Test strips and biosensors

Test strip- def

Test urine for presence of glucose, pH, ketones and proteins

Used to test presence and conc of glucose in urine

Biosensors- def

Device that uses biological material (enzymes) to measure conc of a chemical compound

How test strips work

2 enzymes r immobilised on a small pad at one end of the test strip

Enzymes: glucose oxidase, peroxidase

NB: pad has a cellulose membrane covering it- only allow small molecules from blood to reach enzymes

Pad is immersed in the urine sample for a short time.

Glucose present- glucose oxidase catalyses a reaction where glucose is oxidised to form GLUCONIC ACID and H20

Peroxidase catalyses a reaction between H202 + colourless chemical (chromogen)=== forms a BROWN compound and water

Colour of pad compared to colour chart- diff colours= diff conc of glucose

Higher conc= darker colour

Equations

Glucose + oxygen ——> gluconic acid + hydrogen peroxide

Hydrogen peroxide + chromogen ——> oxidised chromogen + water

How biosensors work

Glucose oxidase is immobilised on a recognition layer

Covering this layer is a PARTIALLY permeable membrane

Small sample of blood tested

Glucose oxidase catalysed reaction in which the glucose in the blood is oxidised to form GLUCONIC ACID + H202.

H+ ions released from gluconic acid.

Gives a positive charge

Current flows. Size of current is proportional to conc of H+

H202 produced is oxidised at an ELECTRODE which detects electron transfer.

Electrons flow proportional to glucose conc of blood sample

Biosensor amplifies current which is read by a processor and then produced as a digital reading

Urine analysis in diagnosis of diabetes

Glucose/ketones present in urine- Indicates that person may have diabetes

If BGC increases above renal threshold- means that not all glucose from filtrate in PCT was reabsorbed

Presence of proteins- kidney disease

Problem w urine tests

Don’t indicate current glucose conc

Only show whether conc was higher/lower than renal threshold in the time when urine collecting in the bladder

Specificity + immobilised enzymes

Glucose oxidase- specific for glucose. Doesn’t work on fructose lactose or sucrose- gives -ve results for these

Adv of dip sticks

Non-invasive hence less painful

Less risk of infection

Easy to use

Disadv of dip sticks

Requires use of colour chart- subjective

Doesn’t give exact BGC/ no digital reading

Can’t be reused

Adv of biosensors

Used more than once

Don’t rely on subjective interpretation of colour chart

Gives a specific reading not a range of values

Accurate, as digital reading given

Gives current BGC

Doesn’t involve colour matching

Disadv of biosensors

Invasive

Risk of infection