When a patient exhales with the smell of rotten apples, it mostly indicates the possibility of diabetic ketoacidosis, and the diagnosis can be made clearly based on the patient’s past medical history and the improvement of blood glucose, blood ketones, electrolytes, blood gas and other tests. The smell of rotten apples is mainly because the patient’s insulin secretion is insufficient, causing the body carbohydrates can not effectively decompose to provide energy, which mainly relies on the decomposition of fat, resulting in the increase of its metabolite ketone bodies, causing acidosis. The high concentration of ketone bodies in the blood can be exhaled directly through the respiratory system, and the gas smelling like rotten apples is the smell of acetone in ketone bodies. When you smell the special smell of rotten apples clinically, you should think about the possibility of this disease, because ketoacidosis is a critical disease that requires emergency treatment to avoid its adverse consequences.