10 minutes before mealtime when blood glucose is in the target range; however, provided there is no hyper- or hypoglycemia present, these insulins may be administered at any time from 10 minutes prior to meal consumption to just prior to eating, or even immediately after eating if one is not sure that the planned meal will be consumed (e.g., in finicky eaters, babies, or the elderly). Regular insulin administered subcutaneously is best administered at least 20 to 30 minutes before eating if blood glucose levels are within target, keeping in mind that if the meal is delayed hypoglycemia may ensue. When blood glucose levels are above a patient’s target range, the lag time should be increased to permit the insulin to begin to have an effect sooner so that the hyperglycemia may be partly reduced before meal carbohydrate elevates blood glucose further. In this case, rapid-acting acting insulin analogues can be given perhaps 15 minutes and regular insulin perhaps 30 to 60 minutes before the meal. On the other hand, when premeal blood glucose levels are below target range, administration of regular insulin should be delayed until immediately before eating, and injections of rapid-acting insulin should be postponed until after some carbohydrates have been consumed.

It should also be mentioned that patients who test their blood glucose frequently or use real-time continuous glucose monitoring (see below) are best able to gauge best possible lag times by knowing the glycemic trend when the meal is about to be consumed. For example, someone with a blood glucose level of 130 mg/dL but a steep trend upwards would ideally like to use a longer lag time than someone with the same blood glucose but a steep trend downward. It becomes easy to see why frequent SMBG (or continuous monitoring) assists in determining lag times better than simply measuring glucose levels before meals.