Specimen Requirements

Patient Preparation

Patient should be placed on a diet containing 150 g of carbohydrate per kg of body weight for 3 days prior to testing. Two additional days of this diet are essential if the patient has not been on a diet sufficient in carbohydrates. Test should be started in early morning, after an overnight fast. Water may be taken during the fast. Patient should remain at rest during the test and refrain from smoking and eating. The patient is drawn for a fasting glucose. If the result is ≤150 mg/dL, the laboratory will proceed with the test. If the fasting glucose result is >150 mg/dL, the laboratory will notify the physician of the result, and await further direction (ie, proceed or cancel). If the fasting glucose is >250 mg/dL, the test is cancelled and the physician notified. Test should not be performed on those patients who have had an illness during the prior 2 weeks, endocrine disorders, anorexia, or patients that are non-ambulatory.

Note: It is recommended that this testing be started prior to 9 am due to diurnal effects.

Test Dosage

Non-pregnant adult: 75 g of glucose solution

Pregnant patient: 100 g of glucose solution

Children: 1.75 g/kg of body weight up to a maximum of 75 g.

A routine GTT on a non-pregnant patient consists of 3 specimens: fasting, 1-hour, and 2-hour post dose specimens. The standard GTT for a pregnant woman consists of 4 specimens:  fasting, 1-hour, 2-hour, and 3-hour post dose specimens. A physician may request additional specimens (ie, 1/2-hour, 1 1/2-hour, 4-hour, or 5-hour specimens). The timing of the additional specimens is based from the time the glucose solution is totally consumed.

Dietary and Drug Restrictions

Drugs which may influence the test and should be discontinued for 3 days prior to testing include:  anti-inflammatory drugs (including salicylates), diuretic agents, hypoglycemic agents, anticonvulsants, propanolol, thiazides, beta blockers, lithium, steroids, and all non-essential medications. Additionally, oral contraceptives should be omitted for 1 complete cycle. Cigarette smoking must be avoided during testing.

Specimen Collection

Submit only 1 of the following specimens:

Plasma

Container/Tube: Dark green-top (sodium heparin) tube or light green-top (lithium heparin) tube

Specimen: 1 mL of heparinized plasma

Transport Temperature: Ambient

Collection Instructions: Fasting (8 hour)

Note: 1. Indicate plasma.
2. Label specimen appropriately (plasma).

Serum

Container/Tube: Plain, red-top tube or serum gel tube

Specimen: 1 mL of serum

Transport Temperature: Ambient

Collection Instructions: Fasting (8 hour)

Note: 1. Indicate serum.
2. Label specimen appropriately (serum).