Testing for Ketosis: The Difference Between Blood, Breath, and Urine

If you’re fasting or following a low carb, paleo or ketogenic diet, you are restricting carbohydrates and are very likely producing some ketones.  But are you in a state of nutritional ketosis where you will benefit the most? And at what level of ketosis are you in? The only way to know for sure is to test. Sounds simple enough, right? Well, it is, once you understand the various ways to test for ketones and learn how to get the most accurate results. And that’s what you’ll learn here. 

Why Test Your Ketones?

A ketogenic diet is specific; the entire point is to eat foods that get you into ketosis and keep you there. But it’s not just eating keto-friendly foods that influences your ketone levels. The amount of food you eat and your specific reactions to foods and activities can affect your ketone levels and blood-glucose levels, which in turn can affect your success with ketosis. This means you can be doing everything “right” and still struggle to reach the levels of ketosis you desire. The only way to find out is to test your ketone levels. 

Monitoring your ketones gives you insight into how your body is responding. It also gives you clues and opportunity to adjust your diet a little more. Perhaps you want to tweak you carbs or you want to  identify trigger foods. Testing allows you to see how minor adjustments affect your results and allows you the capability to discover which foods have the most impact on your ketone levels, good and bad. Or perhaps you want to see how your ketone levels respond to various stimuli, such as exercising or eating a specific type of keto-friendly sugar. Again, the only way to find out is to test. 

But which test is the best for the most reliable results? Is it blood testing? breath testing, or urine testing? The unequivocal answer is blood-ketone testing; it’s the gold standard for testing your ketones. Here’s why:

Three Types of Ketones

On keto, your body will produce three types of ketone bodies. It’s important to understand their differences because each method of testing measures something different – urine tests measure acetoacetate, breath tests measure acetone and blood tests measure beta-hydroxybutyrate.  


  • Acetoacetate (AcAc): When glucose is no longer available as a primary source of fuel, fat is broken down by the liver into fatty acids, which are further broken down into ketone bodies. Acetoacetate is the first ketone body that is produced and is spilled into your urine, especially in the early stages of ketosis, but rarely in later stages.


  • Acetate (Acetone): Acetoacetate, the simplest beta-keto acid, is converted into either Beta-hydroxybutyrate (BHB) or acetone (aka acetate). Acetone is the least abundant ketone body and is exhaled through the lungs as a waste product rather than used as a source of fuel. It is sometimes called the “exhaust” ketone.


  • Beta-hydroxybutyrate (BHB): BHB is the most prevalent and stable ketone body in the blood and is readily transported to cells to use as fuel. It can fuel most of the brain’s energy needs along with organs and muscles and represents approximately 70% of the available ketone energy. 


Now that you know about the three types of ketones, let’s review the various testing methods, which type of ketones they test, and which results are more and less accurate.

Urine Strips: The Safety Check for Diabetics

Urine strips are widely available in drug-stores and supermarkets. These work by dipping the strip in a urine sample for a few seconds and comparing the color of the strip with the colors on the package to determine the concentration of ketones in your urine (results range from no ketones to highly concentrated amounts). The darker the color, the more ketones are present in your urine. 

Urine strips were designed primarily to monitor diabetic ketones in order to help prevent diabetics from unknowingly getting into ketoacidosis. Not to be confused with nutritional ketosis, ketoacidosis is a very serious metabolic state caused by insulin deficiency that occurs when high ketone levels are present with very high glucose levels. As a result, urine strips are effective in helping diabetics avoid a dangerous state. However, because urine strips measure excess or excreted ketones in the urine, they are inaccurate in determining nutritional ketosis and how well your body is utilizing ketones as fuel. 

When first transitioning to ketosis, sometimes the body will make excess acetoacetate ketones, which will show up in your urine and on urine-strip tests. If you get a reading for high ketones early on with a urine strip, you may excitedly think you’re getting a reading of your level of ketosis. Unfortunately, that’s not the case: the reading is simply a marker of the excess ketones you are peeing out, not an accurate account of your level of ketosis (ketones in your blood). Try the same test a few weeks later while maintaining a keto diet and your urine test may reveal no ketones, even if you are actually in ketosis. As your body becomes fat-adapted, it converts ketone bodies more efficiently and is less likely to expel them. 

Accuracy can also be affected by your level of hydration; varying levels of hydration may result in inconsistent results. In addition, the readings are somewhat subjective when comparing the color on the strip to the color on the package. And the reading you get is not a reading at the moment in time, it’s the sum of ketones present since your last urination.

Strips are generally only good for 30 days from the date of opening the vial as they are very sensitive to air and humidity, so if you do purchase urine strips, only purchase what you will use in 30 days.

Bottom line: urine strips are good as a low-cost option when first starting your keto journey and for their primary medical safety purpose: to monitor urine ketones for diabetics in order to prevent diabetic ketoacidosis. However, they are not as accurate for measuring nutritional ketosis as other methods. 

Breath Test: The Acetone Indicator

The key to understanding the breath test is to understand how ketones are metabolized in the body. Breath meters test breath acetone (parts per million) in a non-invasive manner. Acetone production is a byproduct of the fat metabolism process. Because of its small size, acetone appears in exhaled breath.

There are outside factors that can affect your level of acetone and could potentially cause the sensors to fail or clog, including: breath mints, chewing gum, some sugar substitutes, tobacco and e-cigarettes, cough drops, lip balm, mouthwash, toothpaste, foods like garlic, drinks like alcohol, coffee, green tea and fermented drinks, drugs such as disulfiram, cleaning solutions, disinfectant sprays, and environmental factors like breath temperature. Finally, acetone levels can also vary depending on your level of activity and the time of day, simply because of how you’re breathing and your respiratory rate; this can affect how our acetone levels are reflected in our breath. 

It’s important to consider the following when testing breath: device calibration, sensor expiration, breath technique, and environmental conditions. Due to their sensitivity for sensor failure, it is important to be able to calibrate your meter to a known control. Calibration requirements and frequency of calibration vary by device manufacturer. (For example: there may be required firmware or specific gas to calibrate or a recommended process and frequency). Breath acetone sensors do expire due to testing frequency and temperature, so always refer to the manufacturer’s recommended sensor replacement schedule to ensure you’re working with a current product. 

Repeatable breathing patterns are important in achieving accurate and reliable breath samples; inhale (hypoventilation) and exhale (hyperventilation) techniques vary from device to device. For the best bet with a breath-test meter, look for meter brands that routinely test the clinical accuracy of their sensors against a mass-spectrometer.

Bottom line: the breath test measures acetone, which is the byproduct of acetoacetate breaking down and not a measure of the ketones your body uses as fuel. Readings can also fluctuate based on many variables. If you do choose a breath meter, make sure you choose one that allows you to replace the sensor and can calibrate to a known control. Otherwise, you could end up with an expensive paperweight.

The Blood Meter: The Gold Standard in Ketone Measurement

The blood-ketone test does exactly what it sounds like: tests your blood for ketone levels. It’s easily conducted at home with a blood-ketone meter and gives results quickly and accurately. In fact, it’s by far the most accurate test for measuring your level of ketosis. 

The blood meter measures beta-hydroxybutyrate, a ketone body synthesized in the liver, and the most active form of ketones in your blood. Beta-hydroxybutyrate is the main ketone your body utilizes in ketosis and therefore is the most important one to measure. The number that registers on your blood meter when you test reflects the ketone level in your body, right then and there. Consider it your fuel tank. No estimates, no deciphering, just an accurate blood-ketone reading.  

Bottom line: The blood meter is the gold standard for measuring your level of ketosis because it measures the predominant ketone body used as fuel for the body. It’s a quantitative measurement providing real-time insights into your metabolism and considered the most accurate of all testing methods.

The Final Word

Whether testing ketones to confirm the effectiveness of a nutrition, exercise, or wellness plan, the most accurate and reliable way to accurately test your ketone levels is with a blood meter. Although urine and breath tests show ketones, these tests don’t give accurate depictions of your blood-ketone levels and they don’t test the right kind of ketones. This inaccuracy can lead to false conclusions of whether or not you are in ketosis as well as a false understanding of how your body is responding to different variables while on a ketogenic diet or therapies. 

Watch this video as Thomas DeLauer, Celebrity Trainer and Health Author explains the difference between blood, breath, and urine strips. 




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