REDLAN
04-15-2008, 04:13 PM
seems to me that there have been a spate of peeps posting about meter accuracy.
so I thought I would muse on why it is that meters have an accuracy of around +/- 20%.
The first concept to grasp is the idea that there is no such thing as a blood sugar level - rather what we have are blood sugar levels - it will vary depending on which tissue it is measured in. whether we are talking venous, arterial, or capillary blood. level of blood supply, metabolic activity of tissues. In short no 2 fingers even if they are next to each other will have the same BG.
then there are issues with how the blood is collected - they discovered that squeezing causes plasma to appear, lowering the apparent reading. Question how do think depth of penetration of the lancet will change things? How many people use a blunt lancet - tearing of tissues will alter BG reading.
now the meters, and the illusion of accuracy - we get sucked into believing that when our meter says 8.1 mmol or 146 mg/dl that our BG really is this value. Because why would it say 8.1 if it was something else - really we're talking marketing influcences here - what would you prefer a meter that said 8.1 mmol, or one that said 8 mmol?
onto meters...
And what does a meter measure?
blood glucose of course!
actually the one thing that it doesn't measure is blood glucose. What you actually own is a sophisticated voltmeter. On the older style meters (I understand how those one's work, but the principle is the same for the newer meters)
you have a test strip. One surface of it is impregnated with glucose oxidase and a dye The dye for now is colourless. Essentially glucose oxidase breaks down glucose, which then changes the colourless dye to blue.
the strip usually has a very thin gauze strip, which ensures that a set volume of blood is brought into contact with glucose oxidase.
the meter has a little LED which emits light in the red wavelength, and a photoelectric sensor. So...
when you plug the strip in, it knows the strip is there as a voltage will be registered at the photoelectric sensor. When you add blood, the strip will start to turn blue, which will change the voltage registered by the sensor as the amount of light reflected by the strip will change. It knows that you've added blood, because it will flash the led every half a second or so. It then waits a set time, and records the voltage from the sensor (my one did a series of flashes). From this voltage it then applies an algorythm, and then confidently displays....
8.1 mmol on the LCD screen.
so just how many errors could there possibly be? These are ones just off the top of my head...
1) Any variation in blood volume on the test strip will alter the result - increase the blood volume on the strip, and the glucose oxidase has a greater supply of substrate, and will turn more of the dye blue resulting in a higher reading.
2) variation in ambient temperature - for each 10 degree Centrigrade rise in temperature there is a doubling of reaction rate. Test on a hotter day, strip turns bluer and your BG will be apparently higher.
3) timing pulse - remember the pulsing LED... variation in when you put your finger on the test strip, will change the result. give glucose oxidase half a second longer to react it will convert more glucose, and so you will get a higher reading.
4) any variation in battery power will affect the amount of light emitted by the LED, and hence the result. Bit of dirt on the photoelectric cell? same effect.
And why might a high BG be less accurately measured than a low one?
it's all down to glucose concentraion and timing pulses.
the voltage reading is made after a set time. The meter does not wait until all the glucose has been used up, but measures it at a point sometime before.
When you plop the blood on, the reaction rate is relatively high, and then drops as the glucose is used up. For low BG's nearly all of the glucose has been used up by the time the machine makes it's measurement, so any slight differences in timing will make only a very small difference to the reading.
with a high blood sugar, then relatively more glucose will be available, so timing now has a bigger impact as there is more colour change from one second to the next.
PS theses are all my musings - I didn't research them, I just applied a few principles to my understanding of how meters work.
the point is this.
for the amount of money you are prepared to pay for a meter, then it's a miracle quite frankly that they are as accurate as they are!
Even the HBa1c machines give you an A1c which is +/- 0.2 to 0.3.
so I thought I would muse on why it is that meters have an accuracy of around +/- 20%.
The first concept to grasp is the idea that there is no such thing as a blood sugar level - rather what we have are blood sugar levels - it will vary depending on which tissue it is measured in. whether we are talking venous, arterial, or capillary blood. level of blood supply, metabolic activity of tissues. In short no 2 fingers even if they are next to each other will have the same BG.
then there are issues with how the blood is collected - they discovered that squeezing causes plasma to appear, lowering the apparent reading. Question how do think depth of penetration of the lancet will change things? How many people use a blunt lancet - tearing of tissues will alter BG reading.
now the meters, and the illusion of accuracy - we get sucked into believing that when our meter says 8.1 mmol or 146 mg/dl that our BG really is this value. Because why would it say 8.1 if it was something else - really we're talking marketing influcences here - what would you prefer a meter that said 8.1 mmol, or one that said 8 mmol?
onto meters...
And what does a meter measure?
blood glucose of course!
actually the one thing that it doesn't measure is blood glucose. What you actually own is a sophisticated voltmeter. On the older style meters (I understand how those one's work, but the principle is the same for the newer meters)
you have a test strip. One surface of it is impregnated with glucose oxidase and a dye The dye for now is colourless. Essentially glucose oxidase breaks down glucose, which then changes the colourless dye to blue.
the strip usually has a very thin gauze strip, which ensures that a set volume of blood is brought into contact with glucose oxidase.
the meter has a little LED which emits light in the red wavelength, and a photoelectric sensor. So...
when you plug the strip in, it knows the strip is there as a voltage will be registered at the photoelectric sensor. When you add blood, the strip will start to turn blue, which will change the voltage registered by the sensor as the amount of light reflected by the strip will change. It knows that you've added blood, because it will flash the led every half a second or so. It then waits a set time, and records the voltage from the sensor (my one did a series of flashes). From this voltage it then applies an algorythm, and then confidently displays....
8.1 mmol on the LCD screen.
so just how many errors could there possibly be? These are ones just off the top of my head...
1) Any variation in blood volume on the test strip will alter the result - increase the blood volume on the strip, and the glucose oxidase has a greater supply of substrate, and will turn more of the dye blue resulting in a higher reading.
2) variation in ambient temperature - for each 10 degree Centrigrade rise in temperature there is a doubling of reaction rate. Test on a hotter day, strip turns bluer and your BG will be apparently higher.
3) timing pulse - remember the pulsing LED... variation in when you put your finger on the test strip, will change the result. give glucose oxidase half a second longer to react it will convert more glucose, and so you will get a higher reading.
4) any variation in battery power will affect the amount of light emitted by the LED, and hence the result. Bit of dirt on the photoelectric cell? same effect.
And why might a high BG be less accurately measured than a low one?
it's all down to glucose concentraion and timing pulses.
the voltage reading is made after a set time. The meter does not wait until all the glucose has been used up, but measures it at a point sometime before.
When you plop the blood on, the reaction rate is relatively high, and then drops as the glucose is used up. For low BG's nearly all of the glucose has been used up by the time the machine makes it's measurement, so any slight differences in timing will make only a very small difference to the reading.
with a high blood sugar, then relatively more glucose will be available, so timing now has a bigger impact as there is more colour change from one second to the next.
PS theses are all my musings - I didn't research them, I just applied a few principles to my understanding of how meters work.
the point is this.
for the amount of money you are prepared to pay for a meter, then it's a miracle quite frankly that they are as accurate as they are!
Even the HBa1c machines give you an A1c which is +/- 0.2 to 0.3.