Ketones are a sign of starvation: if the body senses it is starving it will break down body fat and these fats are converted into ketones that are tested for in the urine. Ketones should be specifically tested in two main situations.
Pregnancy needs extra calories for both mother and to feed the growing fetus. Since obesity is usually an issue in Type 2 diabetes and some patients try to restrict their calories, not enough food is sometimes an issue in pregnant Type 2 diabetic patients. If you are not gaining weight appropriately it is worthwhile checking the urine for ketones first thing in the morning and if positive it is a sign more calories are needed particularly at bed time. Overweight women need to gain less weight than their lean counterparts during pregnancy.
If the glucose is uncontrolled in the mother because of a lack of insulin especially in the presence of infection, the body senses this as starvation, breaks down fat, forms ketones which in excess will build up as diabetic ketoacidosis (DKA), a very serious situation for the mother and baby. This is much more an issue for people with Type 1 diabetes but can occur in Type 2. Thus if the sugars start to rise unexplainedly as may happen if you have an infection, you should check your urine ketones and if positive take extra insulin and fluids. If the situation persists for more than three hours you should go to the emergency room or talk to your caregivers.
In this setting, if you start vomiting and can’t keep fluids down for more than an hour you need intravenous fluids at the hospital. Do not delay.
Outside these two times the testing for ketones once a week or so is reasonable.