|Written by health professionals: Doctor, M.D., CphT, and others - but easy to read.|
Glucose Tolerance Test
The Glucose Tolerance Test plays a vital role for those people whose lifestyle places them at risk of high cholesterol, hemorrhoids, diabetes and cardiovascular failure.
A glucose tolerance test is one of the milder tests the medical community ever came up with to check for proper sugar metabolism.
It's a laboratory blood test that checks how well the body breaks down blood sugar, and it's used to test for diabetes, insulin resistance and reactive hypoglycemia.
Each of those conditions is covered in more detail below, but the important thing is that glucose tolerance tests allow your doctor to get a clear picture of how your body is dealing with glucose itself.
A glucose tolerance test is most often given orally, so the common thing you're going in for will be designated as a OGTT, or oral glucose tolerance test.
The test may be performed by itself or as part of a panel of tests intended to check your overall metabolism out.
The Glucose Tolerance Test is also commonly given around the 28th week of pregnancy to check for gestational diabetes.
What Happens During A Glucose Tolerance Test?
You should continue to eat your normal diet before taking the test - no food when fasting of course.
You'll probably be asked to NOT eat anything after midnight the night before the test is schedule.
Glucose tolerance testing should never be done if you're ill, as your metabolism acts differently during periods of illness and can give false results.
In addition, the full adult dose of glucose, which is to say up to 75 grams worth, should not be given to people who weigh less that 43 kg (94 lbs) because such a high dose per kilogram will probably produce a false response.
The correct dose for adults of any size is 1.75 grams of glucose for each kilogram of your body weight.
The test is usually done in the early morning for two reasons:
What you may experience during a glucose tolerance test:
The glucose tolerance test takes two hours sitting still in a waiting room - no walking (except to go to the toilet). I (Donald) have had several of these, and found it a very good idea to buy two or three interesting magazines to read.
Fair warning, glucose tests on an empty stomach may make you feel absolutely miserable. I (back to Loni) had to have four of them testing for gestational diabetes with my two children. I was borderline with both kids, which meant I had to go in and do it again both times.
They give you the glucose to drink, I got the lemon/lime flavor but I've heard it comes in orange too. Whichever flavor you get, the stuff is sickly sweet and you have to drink it all down.
About an hour after that, you may find yourself getting dizzy, lightheaded, sweating profusely, nauseous, and so on. All of those are your body's natural responses to being on a sugar high, which 75 grams of glucose will sometimes certainly produce.
You may also feel the desperate need to eat something more solid, but you can't until the test is over.
You may not experience any of these symptoms, but if you do they aren't necessarily indicative that something is wrong, just that you're on a sugar high or the resultant sugar crash.
The glucose tolerance test blood sampling:
Before taking your glucose drink, the first blood sample is drawn. The laboratory technician will do a quick glucose test using a drop of your blood - it will alert them if your blood sugar has fallen too low as a result of the fasting.
The remainder of the blood sample is placed in a vial, and this will be the basis of what is called a baseline blood sample, and will serve as a comparison to the subsequent ones.
They will then give you your oral dose, the entire dose must be drunk down completely within five minutes. If you don't manage this, you'll have to try again on a different day.
After you drink it all down, more blood samples will be drawn at hour intervals to test for glucose and occasionally insulin levels. The exact time intervals and the number of samples drawn will vary according to the purpose of the test.
For most oral glucose tests adminstered for diabetes, the most important samples are the baseline and the one they get after two hours have passed.
Cost cutting has seen an end to the middle (1st hour) blood sample being taken on most counts, but my specialist insists it's a vital measure, and insists that they take it.
The glucose tolerance test and renal glycosuria:
If you have or suspect a rare condition called renal glycosuria, which means that sugar gets excreted in your urine even when you have normal glucose levels in the blood, then the laboratory will collect urine samples at the same intervals as the blood samples. They do this to get a clear overall picture of where the sugar in your body is going.
What Do The Glucose Tolerance Test Results Mean?
Baseline - the first blood sample
The baseline sample they take before giving you the glucose should have between 60 milligrams of glucose per decaliter of blood and 110 mg/dL if you have no metabolic problems.
Fasting samples that test between 110 mg/dL and 126 mg/dL are considered to be borderline and may indicate a pre-diabetic state.
Fasting samples that are repeatedly above 126 mg/dL are considered to be indicative of diabetes.
Middle - the second blood sample - often not taken
The sample taken at one hour is considered by many to be not so important, but it should test as having less than 200 mg/dL glucose concentration to be healthy.
The final blood sample - the two hour one
The two hour blood sample is, of course, the most important when looking for diabetes.
If you're completely healthy, the two hour sample should have a glucose concentration of less than 140 mg/dL. If the concentration is between 140 mg/dL and 200 mg/dL, you're considered to have impaired glucose tolerance, which is a pre-diabetic state, and if it's above 200 mg/dL then you probably suffer from full blown diabetes.
This two hour oral test is considered pretty concrete evidence to diagnose or exclude all forms of diabetes at all but the very earliest stages of development.
The intravenous glucose tolerance test (IVGTT)
However, the intravenous glucose tolerance test (IVGTT) has been developed and used to investigate abnormalities in insulin secretion in pre-diabetic states and may be able to catch diabetes even earlier than the oral tests.
The intravenous method involves direct IV injection with glucose and blood samples taken at one and three minutes instead of multiple hours. I couldn't find guidelines on what's normal and what's not for the IV version, unfortunately.
What Conditions Get Checked with a Glucose Tolerance Test, and What Are They?
Tests longer than two hours are often used to check for reactive hypoglycemia, which is a state of extremely low blood sugar often triggered by eating instead of being chronic.
Reactive hypoglycemia generally occurs two to four hours after taking the glucose or eating a meal high in carbohydrates, and researchers think that it's a problem created by excessive insulin release in response to the carbohydrate or sugar.
Basically, the insulin secretion glands in the pancreas get the message "Hey, we ate something, make with the insulin to metabolize it!" The pancreas obliges, but with way too much insulin. The meal gets digested, but the excess insulin is still floating around in the blood looking for sugar to metabolize, and blood sugar levels fall too low.
Symptoms of hypoglycemia include fatigue, dizziness, light-headedness, sweating, headaches, palpitations, depression, nervousness, irritability, tremors, flushing, sugar cravings, increased appetite, nose irritation, nausea, vomiting, and even epileptic-like responses to rapidly flashing bright lights.
However, the oral glucose tolerance test is not as much of a definite marker on reactive hypoglycemia as it is for diabetes, so remember to keep test results in their proper context.
The other condition that an oral glucose tolerance test is often used for is insulin resistance, which means that while the pancreas is producing enough insulin, there's not enough response from muscle, liver and fat cells.
This is indicative of pre-diabetes leading up to Type II diabetes mellitus. Type I is genetic, and it means your body just doesn't produce enough insulin, period. Type II is the kind you can get later in life from eating poorly and leading a sedentary lifestyle. You produce enough insulin, but your body is desensitized to it and doesn't react correctly.
Insulin resistance will also tend to make overall cholesterol levels higher all by itself, even if it never proceeds to Type II diabetes. While it may not be good news, it's handy information to have.
Diabetes mellitus, the most problematic condition, is a metabolic disorder that has both hereditary and environmental causes.
Diabetes mellitus is caused by a problem with your insulin, the chemical your body produces to metabolize sugar.
Diabetes usually results in abnormally high blood sugar levels, because the insulin cannot break it down and use it for some reason.
The two most common types of diabetes are Type I, where your body doesn't produce enough insulin, and Type II, where your body doesn't respond to insulin very well if at all.
Type I is usually genetic, while Type II may have some genetic tendencies but is more likely to result from environmental factors. Type I is also referred to as juvenile diabetes, and is by far the more difficult to manage.
However, neither of them have cures, though they can be managed with a carefully controlled diet and medical insulin. (Since this was written, a cure for type 2 diabetes has been found).
The acute signs of diabetes are excessive urine production, thirst, increased fluid intake, blurred vision, unexplained weight loss, lethargy and extreme changes in energy levels.
Of course, diabetes can lead to coma and death in a fairly short period of time if not treated correctly.
However, be aware that if sloppily managed, diabetes also has long term complications including cardiovascular disease, retinal damage and blindness, chronic kidney failure, nerve damage, and even microvascular damage. This last can lead to impotence, poor wound healing, and eventually even gangrene if too many microvesicles in an area get damaged and are unable to keep up with circulation. This last effect is most often seen in the extremities first.
These long term effects can hit anyone who suffers from diabetes, but sloppy management will only make them more likely.
Can the Glucose Tolerance Test Give False Results?
Keep in mind that there are several things that can create false results on a glucose test.
If you're experiencing acute stress, have recently done vigorous exercise, are on thiazide diuretics such as hydrocholorthiazide (HCTZ), beta blockers, oral contraceptives, corticorsteroids, like prednisone, or psychiatric medications, then you'll probably get false results.
The solution is to tell your doctor and laboratory if you're experiencing acute stress or are taking any of those medications. And, of course, don't go jogging the morning before your glucose test.
The Glucose Tolerance Test in Conclusion
If you suspect diabetes, the oral glucose tolerance test is one of the first lines of investigation and diagnosis. If it comes up clear, breathe a sigh of relief.
However, you still may be suffering from a pre-diabetic state or from something else entirely, so don't take this one test as the end all and be all of your health.
Maintain a communicative relationship with your doctor, and take the responsibility for discovering the source of any symptoms. By getting involved with your health, you take a huge step towards living a long and happy life.
Main Research and write by Loni Ice, Editing by Donald Urquhart
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