There’s some bad info floating around about how to convert units for ketones. The strips show the measurements in mg/dL but some articles about diabetic ketoacidosis list their measurements in mmol/L.
The typical advice is to divide by 18. This is wrong. That technique works for measuring glucose. It doesn’t work for ketones.
mmol means milli-mole. A mole is 6.02214076×1023 particles. A millimole is 1/1000th of a mole, or 6.02214076×1021 particles. Diabetics are looking at two different particles: glucose molecules, and “ketones”. Ketones are one of three different molecules.
All these molecules have different weights, and this affects the conversion.
1 mole of glucose doesn’t have the same weight as 1 mole of acetoacetic acid.
Another way to think about it, is that we’re going from mass/volume (mg/dL) to number-of-particles/volume (mmol/L). To perform the conversion, we need to know the mass of each particle.
I was not good at chemistry, so I won’t try to explain this further. I just used the calculator linked above to make a conversion table for my ketone test strips:
Ketone test strips have the following traditional units, and this table shows the equivalent SI units.
5 mg/dL = 0.4898 mmol/L
15 mg/dL = 1.4693 mmol/L
40 mg/dL = 3.9182 mmol/L
80 mg/dL = 7.8364 mmol/L
100 mg/dL = 9.7955 mmol/L
For a more complete table, see Ketone Conversion Table on this blog.
Blood ketone testing is better. Urine testing is subject to some issues, because it’s testing urine that’s accumulated for hours.
However, if you have strips, that’s better than nothing.
(Also, I’m not sure how the calculator is making this conversion, because “ketones” are three different molecules, according to the information on the calculator.)
According to the NHS, the measurements associated with DKA are as follows:
Check your blood sugar level if you have symptoms of DKA.
If your blood sugar is 11mmol/L or over and you have a blood or urine ketone testing kit, check your ketone level.
If you do a blood ketone test:
- lower than 0.6mmol/L is a normal reading
- 0.6 to 1.5mmol/L means you’re at a slightly increased risk of DKA and should test again in a couple of hours
- 1.6 to 2.9mmol/L means you’re at an increased risk of DKA and should contact your diabetes team or GP as soon as possible
- 3mmol/L or over means you have a very high risk of DKA and should get medical help immediately
If you do a urine ketone test, a result of more than 2+ means there’s a high chance you have DKA.
Translated to the TRUEplus Ketone test strips, this is:
If your result is darker than the “SMALL” “15mg/dL” reading, there’s a high chance you have DKA.
Also, if you test at “MODERATE” “40mg/dL” you have a very high risk of DKA and should get medical help immediately.
The Erroneous “18” Conversion is Dangerous
To underscore the risks of that bad advice, consider this.
If you did a conversion of 40mg/dL to mmol/L by using the “18” method, you would end up with “2.2mmol/L”.
In our table above, the true measurement is 3.9mmol/L.
Applying the advice above, the “40mg/dL” “MODERATE” reading and the erroneous conversion would say we’re only at 2.2, leading us to the advice of “increased risk of DKA.”
The correct advice, using the calculator’s numbers, would mean “very high risk of DKA and get medical help immediately.”
Additionally the Ketone Test Strips Don’t Measure All Ketones
The strips measure acetoacetate (AcAc), but in diabetic ketoacidosis the ketone 3-b-hydroxybutyrate (3BH) increases drastically. Please read Ketone Bodies: a Review of Physiology, Pathophysiology and Application of Monitoring to Diabetes, and the Wikipedia article about Ketone bodies to learn more.
(I suspect this change in proportion is probably why the NHS advises medical attention when the test reports more than 2+ mmol/L, while, for the blood test, they aren’t as adamant until you are at 3 mmol/L.)