Dr. Yousef Al Adbai.
GLUCOSE TOLERANCE TEST
1. Purpose:
To recognize milder cases of diabetes and renal glycosuria. Quantitative estimation of glucose in human plasma by enzymatic method (GOD-POD). Plasma glucose determinations are useful in the diagnosis and treatment of diabetes mellitus and in monitoring the response to treatment of diabetes mellitus with insulin or oral hypoglycemic agents. Elevated glucose levels may be associated with pituitary or thyroid dysfunction, renal failure, and liver disease, whereas low glucose levels may be associated with insulinoma, hypopituitarism, or insulin-induced hypoglycemia. CSF and fluids have increased glucose in diabetic condition.
2. Principle:
Glucose oxidase (GOD) converts glucose to gluconic acid. Hydrogen peroxide formed in this reaction in the presence of peroxidase (POD) oxidatively couples with 4-aminoantipyrine and phenol to produce red quinoneimine dye. This dye has absorbance maximum at 505 nm. The intensity of color complex is directly proportional to the concentration of glucose in specimen.
3. Performance specifications:
3.1. Linearity: Up to 600 mg/dL of plasma.
3.2. Measurement range: 25–600 mg/dL.
3.3. Sensitivity: The minimum detection limit 40 mg/dL.
4. Primary sample:
4.1. Use only plasma as specimen for the test.
4.2. Collect 2 mL of venous blood in a fluoride–oxalate mixture tube.
4.3. Do not use lysed plasma for testing as it may give false results.
4.4. Do not use contaminated/turbid samples for testing.
4.5. Process the sample on the same day within 3 hours of collection.
5. Type of container and additive:
Fluoride—EDTA mixture tube.
6. Reagents/Consumables:
For patient use, commercially available glucose (75 g) mixed with water.
7. Instrument:
Semi-autoanalyzer.
8. Procedures:
Instructions to be given to the patient: These instructions should be given to the patients by previous day of the investigation.
Biochemistry
8.1. The patient should not take any food after 9 pm the previous night till the test is performed.
8.2. The subject should have normal diet for at least 3 days prior to the test.
8.3. He/she should not have taken drugs which affect blood sugar.
8.4. In exceptional cases, when the patient has to come from a distant place, light tea without sugar may be allowed (2 hours before collection).
Method: Upon arrival of the patient, the following should be done:
8.5. Body weight should be noted down.
8.6. Fasting blood sample should be collected and glucose estimation should be performed.
8.7. Specimen of fasting urine is collected and test for glucose, albumin, and acetone to be done.
8.8. 75 g of glucose dissolved in 300 mL of water should be given orally.
8.9. Blood and urine samples will be collected for every half an hour interval for 2 hours after the glucose has been taken.
8.10. It is not always possible to collect urine at every half hour interval. In such cases, urine sample can be collected for every 1 hour interval.
Glucose estimation: As per the method given in this manual.
Urine sugar: As per the method given in this manual, with the standard curves.
Normal responses:
Fasting glucose within normal limit. Maximum blood glucose is reached either half or one hour after taking the glucose. The blood glucose then returns rapidly to the normal fasting limits, which are often reached in one and a half hour and almost always at two hours. There should be no sugar in any of the urine specimens.
9. Reference:
The GTT curve will be interpreted with the standard curves.
10. Critical/Alert level values:
Below 40 mg/dL, above 400 mg/dL.
11. Potential sources of variability:
11.1. Do not use if the absorbance of the blank reagent is greater than 0.150 at 500 nm as it indicates deterioration of the reagent.
11.2. Check if the patient has followed the instructions regarding preparation before collecting samples.
BIBLIOGRAPHY:
- Harold Varley. Practical Clinical Biochemistry, 5th ed., 1980;1:406–10.
- Trinder P. Ann Clin Biochem 1969; 6: 24.
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