Glucose Tolerance Test: Phlebotomy Procedure and Timing

What the GTT Tests and Why It Matters

The glucose tolerance test measures how well the body processes glucose over time. It is used to diagnose gestational diabetes in pregnancy, type 2 diabetes, prediabetes, and impaired glucose metabolism in patients who have borderline fasting glucose levels.

The test works by giving the patient a standardized glucose load and then drawing blood at specific time intervals to see how quickly the body clears the glucose. Phlebotomists perform the collections throughout this test, and your timing and attention to detail directly affect whether the results are interpretable.

Types of GTT

75-gram OGTT (2-hour): The standard test for diagnosing diabetes and prediabetes in non-pregnant adults. The patient drinks a solution containing 75g of glucose, and blood is drawn at fasting and 2 hours after the glucose load.

100-gram OGTT (3-hour): Used for gestational diabetes diagnosis when the 1-hour glucose challenge screen is abnormal. Blood draws at fasting, 1 hour, 2 hours, and 3 hours.

50-gram glucose challenge test (GCT): A screening test for gestational diabetes. No fasting required, single blood draw 1 hour after drinking the glucose solution. This is the first step. If it is abnormal, the patient proceeds to the 100g OGTT.

For the NHA CPT, the most commonly tested aspects are the fasting requirement, the draw sequence, and the timing rules.

Patient Preparation: Fasting

Before a full GTT, the patient must fast for 8 to 12 hours. This means nothing by mouth except water. No coffee, no gum, no candy. Even small carbohydrate or calorie intake can alter baseline glucose and invalidate the test.

Medications are generally taken as prescribed unless the physician specifically instructs otherwise. Do not advise patients to skip medications without physician direction.

Patients should be told to avoid strenuous exercise for 24 to 48 hours before the test. Exercise affects glucose metabolism and can skew results.

When the patient arrives, verify that they have fasted appropriately. Ask directly: "Have you had anything to eat or drink other than water since last night?" Document the fasting duration.

The Collection Sequence

Here is the standard procedure for a 3-hour 100g GTT (gestational diabetes):

1. Fasting draw: Collect the baseline blood specimen. This is typically a gray top (sodium fluoride/potassium oxalate) tube for glucose. Some protocols also include additional tubes for a full metabolic panel.
2. Administer the glucose load: The patient drinks the standardized glucose solution. Note the exact time the patient finishes the drink. Timing starts when the patient finishes, not when they begin drinking.
3. 1-hour draw: Collect at exactly 60 minutes after the patient finished the glucose drink.
4. 2-hour draw: Collect at exactly 120 minutes.
5. 3-hour draw: Collect at exactly 180 minutes.

The timing intervals are from when the patient finished drinking, not from when you started the test or administered the glucose. Write down the exact finish time and calculate your draw windows from there.

A tolerance of plus or minus 5 minutes around the target draw time is generally accepted. If a draw is significantly late, document the actual time drawn on the label and notify the ordering provider. Late draws may compromise result interpretation.

Gray Top Tube: Why It Is Used

Gray top tubes contain sodium fluoride and potassium oxalate (or sometimes just sodium fluoride and EDTA). Sodium fluoride is a glycolytic inhibitor. It stops red blood cells from continuing to metabolize glucose after the blood is drawn.

Without a glycolytic inhibitor, glucose in a specimen tube continues to be consumed by red blood cells at room temperature. Glucose levels can drop by 5-7% per hour in an unpreserved specimen. For a glucose tolerance test, this drop would cause falsely low results that could be misinterpreted as normal glucose metabolism.

Gray top tubes preserve glucose levels for accurate measurement. Use them for all GTT draws unless your facility protocol specifies otherwise.

What the Patient Must Do During the Test

Once the glucose drink is administered, the patient must:

• Stay seated for the duration of the test. Walking or physical activity affects glucose metabolism and invalidates results.
• Not eat or drink anything other than water during the test.
• Not smoke during the test. Smoking affects insulin sensitivity and glucose levels.
• Not chew gum. Sugar-free gum stimulates insulin secretion. Regular gum contains sugar. Neither is acceptable.

Explain these restrictions clearly at the start. Patients sometimes walk to the bathroom and take a stroll down the hall, not realizing this matters. Tell them specifically: seated, no food, no drinks other than water, no smoking, no gum.

If the Patient Vomits

Vomiting after ingesting the glucose solution is not uncommon, particularly in pregnant patients who already have morning sickness. If the patient vomits within 30 minutes of drinking the glucose solution, the test usually needs to be rescheduled. The glucose load has not been absorbed adequately, and results will be inaccurate.

Document the time of vomiting and notify the ordering provider or nursing staff. Do not continue drawing timed specimens if the glucose load was lost. Some providers will allow the test to continue if vomiting occurred more than 30 minutes after the drink and the fasting draw was already done, but this is a clinical decision, not yours to make independently.

Labeling Timed Specimens

Every tube drawn in a GTT series must be labeled with the exact time drawn, in addition to the standard label information. The time must be written as the actual time of collection, not the target time.

Mark the tube clearly: "Fasting," "1 hour," "2 hour," "3 hour" (or 30 min, 1 hr, etc., depending on the protocol). Include the clock time (e.g., 08:45) so that lab personnel can verify the intervals.

If tubes from the same patient are mixed up or time points are unclear, the entire test series may need to be rejected. Careful labeling prevents this.

Other Timed Draw Tests

The GTT is the most commonly tested timed draw on the NHA CPT, but the same timing principles apply to other timed collections:

• Cortisol levels — drawn at 8 AM and 4 PM for diurnal variation
• Therapeutic drug monitoring — peak and trough levels relative to medication dosing times
• Blood cultures — collected during fever spikes or at defined intervals

In all timed draw situations, accuracy of the time and clear documentation on the label are the core competencies being tested.

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