If I can hazard my reasoning... the home fingertip (capillary) sample meters we use are programmed to read a higher value "equivalent" to the venous readings that would be taken by a lab.
The venous bg numbers are measurably higher than the fingertip numbers. See e.g. Comparability of venous and capillary glucose measurements in blood. My understanding is that the bg meters you and I use, take the capillary numbers and add/multiply/(or look up in a table), the correction factor to convert to equivalent venous numbers. So a good chunk of the difference between 6.67 and 8.1, is explained by the meter converting a capillary reading to a venous reading. And I'm sure the "range of acceptable readings" for test solutions also includes this conversion factor.
Confusing enough? So 6.67 mmol/l on the test solution, may very well be the same as the 8.1 mmol/l venous value that the meter is calibrated and programmed to display.
The literature references the "the published WHO capillary blood equivalence values for venous plasma values" but googling hasn't turned up this equivalence table yet.
Thanks! That is very interesting and at least gives me a new line to investigate. It would be easier if Roche would take the trouble to explain.
It still doesn't explain the discrepancy between my home test readings and the hospital reading, though. Significantly, that error margin corresponds closely with the control v blood glucose error.
The investigation you cite seems a bit inconclusive, though:
RESULTS:
The relationship between venous and capillary glucose values varied, with venous plasma being higher than capillary blood for random and fasting samples but lower for sampling 2 h after oral glucose. Discrepancies were observed between measured capillary blood values and the published WHO capillary blood equivalence values for venous plasma values for all except a fasting venous value of 7.0 mmol/l. For example, for a fasting venous plasma glucose of 6.1 mmol/l the WHO equivalent value is 5.6 mmol/l, while the measured value was 5.2 mmol/l, and for a 2-h venous plasma glucose of 11.1 mmol/l the WHO value is 11.1 mmol/l, while the measured result was 11.7 mmol/l.
CONCLUSIONS:
These results highlight the difficulty in equating glucose levels from one sampling and measuring procedure to another, and raise uncertainties about current published equivalence values which could lead to misclassifications in glucose tolerance status.
Out of interest, I am just reading about this. Apparently, after meals, or more generally non-fasting, arterial and capillary (finger-prick) levels closely correspond, but venous glucose (as measured in hospital) is lower.
So that would be a potential explanation for my home v hospital discrepancy.
The trouble is, however, that fasting concentrations tend to be pretty much the same for venous, capillary and arterial values. My hospital measurement was FBG.
Back to square one! My venous, capillary and arterial levels should all be about the same for FBG. Instead, my home FBG capillary test reads 1.3 mmol/L (23.4 mg /dL) higher than the hospital venous reading. I can't see the Accu-Check makers building in an error that would throw the FBG reading out by that much on a regular basis.
Quote:
1. Glucose test values may not match with different blood samples because glucose is being consumed by the body
Glucose diffuses through the capillaries and is consumed by the cells, so arterial glucose concentration (the capillaries' source) should be higher than venous glucose concentration (the capillaries' drain) unless capillary diffusion or muscle glucose consumption has been stopped. It has been shown that in fasting subjects the glucose levels in arterial, capillary, and venous samples are practically the same (venous glucose is generally 2-5 mg/dL lower than fingerstick capillary or arterial blood glucose)[5,6]. It is only after meals, when glucose uptake in the periphery is rapid, that glucose levels in fingerstick capillary blood samples can exceed those in concurrently drawn venous samples. A typically quoted value is up to 80 mg/dL difference between venous and fingerstick capillary blood glucose values one hour after ingestion of 100 grams of glucose[2].
