All ketones are not dangerous.
If you are insulin dependent and are running high blood sugars and/or not taking your insulin as appropriate you may go into a state of ketoacidosis which is a problem that needs immediate attention (fluids and insulin); this is indeed marked by ketones in the urine/blood.
If you are low carbing or fasting and have adequate insulin present then moderate levels of ketones in the urine/blood are no more than an indication that you are metabolising fat; which is a good thing surely?
Below is the post from the sticky in the low-carb section of this forum that explains the difference.
Dillinger
Ketosis / Ketoacidosis
Diabetics, and even some health professionals, often confuse two quite distinct metabolic processes - ketosis and ketoacidosis. Ketosis is a perfectly natural and healthy state during which the body uses stored or dietary fat for fuel. In order to enter this state, carbohydrate intake needs to fall below a certain level. Ideally, a healthy metabolism should regularly use ketosis, while fasting overnight for example, to fuel the body's processes and utilise stored fat reserves. Most of the body's organs, the heart for example, in fact run very efficiently on ketones.
Ketoacidosis is quite different and is typically the result of a chronic lack of insulin, not a lack of carbohydrate. With insufficient insulin, the body attempts to fuel itself by breaking down fat and protein stores in an uncontrolled way, a process which results in the blood becoming dangerously acidic. In short, ketosis usually occurs when blood sugars are at the lower end of the normal range, and ketoacidosis occurs when blood sugars are dangerously elevated. Ketosis is a result of low carbohydrate intake, ketoacidosis is a result of inadequate insulin levels.