Measuring Ketones: What, When, Why, and How

Keto is unique compared to other diets because there is an objective marker that tells you if you’re on the right track. With an easy at-home test, you can confirm that you are, in fact, in a state of ketosis.

Regular readers probably know I’m not a big data tracker. My energy, sleep, workout performance, stamina, and enjoyment of life tell me almost everything I need to know about how well I’m doing. Nevertheless, I get that some people love to play the self-quantification game. In some medical situations, measuring ketones is advisable, even necessary, as well. I’m not a total curmudgeon about it. Heck, I’ve been known to check in on my blood glucose and ketones from time to time.

If you’re thinking about testing, you should become familiar with the three different methods. Each has its own pros and cons. You’ll want to pick the option that’s right for you.

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What Exactly Are You Measuring?

Let’s back up and do a quick refresher on ketogenic diets. These are any diets where carbohydrate intake is restricted below about 50 grams of total carbs per day. When you restrict carbs, you are really restricting how much glucose the body has to meet its energy needs. Without much glucose coming in, the body needs an alternative fuel source, especially for the brain, which doesn’t run well on fat. That fuel source is ketones.

The liver produces ketones from fatty acids when insulin levels are low and liver glycogen (stored glucose) becomes depleted. This happens when you follow a very-low-carb diet, fast, or engage in sustained exercise.

When your liver is producing measurable amounts of ketones, you are said to be in ketosis. This is not to be confused with ketoacidosis, a potentially fatal medical emergency. Keto diets don’t lead to ketoacidosis because the body has a safeguard that prevents ketone levels from becoming dangerously high: insulin. When ketones rise, the pancreas releases insulin, which in turn hinders the release of fatty acids from stored body fat. Fewer fatty acids in the bloodstream mean less substrate (material) for the liver to turn into ketones.

This feedback mechanism keeps ketone levels in safe ranges unless your body can’t produce insulin. Individuals with type 1 diabetes and very advanced type 2 diabetes are at risk for developing ketoacidosis for this reason. Diabetics often monitor their ketones to make sure they are within safe ranges.

For the rest of us, measuring ketones is just a way of checking whether or not we are actually in a state of ketosis. We might want to know that for a number of reasons, discussed later.

Meet The Ketones

In ketogenesis, fatty acids are metabolized in the liver to create ketone bodies. The primary ketone body is acetoacetate (AcAc), which can convert into beta-hydroxybutyrate (BHB). AcAc also spontaneously breaks down into a third ketone body, acetone.

The body primarily uses AcAc and BHB for energy. Although acetone can be converted into pyruvate, it’s generally considered a waste product.

How to Test Ketones at Home

There are three ways to test your ketones at home:

1. Urine test strips, which measure AcAc
2. Blood tests, which measure BHB
3. Breath tests, which measure acetone

Urine Ketone Test Strips

How it works:

You can purchase urine ketone test strips online or in many pharmacies. They are cheap, costing only pennies per strip. Don’t confuse them with urine pH strips.

Simply collect your urine in a sample cup or pee directly on the strip. After a set time—usually 15 seconds, but some strips take longer—the end of the strip will change color. Compare the color on the strip to the key on the package to get your ketosis level. Rather than giving you an exact readout, the color tells you if your urine does not register any AcAc, or if it shows low, medium, or high levels.

It’s straightforward but not foolproof. For one, if you let the strip sit for too long before you interpret the results, the test can be inaccurate. Urine strips also tend to overestimate the amount of AcAc present and can give false positives. Results can be affected by how well hydrated you are, too.

Although urine tests are shown to correlate decently well with blood and breath tests in diabetics and people new to the keto diet, AcAc in the urine is considered “spillover.” When you first start a keto diet, your cells aren’t great at utilizing ketones, so some get excreted. You’re measuring ketones the liver made but the body can’t use. As you become more keto-adapted, there should be less spillover.

Most keto dieters do find that their urine ketones decline over time. That’s a good thing, indicating less waste; but it also means the urine tests become less useful. It’s very common for experienced keto dieters to have low or no measurable ketones in their urine despite having plenty in their bloodstreams.

Pros:

• Least expensive method of testing
• Does not require blood—no finger pricks

Cons:

• Least accurate, especially after keto-adaptation period
• Affected by how well hydrated you are
• Does not tell you exact ketone levels

Blood Tests for Ketones

How it works:

Blood tests measure the level of BHB in the bloodstream. This is considered the gold standard in ketone testing. They require an initial investment in a meter, plus ongoing purchases of test strips. You also need a lancing device and sterile lancets to prick your finger and draw a droplet of blood. If you’re planning on testing several times a day, it can get expensive fast, not to mention your sore fingertips.

The two most popular meters in the U.S. are the Keto Mojo(™) and Precision Xtra. Both also measure blood glucose, but you need separate test strips. The Precision Xtra meter runs around $25 depending on where you purchase it. Ketone test strips cost about $1.20 each, and glucose test strips about $0.65.

You can get a Keto Mojo starter kit on the company’s website that includes the meter, lancing device, 10 lancets, 10 ketone test strips, 10 glucose test strips, and a travel case for $59.99. Additional ketone test strips cost $49.50 for a 50-pack. Glucose test strips are $14.99 for a 50-pack. They also offer a Bluetooth connector for $9.95 that allows you to upload your test results to an app.

Understanding blood test results:

Your meter will give you a reading of 0 or “Lo” if you aren’t in ketosis. On a typical keto diet, you might be anywhere from 0.3 to around 2.0 mmol (millimole). Fasting and exercise can each drive BHB up to 4.0 mmol or higher. Ketoacidosis occurs above 10 mmol.

In The Art and Science of Low Carbohydrate Living, renowned researchers Jeff Volek and Stephen Phinney proposed that ketosis begins at 0.5 mmol. The designated 1.0 to 2.5 mmol as the “optimal ketone zone.” Don’t get too hung up on these numbers, though. Some individuals produce high levels of ketones on a normal keto diet, while others barely register any unless they fast or use exogenous ketones (which contain BHB). I have friends who have been strictly keto for years and consistently hover around 0.3 or 0.4 mmol. We don’t really understand why these individual differences exist. I’ve offered some hypotheses, but it’s still a bit of a mystery.

Anyway, higher numbers don’t mean that you’re doing better than the next person. In certain medical situations, such as for seizure control, high BHB levels are desirable. For the average person doing keto for weight loss or general wellness, there’s no evidence that it makes a big difference. More recently, Phinney and Volek have started to talk about the “effective therapeutic range”—where you can expect to reap benefits from being in ketosis—as being anywhere between 0.5 and 4.0 mmol.

Pros:

• Most accurate
• Used in therapeutic settings

Cons:

• Requires a finger prick and blood sample
• Cost can add up quickly

Ketone Breath Tests

How it works:

Breath tests measure acetone. Like AcAc and BHB, acetone travels in the bloodstream. The molecules are small enough to diffuse into the lungs, where they are exhaled. This is why some people develop the distinctive fruity or metallic smell also known as keto breath.

Breath meters measure acetone in parts per million (ppm). Everyone, no matter their diet, exhales a small amount of acetone. The average non-keto dieter might only register 1 ppm. Someone following a keto diet would normally have up to about 40 ppm. Fasting can drive breath acetone up to around 170 ppm. Ketoacidosis is indicated by breath acetone concentrations of 1,250 ppm.

Breath acetone does reliably correlate with blood BHB. Multiple studies also show that acetone readings are correlated with weight loss when participants follow a calorie-restricted diet. This doesn’t mean that higher breath acetone readings somehow cause weight loss, though.

Acetone readings go up when the body is burning more fat, that’s true. That’s why they also go up after exercise, when the body is tapping into fat stores for energy. However, these devices can’t distinguish whether you’re burning body fat or the fat from your plate. Participants in these studies were burning body fat because they were in an energy deficit. Depending on how much fat you eat, you can maintain or even gain weight on a keto diet. Ketosis never guarantees body fat loss, no matter how you measure your ketones.

The biggest upside to breath testing is that once you have the meter, you can test as many times as you want each day for no additional cost per test. This is great for people who want to test multiple times per day.

Devices:

Although breath testing does seem promising, there’s a catch. The research studies I reviewed all used mass spectrometry-based devices. These are sophisticated pieces of equipment that you won’t have at home. While there are quite a few handheld devices available online, it’s not clear how accurate they are.

The most popular device seems to be the Ketonix(R), which starts at $79. You blow into the device, which looks like a pen, and the ketone reading syncs with an app on your phone. Reviews suggest that it can take a while for the app to connect to the device and that it’s glitchy, so do your homework before purchasing. There is also the considerably more expensive LEVL device (prices aren’t currently listed on their website). In addition to the up-front cost, LEVL requires monthly sensor replacements that aren’t cheap, and it’s more finicky to use.

Pros:

• Does not require blood or urine
• Unlimited tests once you own the device

Cons:

• Questions about accuracy and efficacy of home-use devices
• Not reliable if you’ve recently consumed alcohol

When to Test

There’s no right or wrong time to test. For general tracking, the most important thing is that you test around the same time each day. Try to keep conditions the same, too. Ketone levels are affected by time of day, how recently you ate and what you ate, exercise, and a number of other factors. Don’t test right after a meal one day, at the end of a 16-hour fasting period the next day, and after a 10-mile run on the third day.

I think it makes the most sense to test first thing in the morning or at bedtime. If you consistently break your overnight fast at the same time, you can also test right before eating.

If you’re looking for high ketone readings to stay motivated, test at night. A test of 12 healthy men and women found that both blood and urine measurements were lowest at 10 a.m., following breakfast Blood tests were otherwise fairly steady throughout the day, with a modest decline in the afternoon. Urine tests showed higher ketones as the day progressed, also with a small mid-afternoon dip. The highest levels occurred before bed, at 10 p.m.

Do You Need to Test?

No, but there are some reasons you might want to.

As I said up top, those using a ketogenic diet therapeutically might need to track ketone levels. For certain conditions like epilepsy, patients might aim for ketone levels of 4.0 mmol or higher. Some folks also monitor their glucose-ketone index (GKI). This ratio reflects the relative availability of the two fuels and is especially used in cancer treatment. A blood BHB measurement is required to calculate GKI.

You might want to track your ketones if you’re running an n=1 experiment. Maybe you want to see what happens when you eat more protein or carbs, or you’re gauging your reactions to certain foods. Athletes might test their ketones to dial in their fueling strategy.

I’ve also had readers tell me that tracking their ketones helps them stay motivated. That’s great as long as tracking doesn’t add stress or anxiety to your life. If you can absorb the information impartially, go for it. If a low ketone reading is going to send you into an emotional tailspin, it’s not worth it. Testing is not exact anyway. There’s always a margin of error.

Just remember that whether you’re trying to lose weight, live to 120, reduce inflammation, or whatever else, your diet is only one of many factors that play into your success. Chasing high ketones is almost never necessary. I know some people swear that they feel better above a certain level. If it’s true that they can feel a noticeable difference, though, they shouldn’t need to test for confirmation. Aim for a feeling, not a number.

Most of all, don’t let the results of any ketone test take precedence over your subjective experience. If you are firing on all cylinders—sleeping great, positive mood, lots of energy, feeling strong—who cares what your blood BHB is?

All that said, if you’re going to track ketones, I say don’t bother with the urine strips unless you’re only going to test for the first few weeks. If you’re planning on making it a regular thing, go ahead and invest in a blood meter. They seem to be more reliable than handheld breath meters at this point. They also measure blood glucose, which can be useful for a variety of reasons. If you don’t want to track, that’s fine too. It’s always an option later if you want more information.

Informal poll: How many of you who follow a keto diet track your ketones? What device(s) do you use? What’s your main purpose for tracking, and do you find it useful?

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