Oral Glucose Tolerance Test

Hello everybody!

This is Hakim for another round of blogging.

NB: This is an edited entry. I have not removed anything, but have added in a few more points.


PURPOSE OF OGTT
The oral glucose tolerance test (OGTT) is a test performed to determine if the patient has diabetes. In my lab, majority of the patients who undergo this test is pregnant. This is because pregnant women could acquire gestational diabetes, which is a form of type II diabetes that is first diagnosed or begins during pregnancy. This condition usually ends soon after the person gives birth.

In a nutshell, the patient would have to give a urine sample and have their blood drawn out. After this, the patient is given a large dose of glucose and subsequently, urine is collected and blood is drawn after a period of time (depending on the GP's request). After the final urine collection and blood drawing, the urine samples are tested for glucose and ketones by using the dipstick. The blood tubes are centrifuged and its plasma is put in an analyzer to test for glucose levels.

PRINCIPLE OF TESTING PLASMA GLUCOSE LEVELS
Our lab is using the COBAS Integra 400 plus. It uses the principles of light absorbance and colorimetry. Glucose in the plasma is oxidised by glucose oxidase, producing gluconic acid and hydrogen peroxide. Peroxidase converts the hydrogen peroxide into oxygen and water. The oxygen liberated in the reaction is taken up by 4-aminophenazone with phenol and forms a pink-coloured substance. The substance can be measured at a wavelength of 515mm. A halogen lamp will produce a light beam and passes the cuvette. A photodetector would sense the amount of light passed through the cuvette (and also the amount of light absorbed by the substance). It is important that calibration is done frequently so that the results would be accurate.

PROCEDURE OF OGTT
It is important to note that the patient is required to fast for 8 hours before the test commences. This is done so as to prevent any pre-analytical variations. For example, a high plasma glucose after a meal could misdiagnose the patient as having impaired glucose tolerance (IGT). The patient is allowed to drink water during the fasting, but they are advised to try to drink as little as possible as the blood could be diluted if they take in large amounts of water. Also, the tube collected is a fluoride tube so that it will reduce the rate of glycolysis by the red blood cells so as to stabilize the levels of glucose. So after the first round of specimen collection, the patient is given a 75 gram glucose drink to drink. She has to finish the whole bottle. It tastes like concentrated F&N Orange. From my own experience, the drink is extremely sweet. I couldn't even finish the whole bottle (which is about 350mL) so I'm not sure which is more painful for the pregnant woman; childbirth or finishing up the whole bottle. There are 2 different timeframes for blood to be drawn, depending on the GP's request. It's gonna be quite confusing, so let's give an example. - Patient A is requested to have her blood and urine collected 1 hour AND 2 hours after drinking the glucose - Patient B is requested to have her blood and urine collected only 2 hours after drinking the glucose So after both patients finish the bottle, she comes back in either 1 hour (for patient A) or 2 hours time (for patient B), depending on the GP's request. During this time, she still cannot eat or drink anything (other than water, but in small amounts). Patient A comes back in 1 hour to have her blood and urine collected. At 2 hours after drinking the glucose, BOTH Patients A & B comes back to have their blood and urine collected. Only after this is done, they are able to eat and drink. For urine sample: The dipsticks are dipped into the urine to check for glucose and ketone levels. A high level of glucose in the urine would mean that there is a high level of glucose in the patient's bloodstream and the levels have exceeded the kidney's threshold. So the kidneys aren't able to reabsorb any more glucose and is then excreted through the urine. Ketone bodies are a result of lipid metabolism. This is caused by starvation. Ketone bodies would be present in diabetics. For blood sample: The blood tubes are centrifuged. The blood tubes MUST be labeled (0 hr, 1 hr, 2 hr) so as to prevent any confusion. The tubes are then put in an analyzer machine. For our lab, we are using the COBAS Integra 400 plus.

Note: For some reason, I can't upload pictures, so here is the link to the website that contains the picture of the analyzer.
http://labsystems.roche.com/content/products/integra_400plus/introduction.html
REFERENCE RANGE OF OGTT If the levels of plasma glucose is higher than normal, then we can say that the pregnant woman is suffering from gestational diabetes. There are many diagnosing criteria for OGTT. The World Health Organization's (WHO) guidelines for diagnosing GDM is ≥5.3 mmol/L for 0-hour plasma glucose, OR ≥10.0 mmol/L for 1-hour plasma glucose, OR ≥8.6 mmol/L for 2-hour plasma glucose. However, some doctors do not request for a 1-hour plasma glucose because it is not as clinically significant as the 0-hour and 2-hour plasma glucose levels.

They are advised to maintain a healthy lifestyle and have a proper suitable diet. They can also take insulin.

Ok that's all from me. If you have any questions for me, I'd be glad to answer them as fast as possible.

Happy SIP-ing!
Hakim
0703555C

13 more weeks!!!