Which is more accurate for testing ketones--blood or urine?

Updated 5 months ago by Erika

There's a reason we launched a company around blood-testing for ketones, and it's a good one: testing urine for ketones is far less accurate and reliable than testing blood. Allow us to explain: 

A number of things that can cause urine strips to give false results:

  • Certain medications can inspire false readings when testing with urine. For example, 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 nitroprusside (a vasodilator that relaxes the muscles in your blood vessels to help them widen). Highly colored urine, as when taking Pyridium (for painful urinary tract infections; it turns urine purple) can cause a false reading and so can 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.

Urine strips don't measure the same thing as blood strips, even though they look for ketones. There are three different types of ketones in your body: acetoacetate, acetone, and beta-Hydroxybutyrate. Urine strips measure acetoacetate (however this can be measured directly in the urine if the acetoacetate  is in excessive levels), breath meters measure acetone, and blood meters measure beta-Hydroxybutyrate.

Urine strips were never designed for people on the ketogenic diet. They were designed to help diabetics ensure they don't have elevated ketone levels, which means urine strips are not nearly as accurate for keto dieters. 

Urine strips are unreliable for testing acetoacetate.
 As your body begins producing ketones (acetoacetate, in particular) they will not all be used up; they will spill over into your urine. The way to measure these excess amounts of ketones is through a simple urine strip, where you can note the level through the change of color on the urine strip. However, this test is not always reliable; Once your body is in a “keto-adapted” state, it will be very efficient and thus using the acetoacetate that it produces from your fat. As a result, a urine test will show a lower level of ketones, which is misleading. You are actually deeper in ketosis but the strip will say you are less so.

Electrolyte and hydration levels can modify the acetoacetate in your urine, and thus influence your test results; electrolyte and hydration levels affect the solution of acetoacetate in your urine. and thus, your test results.

Blood strips measure beta-Hydroxybutyrate (BHB), which is a ketone body produced by the liver, from fat, for energy when glucose isn't available.This measurement is considered  to be the more accurate measurement by far of ketone levels. For this reason, all clinical studies are done with blood testing. 

Thus, when considering urine testing versus blood testing, ask yourself these questions: Do I want my measurements to be as accurate as possible? Once I become fat adapted on my keto diet, do I still want to measure my ketone levels? If the answer to either of these questions is yes, you want to test with blood strips because blood strips are more consistently accurate for testing ketones.

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