Urine vs Blood
Obviously, we're a little biased on this one, but let me give you a few facts about the difference between measuring ketones in your urine and measuring them in your blood.
We'll start with the urine strips. First, there are a number of things that can cause these strips to give false results as listed below:
Certain medications can give false readings when testing with urine. Levadopa taken for Parkinson’s or Restless Leg Syndrome can cause a false negative result. Ace Inhibitors such as Lisinopril can cause false positives when using urine tests containing nitroprussides. Highly colored urine as when taking Pyridium (for painful urinary tract infections – turns urine purple) can cause a false reading as well as using a medication called Depakote. Urine can be inaccurate due to sitting in the bladder for an extended period or remaining in the ambient air for a long time. Urine concentration can cause inaccurate results. Menstrual blood in the sample will cause inaccuracy. Excess ascorbic acid or Vitamin C can cause a false positive result. Urine test strip reagents lose potency when exposed to air; bottles of strips open for 90 days can cause inaccurate results.
Next, these strips don't measure the same thing even though they look for ketones. There are 3 different types of ketones in your body, acetoacetate, acetone, and Beta-hydroxybutyrate. The urine strips measure acetoacetate, breath meters measure acetone and blood meters measure Beta-hydroxybutyrate.
Urine strips were never designed for people on the ketogenic diet, they were designed for diabetics to make sure they don't have elevated ketone levels so they are not nearly as accurate for keto dieters. Here is more information on that.
One of the ketone bodies, acetoacetate, can be measured directly in the urine if they are in excessive levels.
The way metabolic substrates get into your urine is only if they are “spilling over” into your urine. If there is a massive excess of a certain metabolic substrate, or if you are not using a substrate, it will spill over into your urine. This is actually a good thing. When carbohydrates increase in your blood stream to an excessive level, insulin is secreted, and then they convert to fat. This process also includes raising of inflammation. When ketone body levels increase, they are just excreted through your urine instead of having to be stored back as fat.
As your body begins producing ketones (acetoacetate, in particular) they will not all be used up. The way to measure these excess amounts of ketones is through a simple urine strip. These will spill over into your urine and you can note the level through the change of color on the urine strip.
However, this is not always a reliable test. Once your body is in a “keto-adapted” state, it will be very efficient and using the acetoacetate that it produces from your fat and you will have a lower level of ketones that are reading on the strip. This is misleading. You are actually deeper in ketosis but the strip says you are less so.
Other factors that can modify your urine levels are electrolyte and hydration levels, which affect the solution of acetoacetate in the urine and can give variable readings.
Then we have the blood meters that measure bhb. They are considered by far to be the more accurate measurements. All clinical studies are done with blood for this reason.
How accurate do you want your measurements to be? And once you become fat adapted on your keto diet, do you still want to measure your ketone levels? If so, you will need to use a blood meter.