Glucose Challenge - 2 Hour

GLUCOSE CHALLENGE (2HR)

Chemistry

 ORAL GLUCOSE TOLERANCE TEST (OGTT) PROCEDURE
 The oral glucose tolerance test (OGTT) may be useful for the further evaluation of patients in whom diabetes is strongly suspected but who have normal fasting plasma glucose or impaired levels of fasting glucose (100 – 125 mg/dL). The OGTT is not recommended for routine clinical use to establish the diagnosis of diabetes but may be required in the evaluation of patients with impaired fasting glucose or when diabetes is still suspected despite a normal fasting plasma glucose.
When a standard dose of glucose (glucola) is given orally, absorption occurs rapidly and the blood glucose concentration increases. This stimulates the pancreas to produce more insulin. As a result, blood glucose levels reach a peak at about an hour and then fall as rate of utilization exceeds that of absorption. This test measures the ability of the individual to remove an added glucose load from the circulation.

Special Notes:

• Patient should be maintained on a normal carbohydrate diet (150-300 gm) for at least three days prior to testing with unrestricted physical activity.
• An overnight fast of 12 hours precedes the test.
• Vomiting of ingested glucose during the first hour will invalidate results.
• The fasting specimen must be tested for glucose prior to giving the patient glucola. If the fasting blood glucose is greater than or equal to 126 mg/dl the test will be discontinued. The Physician must be notified immediately if the patient becomes ill.
• Patients must remain in the phlebotomy waiting during the entire test.
  
  Administration of Glucose Load and Drawing of Specimens
   
• Ideally, the GTT should be scheduled between 7 and 9:00 A.M.
• A fasting plasma/serum sample (zero time) is drawn before a standardized glucose load (75 g, 1.75 g/kg of ideal body weight) is given. If the fasting blood glucose is greater than or equal to 126 mg/dl the test will be discontinued.The standardized glucose load is supplied by the pharmacy or the laboratory depending on the facility.
• One plasma/serum sample is drawn 2 hours after standardized load glucose load. The 2-hour glucose sample is adequate for the establishment of normal, impaired glucose tolerance or of diabetes.

Refrigerate

Room temperature: 2 days
Refrigerated (2-8°C): 7 days
Frozen (-20°C or colder): 1 day

Drugs and Chemicals That May Affect the Glucose Tolerance Test
It should be pointed out that glucose tolerance is affected by many drugs and chemicals. These include the following classes and specific drugs:

• Diuretics and anti-hypertensives: Bumetamide, cholorthalidone, clonidine, diazoxide, ethacryinic acid, furosemide
• Hormonal agents:ACTH, calcitonin, glucagon, glucocorticoids, oral contraceptives, prolactin, somatotropin, thyroxine
• Neurologically active agents:Epinephrine, isoprotererol, Levodopa, norepinephrine phenytoin, propranolol
• Anti-neoplastic agents:Alloxan, L-asparaginase, cyclophosphamide, megestrol acetate, streptozotocin
• Psychoactive agents:Chlorprothixene, haloperidol, lithium carbonate, phenothiazine, tricyclic anti-depressants, marijuana
• Analgesic, anti-inflammatory agents:Aspirin, acetaminophen, indomethacin ,morphine(toxic doses)
• Miscellaneous:Carbon disulfide, cimetidine, EDTA, ethanol, heparin, INH, nalidix acid, nickel chloride, nicotinic acid, niridazole, pentamidine, phenolphthalein, thiabendazol

When the OGTT is used the categories for glucose tolerance are shown below:

          2-hour postload glucose < 140 mg/dL (7.8 mmol/L) = normal glucose tolerance
          2-hour postload glucose 140 – 199 mg/dL (7.8 – 11.1 mmol/L) = impaired glucose tolerance (IGT)
          2 hour postload glucose > or = 200 mg/dL (11.1 mmol/L) = Provisional diagnosis of diabetes (the diagnosis must be confirmed)

Criteria for the diagnosis of diabetes
2 hour plasma glucose	≥ 200 mg/dL (11.1 mmol/L) during an OGTT.
Hemoglobin A1C	≥ 6.5%
Fasting plasma glucose	≥ 126 mg/dL (7.0 mmol/L)
Patient with classic symptoms of hyperglycemia or hyperglycemic crisis, a random plasma glucose ≥ 200 mg/dL (11.1 mmol/L)

The oral glucose tolerance test (OGTT) should be used as a diagnostic test for the following reasons:

• Fasting plasma glucose alone fails to diagnose approximately 30% of cases of previously under diagnosed diabetes.
• An OGTT is the only means of identifying patient with impaired glucose tolerance.
• An OGTT is frequently needed to confirm or to exclude an abnormality of glucose tolerance in asymptomatic patients.

An OGTT should be used in individuals with fasting plasma glucose levels of 110 – 125 mg/dL (6.1 – 6.9 mmol/L) to determine glucose tolerance status.