Introduction
Glucose and Galactose Tolerance Tests are biochemical diagnostic tests that assess the body’s ability to metabolize specific sugars. These tests help evaluate carbohydrate metabolism and liver function in both healthy and diseased conditions.
Glucose Tolerance Test (GTT)
The Glucose Tolerance Test measures how efficiently the body metabolizes a given dose of glucose. It is a key diagnostic tool for identifying diabetes mellitus and other abnormalities in carbohydrate metabolism.
Definition:
Glucose tolerance refers to the body’s ability to utilize or oxidize glucose. A normal response maintains blood and urine glucose within normal limits after a test dose, while deviations indicate impaired or abnormal tolerance.
Types of GTT:
- Oral Glucose Tolerance Test (OGTT)
- Intravenous Glucose Tolerance Test (IVGTT)
Clinical Purpose:
GTT is considered an artificially induced metabolic test used for diagnostic and prognostic purposes in various diseases.
A) Oral Glucose Tolerance Test (OGTT)
In OGTT, the individual’s blood and urine glucose levels are measured at intervals after an oral dose of glucose.
Patient Preparation:
- Overnight fasting for at least 12 hours (post-absorptive state).
- No medication that affects blood glucose should be taken.
- Adequate carbohydrate intake for 3 days prior to the test.
- Fasting blood and urine samples should be collected before glucose administration.
Procedure:
- Administer 75 g of glucose (or 1 g/kg body weight) dissolved in 200 mL of water orally.
- Collect blood and urine samples every 30 minutes for 2.5 hours.
- Determine blood glucose levels and test urine for glucose using Benedict’s test.
- Plot blood glucose concentration against time to obtain an OGTT curve.
Normal OGTT Curve:
In a healthy individual:
- Fasting blood glucose: 60–90 mg%
- Peak glucose (at 1 hour): 110–130 mg%
- Returns to fasting level within 2 hours
- No glucose appears in urine (blood glucose remains below renal threshold ~180 mg%)
Abnormal OGTT Curves
(a) Decreased Glucose Tolerance
Seen in Diabetes Mellitus. Fasting glucose and post-load glucose levels are markedly elevated.
- Fasting blood glucose > 130 mg%
- In mild diabetes: 130–150 mg%; in severe cases: >180 mg%
- Glucose remains elevated beyond 2 hours
- Urine samples test positive for glucose (Benedict’s test)
Causes:
- Diabetes Mellitus
- Liver diseases
- Hyperthyroidism
- Cushing’s syndrome
- Pregnancy
- Old age
(b) Impaired Glucose Tolerance (IGT)
This condition reflects an intermediate state between normal and diabetic glucose tolerance, often associated with urban lifestyle and obesity.
- Fasting glucose: 90–130 mg%
- 2-hour glucose: 140–200 mg%
- Blood glucose does not return to fasting level after 2 hours
Significance: IGT indicates increased risk for Type 2 Diabetes and coronary artery disease.
(c) Increased Glucose Tolerance
A flat OGTT curve is observed due to rapid utilization of glucose.
Seen in:
- Hypopituitarism
- Addison’s disease
- Hypothyroidism
- Intestinal malabsorption
- Renal glycosuria (defective renal glucose reabsorption)
Medical Significance of OGTT
- Primary diagnostic test for Diabetes Mellitus
- Helps differentiate between various forms of glucose intolerance
- Identifies Impaired Glucose Tolerance (IGT), a precursor to diabetes and heart disease
- Useful in evaluating endocrine disorders affecting glucose metabolism
B) Intravenous Glucose Tolerance Test (IVGTT)
The IVGTT is used when the gastrointestinal tract cannot properly absorb glucose.
Patient Preparation:
Same as OGTT (overnight fasting and no interfering drugs).
Procedure:
- Administer glucose intravenously at a dose of 0.5 g/kg body weight as a 25% solution over 3 minutes.
- Blood samples are collected every 10 minutes for 1 hour.
Interpretation:
- Normal: Peak blood glucose occurs within minutes, declines by 20–30 minutes, and returns to normal within 60 minutes.
- Decreased tolerance: Blood glucose remains elevated beyond 1 hour.
T-half value: Time required for blood glucose to fall to half its peak value.
- Normal: t½ < 45 minutes
- Diabetic: t½ > 60 minutes
Galactose Tolerance Test
The Galactose Tolerance Test evaluates the liver’s ability to metabolize galactose, a sugar almost exclusively processed by the liver. Hence, it serves as a liver function test.
Procedure:
- Inject 300 mg/kg body weight of galactose intravenously.
- Collect blood samples at 10-minute intervals for 2 hours.
- Estimate blood galactose levels at each interval.
Results:
- Normal: Galactose half-life = 10–15 minutes.
- Abnormal (prolonged half-life): Indicates hepatic dysfunction such as hepatitis or cirrhosis.
Significance:
- Useful for diagnosing liver diseases.
- Assesses the liver’s ability to metabolize galactose effectively.
- Helps evaluate hepatic cell function in chronic conditions.
Summary Table
| Test Type | Purpose | Normal Response | Clinical Use |
|---|---|---|---|
| OGTT | Assess carbohydrate metabolism | Blood glucose returns to baseline within 2 hours | Diagnosis of Diabetes Mellitus, IGT |
| IVGTT | Used when oral absorption is impaired | Blood glucose normalizes within 45–60 minutes | Assess insulin response, diagnose Diabetes |
| Galactose Tolerance Test | Assess liver function | Galactose half-life = 10–15 minutes | Detect hepatocellular dysfunction (Hepatitis, Cirrhosis) |
Detailed Notes:
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