I am wondering whether anyone else has been puzzled by the following:
The control solution is set at 0.12% concentration. That converts to 6.67 mmol/l or thereabouts. The range of acceptable readings for the same solution, however, is set at 6.7 - 9.0 mmol/l
My first thought was that there must be some mistake. How can the actual value of the solution be right at the bottom of the range of acceptable readings?
Then I tried testing the control solution and indeed, got 8.1, right in the middle of the range. Trouble is, the actual value is 6.67, so all my readings are going to be out by that amount.
I was alerted to this in advance, when I had a fasting BG test result from the hospital. It was 5.3, while my meter had told me it should be 6.7.
I'm still not sure what to make of this. Any thoughts?
If you real blood glucose is 7.85 mmol/l then the expected deviation can be +/- 20%. This means +/- 1.57 mmol/l => lower margin 6.28 = 113 mg/dl
=> upper margin 9.42 = 169 mg/dl
=> pretty much the range of the control solution. Of course the more in the middle the better.
Blood glucose soluted in fluid is not distributed equally. The enzymatic reaction in the test stripe will not always react in the same way. At the end you want to produce cheap test stripes for daily use. The deviation is our price to pay for the revenue of the manufacturers because the stripes are highly overpriced. Most of us pay 1/2 to 1 dollar per stripe but the production value is cents.
I wrote "Blood glucose soluted in fluid is not distributed equally" but actually I meant "Blood glucose soluted in the circulatory system and layers of the skin is not distributed equally".
Right - I am Ok with everything you explained, thanks.
The problem is that my control solution is actually 0.12% glucose, or in UK units 6.67 mmol/l (120 mg/dl in the US). So you would expect the control test to produced results centered on that, no matter what margin is allowed. But it isn't. The error margins for reading the control solution are given as 6.7 and 9.0 mmol/l. And when I run a control test I get 8.1, rather than 6.67.
It's like the manufacturers are saying:
1. This control solution is 6.67mmol/l 2. When you sample it you should get between 6.7 and 9.0
And indeed, you do - you get 8.1
Why is a 6.67 solution producing a result centring on around 8, and by design? Doesn't that imply that my BG readings will also be out by the same amount? The meter is known by the manufacturers to be reading too high, surely, but they don't say so.
Hello. I'm Rob Muller with Roche here in the U.S. Please call customer support in the U.K. at 0800 701 000. I feel sure they will be able to answer you questions about the control solution. That said, please let me know if they don't. :-) Have a good day.
Best regards,
Rob
I have actually called them, and the guy said it was the first time he'd been asked this question. He needs to consult and get back to me.
I am surprised that no-one wants to know the exact concentration of the control solution; it's the only way to see how your meter is performing. All this time I was getting preprandials of around 6.5, and in fact I was 5.3. I needed to know that.
One factor you need to remember is that blood sugar meters measure plasma glucose, not whole blood glucose. To convert a whole blood glucose concentration to a plasma concentration you multiply by 1.15. In this case, the control solution mimics a plasma glucose reading of 6.67mmol/L which actually corresponds to a whole blood reading of (6.67/1.115) mmol/L, which is 5.8 mmol/L, even worse than you thought.
It is quite possible that the information you have on the 0.12% concentration is really not relevant or meaningul. I don't even know what % means, is it by weight or volume? We measure blood sugar in weight(mass)/volume. The control solution is specifically formulated to mimic presumably centered in the expect normal range for the meter, typically something like 120 mg/dl (6.67 mmol/L) or similar.
Ok, so I worked out the math, and that is correct a 0.12% solution by weight corresponds to 120 mg/dl (6.67 mmol/L). So I guess I don't understand either. Perhaps we just need to see what Roche says.
So just to check my understanding, are you saying that medically blood glucose means plasma BG, and the meter measures plasma BG, and also the control solution mimics a plasma 0.12% BG. Therefore, the whole blood BG, which is less, doesn't come into it?
The blood that comes out when you lance is plasma, it has more water than the blood in your veins and arteries and because of that it has a higher glucose concentration. Given the situation, I'm not sure it comes into it.
OK, well just as an update - I have been told on another forum that:
"It's control solution not blood. Meters have to correct for all sorts of blood effects - haematocrit and other endogenous interferents. These don't exist within control solution. All you need to worry about is whether the meter reads within the specified range. It looks as though it reads nearly smack in the middle so that's good. "
Whether that means my meter is accurate with real blood I am still not at all sure. Two readings at home around one at the hospital showed over 1 mmol/l difference.
But that is what technical deviation means. It can deviate from test to test - even with the same blood. With +/-20% deviation you will see differences of +/- 1 mmol/l and more.
Apart from variations due to inaccuracy, though, there is a systematic difference between the reading that control solution and blood will produce, as I understand it.
Well, there is another possibility. Dr. Bernstein in his latest teleseminar talked about the new Aviva plus strips. I believe he said that in order to reduce false lows that were occuring with maltose (?) Roche had reformulated their Aviva strips to create the new "plus" strips. He criticized the plus strips having fixed the maltose counfounding, but ruined the readings by introducting systematic bias causing reading to be low by 12%.
I'm coming back as promised to report on progress - such as it is. Here is the explanation I got from Roche. Remember, all I want to know is how inaccurate my BG readings might be. In response to the below explanation, I then asked them whether the errors to be expected for control solution in non-lab conditions should also be expected for BG readings.
No reply yet!
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Thank you for contacting Accu-Chek.
Please be aware that performing a control test in your home environment cannot by compared to performing a control test in a laboratory, as the strict laboratory testing conditions are not met in the home environment. This means that achieving the target reading of 6.67 mmol/l is not possible. Due to these factors a target range has instead been calculated. This range takes into consideration the effects of temperature deviations and possible sources of contamination. The specific target range for the Accu-Chek Compact Plus control solution is printed on the peel-off label attached to the drum container.
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I'm just following up my enquiry, as I am quite urgently needing to know how to interpret my meter readings. My question is:
Since, as you explained, the measurement of glucose in the control solution is inevitably subject to error because of non-laboratory conditions, is the same true of blood glucose measurement? For example, if 6.7 mmol./l control solution reads out at 8.1 mmol/l, as a result of non-laboratory conditions, might I expect a similar margin of error for blood glucose readings?
I am concerned because the hospital reading was 5.3 for FBG, while my meter read 6.6 at the same time (within twenty minutes either side of the hospital sample).
Told to expect a response a week last Monday, eight days ago, but none arrived, so now I have written to the CEO (MD) of the UK branch. There has to be a simple answer, but they don't seem to want to provide it.
