Dave, here is the basic chemistry of Glucose which meters use to give a blood glucose reading.
Glucose is a simple sugar, a monosaccharide and the most important Carbohydrate in biology. It is used as an energy source by living cells through Respiration and is a major product formed by plants through Phytosynthesis.
Glucose (C6H12O6) is an Aldohexose because it has six carbon atoms and an aldehyde group which exist in an open chain (acyclic) aldehyde form and the ring (cyclic) form. With Aldehydes, the double bonded Oxygen atom and the Hydrogen atom are bonded to Carbon No.1; C-1.
The Anomers of Glucose are α -glucose and β -glucose and differ structurally by the relative positioning of the Hydroxyl group attached to C-1 and the group at C-6. With α -glucose the hydroxyl group is attached to C-1 in the trans position relative to the - CH2OH group at C-5, portrayed as a Haworth projection , while β means it is cis . Another method of identifying α -glucose from β -glucose is by stating if the Hydroxyl group at C-1 is above or below the plane of the ring. Confusion can arise with this method if the Glucose ring is drawn upside down or in the chair conformation.
Within the cyclic forms of Glucose , three Rotamer Conformations exist. Rotation occurs around the O6-C6-C5-O5 Torsion Angle called the ω-angle . The three stable staggered Rotamer Conformations are gauche-gauche (gg), gauche-trans (gt) and trans-gauche (tg).
In solution, the open chain aldehyde form of glucose exists in equilibrium with several cyclic isomers , each containing a ring of carbons closed by one oxygen atom. In aqueous solutions however more than 99% of glucose molecules, at any given time exist as pyranose (Glucopyranose) which is a ring of five Carbons and one Oxygen atom. C-1 to C-4 are linked to a Hydroxyl side group and C-5 being linked to C-6 outside the ring, forming a CH2OH group. The open chain aldehyde form is limited to about .25% and the furanose (Glucofuranose) exists in negligible amounts.
Alpha-D-Glucose has a specific rotation of + 113 and beta-D-glucose only +19 . If either pure form is dissolved in water, the rotation gradually changes until it reaches an equilibrium value of +52.5 . This process of the change in rotation is called mutarotation . In aqueous solutions equilibrium is reached in a matter of hours. Alpha-L-Glucose has a specific rotation of - 113 and beta-L-glucose only -19. L-glucose is not usually found in nature. At room temperature equilibrium takes about 3 hours. As the temperature rises mutarotation increases rapidly.
At pH 7 in aqueous solutions , glucose exists in solution in cyclic hemiacetal form as about 63.6% β-D-glucopyranose and about 36.4% α-D-glucopyranose , the proportion of linear aldehyde and furanose forms are negligible. The glucose oxidase binds specifically to β-D-glucopyranose and does not react with α-D-glucose . It is able to oxidise all of the glucose in solution because the equilibrium between the α and β anomers is driven towards the β side as it is consumed in the reaction.
Glucose oxidase has given way to Glucose dehydrogenase because it has over 25 times higher activity than Glucose oxidase enabling rapid glucose sensing.
As I have said my knowledge is limited to chemistry. When I have made 5mmol/L solutions of Glucose and tested my Performa and Optium meters I got constant but not expected results.
After equilibrium occurred the Performa read 11.8 and the Optium 7.4 mmol/L and an average 9.6 which is almost double the expected reading.
The only way that I can explain the readings compared to blood glucose meter readings is that as Alpha-D-Glucose is utilized and beta-D-glucose mutarotates quickly into Alpha-D-Glucose.
The liver only releases glucose back into the blood as Alpha-D-Glucose.
From my data I believe that the algorithms for the FreeStyle Optium favor Alpha-D-Glucose because the Optium tends to be higher when I have no bolus insulin, have dawn phenomenon, just eaten or I am not active for a period of time. My readings are usually higher during these periods.
The readings for the Accu-Chek Performa will be higher when I have bolus insulin, been physically active and in the Hypo range. If I have a Hypo and both meters give similar readings, I know I have to quickly treat with dextrose where as with a physically induced Hypo, I will eat a meal.
Dave, I mainly use the Performa but will always check with the Optium if I don’t feel right. If meter readings are very different at normal BGL, I will wait until readings are about equal before I decide how much insulin I have to inject.
I have never had a Contour Next meter but I am happy with my meters as I know how they relate to my 3 monthly blood tests.
I stand by what I have said, “When the concentrations of Glucose are changing rapidly or levels are high with limited removal of Glucose, the meter readings may be at the limit of its accuracy range of 15%.
May be when the IR (Infra Red) technology is perfected for blood glucose meters, we will have a more accurate result.