- Blood ketone testing is not affected by water intake as much as urine strips and are more accurate as a result
- Blood ketone testing is accurate at the time taken whereas urine strips test the amount of ketones that the body has previously excreted, possibly some time ago
- Blood ketone testing is more exact than urine testing
- You’re trying to enhance your ketosis
When I first saw the prices for blood ketone testing meters (and worse yet, the testing strips), I didn’t think they would catch on. Urine test strips are cheap and easily portable for testing wherever you are, so there would have to be a really good reason for testing blood instead.
I was getting inconsistent results from my diet and didn’t really know why. Granted, I wasn’t doing a 3:1 or even a 2:1 diet (see “What are the types of ketogenic diets?” for an explanation), but I should have been able to lose more weight on a 1:1 diet. I was using urine test strips daily and was always in ketosis, although not always in optimal ketosis.
Because I was serious about the diet (I had lost weight before starting keto, but wanted to lose a lot more), I decided to get a meter so I would better understand exactly how well I was doing.
I drink a lot of water and I know that urine test strips’ accuracy can vary quite a bit depending on your urine output. This is because your body tries to keep a steady blood volume and when you increase how much you drink, you increase how much you urinate.
However, your kidneys filter out more or less the same amount of other chemicals no matter what your water intake is (including ketones), so if there is more water, the numbers are diluted and if less water, those chemicals are more concentrated. This has no impact on your day to day living, but when measuring the amount of something (think ketones) in urine, the amount of water makes a difference.
If you were to do a blood test, drink a gallon of water, and retest an hour later, your ketone results would be similar, possibly a little lower, but similar. If you were to do the same with urine test strips, the results would vary greatly. For that reason, at any given time, the blood test strips are more accurate than urine test strips.
Timing of testing matters
Blood ketone testing measures the amount of ketones circulating through your bloodstream at the time the testing is done. It is as close to a “live” view of your ketosis status as we can currently get.
Urine ketone testing measures the amount of ketones found in the urine at the time of testing. Because urine is stored in the bladder and fills up over time, urine testing measures the total amount of ketones in the urine that has accumulated since the last urination.
If you test your urine four hours after the last time you urinated, the ketone level measured is really an average of the ketone levels over the past four hours. Although this level of accuracy may be beyond what a casual keto dieter cares about, it is still an accuracy issue.
Blood ketone testing is more exact
Urine ketone test strips are based on a visual scale from tan (negative for ketones) to pink (~5 mg/dL) and getting darker to purple as the concentration increases. Additionally, there is, depending on the manufacturer, a scale of five different colors that each represent a different level.
The largest manufacturer of urine test strips has a scale labeled at 5 mg/dL, 15 mg/dL, 40 mg/dL, 80 mg/dLa nd 160 mg/dL, of which only the first three (and negative) are really relevant for monitoring ketogenic diets. You look at the color of the test strip and compare to the scale and your level is whatever color is closest on the scale.
If the color is tan, you are not in ketosis. If it is light pink, you are “barely in ketosis” or trace ketosis. If it is as dark as the 15 mg/dL, you are at the low end of optimal ketosis. If it is as dark as the 40 mg/dL, you are at the high end of optimal. There is no advantage to having levels higher than 40.
So urine test strips can only tell you whether you are in ketosis or not and whether you are in optimal ketosis or not. Even that is a guess though, as the scale is visual and you are comparing two colors. Trying to figure out exactly how far into ketosis you are is impossible.
Considering the previous four reasons that blood ketone testing is better than urine testing, you can probably see how limited the information is that urine testing gives you. Are you in ketosis, yes or no? If yes, are you in optimal ketosis? That is about it.
If you are trying to enhance your ketosis through experimentation, the only real option is with a meter. If you want to see what the effect of eating at 10 PM has on your ketosis, you’ll need a meter. Want to see how many carbs it takes to kick you out of ketosis, you’ll need a meter.
If you want to lose a lot of weight quickly, but safely, you should get a meter. You will have more control over testing, be able to check if tweaks have helped or hurt, and know where you stand at any given moment. You will not have that option with urine testing.
*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure or prevent any disease.