I'm glad to see somebody here bought a home blood ketone meter! I used to recommend them here and on other forums, but I stopped because nobody ever listened.
My ketone meters are Precision Xtras (I own three). Until recently the Precision Xtra was the only home ketone blood meter on the market. This is the first time I've heard of the Nova Max.
Here's an article I wrote last year about the Precision Xtra:
Precision Xtra on KetoCure.com
You mentioned references; there are plenty in that linked article.
I don't think you need to worry about pathological ketoacidosis. It occurs as a result of disease (e.g. diabetes or alcoholism). I can't find any mention in the literature of it happening to healthy people as a result of diet or fasting, no matter how high their ketone levels climb.
If there were any danger of ketoacidosis in healthy people, I would expect to find it mentioned in Epilepsy and The Ketogenic Diet by Strafstrom and Rho. This is the standard medical textbook on keto diets. It has extensive sections on biochemistry and clinical applications. The book doesn't mention ketoacidosis (I searched a PDF version).
If there were any risk of ketoacidosis, another place I'd expect to see it mentioned is a terrific review paper on fasting from 1982 called "Fasting: The History, Pathophysiology, and Complications" by Kerndt et al. This paper reviews 175 earlier papers on fasting and tabulates every medical complication that had ever been reported. It doesn't mention even a single instance of pathological ketoacidosis.
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Like all home meters, when you use the Precision Xtra to measure glucose, it's not very accurate. But when you use it with ketone strips to measure beta-hydroxybutyrate, it's a whole other story. The Xtra's ketone strips are incredibly accurate. The Xtra's ability to measure blood ketones has been evaluated about a half dozen times in peer-reviewed scientific publications, and it has been repeatedly found to be very nearly as accurate as reference lab equipment costing many thousands of dollars. It's an incredible little machine.
Any device of this type is probably more accurate than urine strips because urine strips don't measure the thing we're interested in, which is the concentration of ketones in the blood. Moreover, urine strips don't measure urine ketones accurately, and they have a truncated range ... purple is not a very high level, but that's the highest they go.
Maybe the Nova Max is as accurate as the Precision Xtra. I don't know.
In order to evaluate your 3.5 mmol/L number exactly we'd have to know two things:
What substance does the Nova Max measure? Is it beta-hydroxybutyrate? There are three different ketones; beta-hydroxybutyrate is one of them.
Is it reporting a plasma or whole blood number?
I'll assume your machine measures beta-hydroxybutyrate and reports in terms of plasma. That's how the Precision Xtra works.
3.5 mmol/L is a pretty high number. But to put it into context, I've gotten as high as 7.7 on a clinical ketogenic diet -- in other words, I was getting ninety percent of my calories from fat.
If you're trying to lose weight, I think 3.5 is a fine number. Ketosis of that level and higher often suppresses appetite and you may be able to get yourself into a virtuous circle where your appetite gets so low that you practically stop eating, in which case you'll be in high ketosis, which suppresses appetite... around and around. Nutrient-dense foods like eggs and liver and kale are your friends if that happens.
If you're not trying to lose weight, I'm not sure 3.5 is healthy over the long run. I've been at that level or higher for a year and a half in order to control my migraines, and I don't like it. My gut feeling is that it's not healthy. I'm only doing it because of my headaches.
There's a lot of scientific literature about the effects of long term ketosis on kids, but very little about adults.
I would watch for any of the following warning signs and reduce the level of ketosis if they appear.
You can reduce the level of ketosis by eating a spoonful of rice syrup mixed in water. I do this a few times a week whenever my ketosis feels too high. The doctors who prescribe keto diets for children tell them to use orange juice for this purpose, but I think rice syrup is healthier since it metabolizes to nearly pure glucose.
Lethargy, listlessness, fatigue.
Dry mucus membranes.
Impaired immmune system.
Elevated or irregular heart beat.
A feeling of stress or fight or flight response.
Severe orthostatic hypotension (the feeling that you might faint when you stand up suddenly).
Another thing to consider is that clinical ketogenic diets have been shown to increase the risk of kidney stones by a pretty large factor. Doctors often prescribe huge amounts of potassium citrate to kids on clinical keto diets as a precaution. My urologist told me the best precaution against kidney stones is to drink lots of water so the urine doesn't become concentrated. I don't know if this is an effective precaution against the type of kidney stone associated with ketogenic diets, but I've been following my doctor's advice for the last year and a half and so far it's working, despite my history of kidney stones.