9.11 - Testing Urine for Protein & Glucose
The tests
Aim
To test urine samples for glucose and for protein. Neither should normally be present, so finding either is a sign that something is wrong. Artificial urine is used in place of real urine.
Background
- A healthy kidney filters the blood, then reabsorbs all of the glucose back into the blood. Its filter is too fine to let large protein molecules through.
- So healthy urine contains no glucose and no protein.
- Glucose in the urine is a sign of diabetes, where the blood glucose is so high that the kidney can't reabsorb it all.
- Protein in the urine is a sign that the kidney's filter is damaged and is leaking protein.
- You can't see glucose or protein in a sample, but each test uses a reagent that changes colour only if its molecule is present.
- So in this practical you add Benedict's solution and Biuret reagent to each sample, and use the colour changes to tell whether glucose or protein is there.
Method
- Set up the samples - Put the same volume of each sample into two test tubes, one for each test. Test a sample known to contain the molecule, and one known not to, alongside the unknowns.
- Test for glucose - Add fresh Benedict's solution to one tube of each sample, and heat it in a water bath at about 75 °C for 5 minutes.
- Test for protein - Add fresh Biuret reagent to the other tube of each sample and mix. It isn't heated.
- Record the colours - Compare each tube with the known samples, to be sure what each colour means.
The colour changes are shown in this table.
| Test for | Reagent | Positive result (present) | Negative result (absent) |
|---|---|---|---|
| Glucose | Benedict's solution, heated | Blue changes through green and yellow to brick-red | Stays blue |
| Protein | Biuret reagent, not heated | Blue changes to purple/lilac | Stays blue |
Safety
- Eye protection - Wear it throughout.
- Water bath - Heat Benedict's solution in a water bath rather than over a flame, so it doesn't spit.
- Biuret reagent - It contains sodium hydroxide, which is corrosive, so handle it carefully and wash off any splashes at once.
- Hygiene - Treat every sample as if it were real urine, and wash your hands afterwards.
Common exam questions
Question 1
Describe how you would test a urine sample for glucose, including the result for a positive test.
Add Benedict's solution to the sample and heat it in a water bath at about 75 °C for a few minutes. If glucose is present, the solution turns from blue to brick-red.
Question 2
One sample turned brick-red with Benedict's and stayed blue with Biuret. A second sample stayed blue with Benedict's but turned purple with Biuret. What does each sample suggest?
The first sample contains glucose but no protein, which suggests diabetes. The second contains protein but no glucose, which suggests the kidney's filter is damaged and leaking protein.
Question 3
Why is artificial urine used instead of real urine?
Real urine could carry microorganisms and is a health risk, so an artificial sample is safer and more hygienic to handle, while giving the same colour results.
Question 4
Why is the Benedict's test heated but the Biuret test is not?
Benedict's solution only reacts with glucose when it's heated. Biuret reagent reacts with protein at room temperature, so it doesn't need heating.