Source: Martina. "Measuring Ketones." Web blog post. KetoDietApp.com. Compumaster Ltd., 30 Nov. 2013. Web. 14 Apr. 2016. <http://ketodietapp.com/Blog/post/2013/11/30/Ketosis-Measuring-Ketones>.
Ketosis is a state at which your body produces ketones in the liver, shifting the body's metabolism away from glucose and towards fat utilisation. The presence of ketones in your body, which is indicative of lipolysis, is a definite proof of ketosis. According to Volek and Phinney in their best-selling book "The Art and Science of Low-Carbohydrate Living", nutritional ketosis is defined by serum ketones ranging from 0.5 to 3.0 mM.
The frequently asked question is whether ketosis is dangerous. Ketosis is not only a great weight loss tool, but it has been used for treating diseases such as epilepsy, Alzheimer's or cancer. You can check out my post here to see more about health effects of ketogenic diets with links to relevant research.
While nutritional ketosis is perfectly safe, ketoacidosis is an indicator of serious health problems. Ketoacidosis only occurs in type 1 and type 2 diabetics and alcoholics and has nothing to do with nutritional ketosis. The level of ketones in ketoacidosis are 3-5 times higher than in ketosis resulting from a ketogenic diet.