The science behind the glucose test strip

There’s a lot of talk in the DOC about making a more accurate BG test strip. We want a test strip that is inexpensive with as little error as possible. That’s not too much to ask. Or is it? I decided to look into how current BG strip technology works and do a few basic calculations to see just how big of a difference there is between blood with a BG of 40 verses that with a BG of 300 mg/dL.

Test strips contain two chemicals; an enzyme called “glucose oxidase” which is transforms glucose into gluconolactone, and a “mediator” (typically ferricyanide) which participates in the reaction and produces ferrocene and releases electrons.  The electrons make up the current that the meter then interprets as a BG value.  When there is a higher concentration of glucose in the blood there will be a higher concentration of electrons produced which will cause an increase in the current sent across the electrode on the strip, and a higher BG reading is obtained.

Other compounds which are often found in the blood also transform the ferricyanide into ferrocene and release electrons.  (Such as acetomenophen, vitamin C, and uric acid.)  To prevent this from skewing the test results, each test strip has two working electrodes (plus a reference).  One working electrode has both of the chemicals I described above, and the other has only the mediator but no enzyme.  Thus both electrodes measure the current that results from acetomenophen, vitamine C, and uric acid, but only the electrode with the enzyme also measures the current produced by the glucose reaction.  This allows the meter to subtract out the current that results from these other compounds in the blood.

Current meters can measure as little as 36,000 molecules of glucose in 30uL of whole blood (uL = microliter, most strips use about 3uL).  A single gram of glucose contains around 3,400,000,000,000,000,000,000 molecules (that’s 3.4×10^21).  After thinking about all of this, I began to wonder just how different 40 mg/dL blood is from 300 mg/dL blood.

I decided the best way to compare would be to figure out what mass percent these numbers translate to.  I found online that the approximate density of blood is 1.05 g/mL which is equal to 1.05 mg/uL (which means that 1uL of blood has a mass of approximately 1.05mg).

A BG reading of 95mg/dL can easily be converted to mg/uL by dividing by 100,000.  So,  95mg/dL = 0.00095 mg/uL (I chose this BG value since it is within range of a nondiabetic person’s average BG thus the density of blood given online should be most correct at this level.) This means that in 1 uL of blood, there are 0.00095mg of glucose  (or 3.23×10^15 molecules).

To get a mass% (the percent of the total mass of the whole blood that is glucose) just divide the mass of glucose in 1uL by the mass of 1uL of blood: 0.00095/1.05 = .000904 or 0.0904%.

Using the same math, a BG of 40 mg/dL = 0.038% and has 1.36×10^15 molecules per uL, and a BG of 300 mg/dL = 0.285% with 1.02×10^16 molecules in 1uL.  These differences are actually quite miniscule if you think about it.

[It is important to note that outside of the range of a non-diabetic blood sugar level, the density is going to change.  At elevated BGs, the actual density might be higher, making the number calculated using the above rational somewhat elevated, and the low BG levels will appear somewhat lower than the actual value. However, with these tiny percentages, the change in density is very likely minimal and can thus be disregarded.]

In my experience as a chemist, I have found that analytical equipment varies in price depending on the level of accuracy.  For example, for every additional decimal place that a balance is capable of measuring, the price of that balance increases exponentially.  We are essentially asking these companies to produce a product that measures the concentration of the glucose in our blood to an additional decimal place.  I would be willing to bet that the technology already exists, but the cost is well outside of what any of us can afford to pay.

Before our meters become more accurate, one of two things needs to happen.  The materials needed for a more accurate test strip using current technology need to get cheaper, or someone needs to figure out an entirely new way of measuring glucose (or at least some new chemical coatings for the electrodes).

So, that’s the way it is.  Now, feel free to form whatever opinion you’d like ;)

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4 thoughts on “The science behind the glucose test strip

  1. Pingback: {Type 1 Tuesday} 11.01.11

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  3. Preparing and applying the precise amount of glucose oxidase and ferricyanide to the strips must be a real trick. The strips themselves look fairly simple. Many thanks for covering the topic so well.

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