Glucose Tolerance Test

Glucose Tolerance Test  

Synonym - Specimen - Reference Value - Method  - Description - Types of Glucose tolerance test and its interpretation.

Synonym             :     GGT 

Specimen            : Plasma (1 mL) collected in sodium fluoride tubes 

                               Serum (1 mL), Plasma collected in heparin tube is also acceptable .

                          It is important to use the same type of collection container throughout the

                               entire test . 

Reference Value : Fasting sample less than 120 mg/dL : 1- hour sample Less than 160

                                   mg/dL 

Method                    : Spectrophotometry 

Description             : Glucose tolerance test is indicated in case of deviation in normal 

                                      Carbohydrate metabolism as in diagnosis of diabetes millitus . 

Types of Glucose tolerance test 

1 - Intravenous tolerance test 
2 - Oral  tolerance test 

Explain 


Intravenous or oral tolerance tests may be used to detect glucose tolerance with persistent  hyperglycemia below the renal threshold (i.e without glucosuria  ). 


If diabetes mellitus has been confirmed , the intravenous GTT combined with measurement of insulin after glucose injection may be used to identify the type of diabetes mellitus . 

The most common use of oral GTT is to evaluate intestinal absorption 


GTT may be indicated in case of hyperinsulinisim  . 
Glucose tolerance is determined by elevated the concentration of blood glucose at specific time before and after the oral or intravenous administration of a given quantity of glucose 


GTT may be oral glucose tolerance test or intravenous glucose tolerance test , both may be used . 


The oral GTT is used for individuals who are not know to have problems with gastrointestinal  malabsorption  . The intravenous GTT is used for individuals who are unable to tolerate oral glucose . 

Oral glucose tolerance test 

when glucose is given by the oral route , alteration of blood glucose concentration takes 3 phases  as follow : 


1 - phase one : occurs during absorption period . The rate of glucose absorption into circulation exceeds that of removal and blood glucose concentration increase . 


2 - Phase two : hepatic output is inhibited and insulin secretion is stimulated .The peak level of glucose is reached and then begins to fall in which rate of removal exceeds that of entry . 


3 - Phase three : glucose level reach to original values . 

Procedure 


1 - Take fasting blood sample . 
2 - 2.2 g of glucose / Kg body weight are given orally 
3 - Take blood samples at 30 - 60 min. intervals for 3 - 4 hours . 

Interpretation 

1 - In diabetic patients, fasting blood glucose usually more than 120 mg/dl and the level exceeds 180 mg/dl during testing by 30-60 min. and not returns to the resting level again .  Prolonged glucose disappearance indicates intolerance
as in diabetes mellitus , hyperadrenocorticism , hyperthyroidism , and hepatic insufficiency . 


2 - Normally , fasting blood glucose is less than 120 mg/ dl , and values , not exceeds 160 mg/dl during testing (1 st hour ) , then the level return to normal by the end of the 2nd hours . 


3 - Failure to reach the maximal levels may indicate intestinal malabsorption . Lower glucose level than peak values may be seen in hypothyroidism adrenal insufficiency , hypopituitarism , and hyperinsulinism .