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When following a ketogenic diet, you’ll want to check that you’re in ketosis, the state when your body burns fat instead of glucose. When your body produces ketones, they’re found in your blood, in your urine and your breath. Checking the levels gives you the information that your keto diet is working and will motivate you to persevere. 

There are three main ways of measuring ketones, and each method measures the levels of a different substance. A blood test will calculate the levels of β-hydroxybutyrate (BHB), a urine test will measure the amount of acetoacetate present, while a breath test will measure acetone.  

There’s also a fourth way: observation. You may notice the fruity smell of acetone in your urine or on your breath. These are signs that your body is most probably in ketosis and is burning fat. However, these signs aren’t always present, and you certainly won’t get an accurate picture from observation alone. 

When you’re in ketosis, all three of these substances will be detectable in your blood. Some molecules are small enough to pass from the bloodstream to the lungs so you can detect them in the breath. Others are excreted in the urine, and measuring them is a proxy guide to blood ketone levels. 

Let’s take a look at the three most reliable options and the advantages and disadvantages of each of them. 

Blood ketone tests
Blood ketone testing is the most precise way of measuring ketones because it measures β-hydroxybutyrate. This substance is present in the highest quantities in the blood. By measuring BHB, you are measuring the ketones that your body will be using, in contrast to the other testing options, which calculate excreted ketones. 

How to measure BHB in blood?
It’s quick and easy to measure BHB in blood with a blood ketone meter. You produce a tiny drop of blood with a fingerstick which you then use to give a reading of the levels of β-hydroxybutyrate in your blood. Some ketone meters, such as the Swiss Point of Care GK dual glucose and ketone meter kickstart set, also allow you to measure blood glucose levels. This can be a helpful way of identifying the effect that certain foods have on your body. You enter a different type of test strip depending on what you want to measure. 

To take the BHB measurement, you’ll need ketone test strips, an alcohol swab to sterilise the area and a lancing pen and lancets to draw blood from your fingertip. Then proceed as follows: 

Blood ketone meters are battery operated and are convenient to carry with you or when travelling. You’ll use a new test strip for every test, plus a disposable lancet. 

Advantages: Blood tests are the most accurate way of assessing ketosis. They’re user friendly and reliable and give you an instant result. And with a dual purpose glucose and ketone meter, you also have the possibility of checking your blood glucose levels. 

Disadvantages: Can be costly if you test frequently. This method of testing might not suit you if you don’t like to prick your finger yourself or if you are uncomfortable with blood.  

Breath ketone tests
Measuring the concentration of acetone in your breath is an affordable and non-invasive way of checking ketosis. However, it’s less accurate than blood testing. 

Acetone is one of the substances created when the body breaks down acetoacetate, and the acetone concentration reflects the rate at which the body is burning ketones. It’s thanks to this process that you can sometimes smell acetone on the breath of someone who’s in ketosis. 

Bear in mind that ketone levels in the breath can be affected by factors such as hydration levels and alcohol consumption, so they don’t always reflect blood ketone levels accurately. 

How to use a ketone “breathalyser”
These devices are battery operated so you can use them anywhere. With set-up complete, exhale deeply into the device. Make sure to force all the air out of your lungs. The ketone level will then appear in the window of the device. 

Even a small glass of wine can influence these devices towards reading much higher ketone levels, so wait until all the alcohol is out of your system before testing. 

Advantages: Breath tests are non-invasive so are perfect for anyone. And once you’ve bought the device, you don’t need to purchase replacement test strips or lancets. 

Disadvantages: Because they measure excreted ketones rather than those in the blood, they aren’t as accurate as blood tests. 

Measuring ketones with urinalysis strips 

Keto Sticks Urine are a low tech option that can be very helpful in the early stages of your keto diet when you need to check whether you’ve entered ketosis. They show the level of acetoacetate – the ketone bodies that are excreted in the urine. 

You can also use urinalysis strips to test if any normally keto-friendly foods might be hindering your progress. They are cheap, readily available and easy to use. The paper strips are impregnated with sodium nitroprusside, which reacts and changes colour to show the ketone levels. 

How to use a urinalysis strip
Dip the strip in urine and then wait for the recommended number of seconds, which varies between different brands. You will see a colour change in the squares printed on the strip that will show whether you are in ketosis. 

Advantages: Cheap, readily available, non-invasive. 

Disadvantages: Less accurate, and can be difficult to read or to interpret .