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Saturday, January 10, 2015

How does glucometer or Glucose monitoring device work?

Glucose testing is an important part of a diabetic’s daily health care. Without testing, a diabetic can easily become ill, because his glucose levels are not where they need to be.



To do glucose testing, a diabetic uses a glucose testing meter, which uses a glucose testing strip.



A glucose meter or glucometer, is a medical device used for measuring the approximate level of glucose in the blood.

Working principle of glucose meter:

The glucose meter, determines the concentration of glucose in the solution. Most glucose meters, are based on electrochemical technology, they use electrochemical test strips to perform the measurement.

A small drop of the solution to be tested, is placed on a disposable test strip, that the glucose meter uses for the glucose measurement.

Glucose meter test strips

Glucose strips, that are used for glucose monitoring from blood. 






In each test strip, there is an enzyme called glucose oxidase. This enzyme reacts with the glucose, in the blood sample and creates an acid called gluconic acid.

The gluconic acid then reacts, with another chemical in the testing strip called ferricyanide. The ferricyanide and the gluconic acid, then combine to create ferrocyanide.

Once ferrocyanide has been created, the device runs an electronic current through the blood sample on the strip. 



This current is then able to read, the ferrocyanide and determine how much glucose is in the sample of blood, on the testing strip. That number is then displayed on the screen of the glucose testing meter.

Methods used in electrochemical measurement of glucose

The two most common methods, used in electrochemical measurement of glucose are:
  • Colorimetric method and
  •  Amperometric method.

Colorimetric method

In this method, the typical sensors such as LEDs or photo sensors form the analog interface. These sensors are followed by a Transimpedance Amplifier (TIA) for the glucose concentration measurement in the solution.

The Colour Reflectance principle is used in this method to sense the colour intensity in the reaction layer of the test strip by the photometry.

The glucose meter generates a numerical value, that is a measurement of the glucose concentration present in the solution.

Amperometric method

In this method, the electrochemical test strip contains a capillary that is used to draw in the solution placed at one end of the test strip. The test strip also contains an enzyme electrode containing a reagent such as Glucose Oxidase. 




Glucose undergoes a chemical reaction in the presence of enzymes and electrons are produced during the chemical reaction. These electrons (i.e., the charge passing through the electrode) are measured and this is proportional to the concentration of glucose in the solution.

An ambient temperature measurement is also made in order to compensate for the effect of temperature on the rate of the reaction.

The solution sample (Blood) is placed on the test strip and the reaction of the glucose with the enzyme takes place.The flow of electrons will correspond to the flow of current through the working and the reference electrodes. This current will change according to the glucose concentration.

The current is measured using a transimpedance amplifier (current-to-voltage converter) and an analogue-to-digital converter (ADC). The output of the transimpedance amplifier will be seen as a variation in the voltage with varying glucose concentrations in the solution. 

Factors affecting Glucose Measurement
  • Temperature,
  • Humidity,
  • Altitude, etc.
This is because the rate of the enzyme reaction depends on these and other factors.

References:
  • Glucose Meter Reference Design, Microchip. 
  • Glucose Meter Fundamentals and Design, Freescale Semiconductor Inc.
  • Other Internet Sources

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