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