Your system produces ketone bodies when your liver breaks down fatty acids in a state of low glucose levels. In total, there are three ketone bodies: acetoacetate (AcAc), beta-hydroxybutyrate (BHB), and acetone. 

AcAc is the primary ketone body in the urine. BHB is the primary ketone body in the blood (and also the most critical and predominant ketone in a healthy state of ketosis). And acetone is the primary ketone body in the breath. 

While urines strips are the most popular ketone testing method, they are, unfortunately, the least accurate as well. This is because urine strips only measure AcAc (acetoacetate), and various factors influence its levels in urine, for instance, your hydration status. Besides, AcAc in the urine may disappear once you become keto-adapted, leading to a false negative test result. 

This accurate way to test ketosis involves measuring acetone levels in your breath.^[1] The great thing is that you only have to buy a ketone breathalyser once and then you can test yourself whenever you want. 

The downside is that breath ketones do not always reflect blood ketone levels perfectly. The reason is that various factors can affect breath ketones, for example, alcohol consumption and hydration status.

This method measures BHB (beta-hydroxybutyrate) levels in your blood and is the most accurate way to measure ketosis. The downside is that these tests can be quite pricey. Besides, they require a finger prick, which may be a problem for some people. If you are okay with the cost and do not mind the finger prick, this is the approach I use and would recommended you use to measure ketosis. 

On a keto diet, the optimal BHB levels are between 0.5 and 3 mmol/L. If you measure ketosis with a blood test and your BHB level is outside this range, adjust your carb intake. If it is below 0.5 mmol/L, you are not in a state of ketosis. Reduce your carb intake by 20g a day and see how that affects your levels (You learn how to overcome some keto challenges later in this book). 

If your BHB levels are above 3 mmol/L, you are in deep ketosis. There are varying opinions of how much more affective higher ketone levels are for your weight loss and health. Do not get caught up with trying to get to 3 mmol/L and above. Just focus on achieving at least 0.5 mmol/L.

Check how you feel and how you are progressing. If you feel well and energetic and you are losing weight at a decent rate (if that is your goal), then keep on going. 

Depending on your goal, you can adjust your diet based on the data you have gathered. For example, if you opt for the low carbohydrate ketogenic carb restriction level (fewer than 50g of net carbs a day), you can check whether you are in ketosis and reduce your carb intake if necessary to reap the benefits.

Also, if you have just started a keto diet, it is especially valuable to test whether you are in nutritional ketosis. For some people, this state can occur within just a day or two after they limit their carb intake. For others, it takes a week or two. If you are not yet in ketosis after two weeks, cut your daily net carb intake by 20g. After a week or so, you should be in ketosis. If not, you are most likely cheating on your diet. (Be honest with yourself!) If you are still not in ketosis, drop your carb intake a bit again until you are at 20g of net carbs a day.

What is also great about testing ketosis is that you can find out in how deep of a ketosis state you feel and perform best. Some people note that their brain function and energy level are best in a deep state of ketosis. Others feel that a moderate state is enough to reap the benefits. By testing and evaluating which ketosis state works best for you – you can adjust your carb intake to maintain that state.