Aaaah so it's not the ketones by themselves that are the problem - but when high and combined with high BG, it's a different story
Almost but not quite. Small amounts of ketones are produced when fat is broken down . This happens everyday, its part of normal metabolism . Insulin is required to regulate this process, if sufficient insulin is present, the breakdown of fats will be controlled and ketones will never get too high.
In the case of a T1 diabetic with no , or far too little insulin in circulation, there is no regulation of this process There is in effect no brake so fat and muscle, are broken down and ketones can rise very quickly ,' poisoning' the blood, causing diabetic ketoacidosis.
Most frequently this happens concurrently with high glucose, because the liver releases more glucose in an attempt to fuel the 'starving' cells. There are times (less commonly) that it can occur with relatively low glucose levels. This could happen for example if the person has had several bouts of vomiting or has previously been fasting, or has been drinking excessive alcohol.
In the case of people with T2 there is normally sufficient insulin to avoid DKA at any glucose level. T2s develop DKA only occasionally it is usualy triggered when a severe infection or some other problem causes a demand for more insulin than is available.