14.1.8 Test Strips & Biosensors

Measuring urine glucose concentration

• People with diabetes cannot control their blood glucose concentration so that it remains within normal, safe limits
• The presence of glucose in urine is an indicator that a person may have diabetes
  • If blood glucose concentration increases above a value known as the renal threshold, not all of the glucose from the filtrate in the proximal convoluted tubule is reabsorbed and some will be left in the urine

• Test strips can be used to test urine for the presence and concentration of glucose
• Two enzymes are immobilised on a small pad at one end of the test strip. These are:
  • glucose oxidase
  • peroxidase

• The pad is immersed in the urine sample for a short time
• If glucose is present:
  • Glucose oxidase catalyses a reaction in which glucose is oxidised to form gluconic acid and hydrogen peroxide
  • Peroxidase then catalyses a reaction between the hydrogen peroxide and a colourless chemical in the pad to form a brown compound and water

• The colour of the pad is compared to a colour chart – different colours represent different concentrations of glucose (the higher the concentration of glucose present, the darker the colour)
• Urine tests only show whether or not the blood glucose concentration was above the renal threshold whilst urine was collecting in the bladder – they do not indicate the current blood glucose concentration

Top: the chemical reactions catalysed by the immobilised glucose oxidase and peroxidase enzymes on the end-pad of a urine test strip. Bottom: an example of the colour chart used to interpret the colour of the end-pad of a urine test strip. In this example, the concentration of glucose is given in mg per cm³

Measuring blood glucose concentration

• A biosensor can be used by people with diabetes to show their current blood glucose concentration
• Similar to the test strips, a biosensor uses glucose oxidase (but no peroxidase) immobilised on a recognition layer
• Covering the recognition layer is a partially permeable membrane that only allows small molecules from the blood to reach the immobilised enzymes
• When a small sample of blood is tested, glucose oxidase catalyses a reaction in which any glucose in the blood sample is oxidised to form gluconic acid and hydrogen peroxide
• The hydrogen peroxide produced is oxidised at an electrode that detects electron transfers
• The electron flow is proportional to the glucose concentration of the blood sample
• The biosensor amplifies the current, which is then read by a processor to produce a digital reading for blood glucose concentration
• This process is complete within a matter of seconds

The parts of a glucose biosensor