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If one is interested in diagnosing diabetes at its earliest stage then a standard OGTT has to be performed under carefully prescribed testing condition. When no other cause for such an abnormal curve exists then diagnosis of diabetes is made. OGTT is still considered to be a gold standard for diagnosing diabetes when other routine tests are negative for diabetes mellitus
There are many factors which modify the test report
Host Factors
1. Diet
2. Weight
3. Age
4. Physical activity
5. Illness and trauma
6. Emotional states
7. Endocrinopathy
8. Pregnancy
9. Drugs
10. Sex
11. Obesity
Technical Factors
1. Time of testing
2. Type of test
3. Size of glucose load
4. Site of blood sampling
5. Types of blood samples
6. Preservation of sample
7. Method of analysis of sample
8. Diurnal variation - glucose tolerance decreases in the afternoon.
Standard glucose load
It is usually 75 gm or 1.75 gm per kg of desirable weight dissolved in 300 cc of water. In pregnancy glucose load is 100 gins. OGTT is done in morning after and overnight fast for 8 to 10 hours.
Patient standardization
This is done by giving 300 gms of carbohydrate per day for atleast 3 days. Inadequate carbohydrate may stimulate hepatic neoglucogenesis. If this is ignored often it may lead to false positive GTT. Prior to test, 8 hours fasting is necessary. Patient can take water to satisfy thirst. Postpone GTT for 3-4 weeks in patients having recent illness, surgery or any major stress. Before calling an abnormal test, diagnostic of diabetes mellitus one has to rule out all of the non diabetic causes of abnormal GTT like.
1. Liver diseases
2. Chronic illness-prolonged inactivity
3. Starvation or under nutrition
4. Acute stress - fever, trauma, major surgery
5. Potassium depletion after diuretic therapy.
6. Chronic renal failure
7. Drugs like thiazide, steroids, oral contraceptives, dilantin, nicotinic acid.
8- Other endocrine disorders, active Acromegaly, Cushing Syndrome,
pheochromocytoma and thyrotoxicosis.
What are the ogtt diagnostic values?
Values based on who national diabetes data group
| |
Status |
Fasting |
2 hours Post Prandial |
| 1 |
Normal
|
<115 mg% |
<140 mg% |
| 2 |
IGT |
<140 |
>140 to >200 |
| 3 |
DM in non pregnant adult |
>140
|
>200 |
4
|
DM in pregnancy
|
>105 |
>165 |
5
|
DM in children
|
>140 |
>200
|
| 6 |
DM in aged |
for each decade over 50 years add 10rng% specially for post prandial values. |
Other Criteria are as follows
A. Fajan-Conn Criteria: for OGTT under 50 years 1.75 gm/K£ body weight glucose
| |
Ihr. |
1 1/2 hr |
2hrs
|
| Blood |
160 mg% |
140 mg% |
120 mg% |
| Plasma |
185 mg% |
165 mg% |
140 mg% |
Definitive Diabetes at least 2 abnormal VALUES
B. Wilkerson's UPSPHS: 100 gm of glucose
| |
F |
Ihr |
2hr |
3hr |
| Blood |
110 mg% |
170 mg% |
120 mg% |
110 mg% |
| Plasma |
130 mg% |
195 mg% |
140 mg% |
130 mg% |
| |
1 point |
1/2 point |
1/2 point |
1 point
|
2 Points or more definitive diabetes mellitus. C Joslin Clinic : 100 gm glucose
| |
F |
Ihr |
2hr |
3hr |
| Blood |
110 mg% |
170 mg% |
120 mg% |
110 mg% |
| Plasma |
130 mg% |
190 mg% |
140 mg% |
130 mg% |
D. Joslin Research Lab
| |
F |
1/2 hr |
Ihr |
ll/2hr |
2hr |
3hr |
| Blood |
110 mg% |
170 mg% |
160 mg% |
135 mg% |
120 mg% |
100 mg% |
| Plasma |
115 mg% |
200 mg% |
190 mg% |
160 mg% |
140 mg% |
130 mg% |
Definite diabetes total increment glucose value 9mg% or greater.
E. University Group Diabetic Program : 30gm/m glucose Definitive diabetes mellitus total
absolute
glucose at F, Ihr, 2hr, 3hr
Blood : 500 mg% or greater
Plasma : 575 mg% or greater
Most commonly used criteria is that of WHO.
Disadvantage of ogtt
1. Non reproducibility.
2. Time taken for test and number of samples to be taken are more.
3. Various host and technical factors can alter it.
Indications of OGTT
1. In high risk group like multiparous women with bad obstetrical history when their is
strong family history, unexplained neuropathy. Where both fasting and 2hr PP blood
glucose are normal OGTT has to be done.
2. In impaired GTT cases as a follow-up to diagnose overt diabetes mellitus developing,
every year OGTT has to be done.
When not to do OGTT
In a known established case of diabetes mellitus OGTT is not required.
IVGTT
Used for research purposes. It can be done in those with abnormal intestinal absorption. 25gms of glucose in 50% solution is given IV over 3 min. Samples collected every lOmins for 1 hour and a graph of blood glucose against time plotted. The rate constant is calculated by a formula K= (0.693 X 100) /1.5. Normally K is 0.9 to 2.3, if K is below 0.9, it indicates diabetes.
Advantages of IVGTT
1. It is a shorter procedure.
2. Eliminates irregular oral absorption.
Disadvantages of IVGTT
1. Frequent blood collection
2. Non physiological, influences of gut hormone on insulin secretion eliminated. Hence
test is less sensitive.
Cortisone gtt
Corticosteroids are diabetogenic as they accelerate, hepatic ghiconeogenesis and decrease the peripheral utilisation of glucose.
50mg of cortisone acetate is given orally and 1/2 and 2 hr before administration of glucose and then standard OGTT is performed. The test is positive if 1 hr glucose values exceed 160mg% and 2 hr value exceeds 140mg%. In those weighing more than 160 Ibs gives 62.5mg instead of 50mg cortisone acetate. This test is unreliable in obese, during pregnancy and elderly individuals.
Conclusion
OGTT though time consuming is still considered to be Gold Standard for diagnosing diabetes mellitus when rest of the investigations are negative.
REFERENCES
1. Diabetes mellitus in clinical endocrinology by PJ Mehta and VJ Ratnam, 2nd Ed- 1988,
page 75-76.
2. Epidemiology and detection of diabetes in Joslin Diabetes Mellitus. Alexander Marble,
Priscilla White, Robert F Bradely and Leo P Krall, Lea and Febiger Publishers, 1972, llth
Ed. Page 27.
3. Prime considerations in diagnosis and treatment chapter 3 in clinical diabetes. Modern
management Edition by Stephen Pudolsky 1980. Appleton Century Publishers.
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