3 Glucose Testing and Timed Collections

Preparation and collection essentials

Reliable glucose results depend on correct patient preparation, precise collection timing, accurate labeling, and prompt specimen processing. Always follow the test order and the performing laboratory’s current instructions, because required tube types and collection protocols can vary.

Before collection, verify the patient’s identity, requested test, fasting status when required, and the correct collection supplies. Use standard venipuncture and patient-identification procedures.

Specimen type, labeling, and handling

Glucose may be measured in plasma or serum, depending on the test. Collect the specimen in the tube specified by the laboratory. At collection, label each specimen with the patient’s identifiers and its exact collection time or test interval, such as fasting, 1 hour, or 2 hours. For a series, label every tube separately rather than relying on collection order.

Blood cells continue to consume glucose through Glycolysis after collection, which can falsely lower the measured concentration. If serum or plasma is collected without an effective glycolysis inhibitor, separate it from the cells promptly according to laboratory instructions. A gray-top tube containing sodium fluoride and potassium oxalate is commonly used for glucose testing, but fluoride’s inhibition of glycolysis is not immediate; prompt processing remains important.

Follow the laboratory’s requirements for centrifuging, separating, storing, and transporting specimens. After collection, mix additive tubes as directed, avoid hemolysis, and promptly deliver or process samples using the required handling procedure.

Fasting and meal-related tests

A Fasting plasma glucose specimen is generally collected after at least 8 hours without caloric intake; plain water is usually allowed. Confirm the required fasting period and tell the patient to follow the ordering clinician’s instructions about medications. Patients should not stop prescribed medication on their own. Record or report a fast that was not completed as directed, since it may affect interpretation.

A Random glucose can be collected without fasting. A Postprandial glucose is collected at a specified interval after a meal; the order may define whether timing starts at the beginning or end of the meal. Record the meal and collection times as required. For every timed test, follow the time point specified in the protocol rather than assuming all tests count time in the same way.

Oral glucose tolerance testing

An Oral glucose tolerance test (OGTT) measures glucose before and at one or more specified intervals after the patient drinks a glucose solution. Protocols differ by purpose, age, pregnancy status, and laboratory. A common adult, nonpregnant protocol uses a 75 g glucose dose; this description does not replace the specific test instructions.

1. Prepare the patient. The patient typically fasts overnight for at least 8 hours, with water allowed. For a diagnostic OGTT, instructions may also call for a usual diet with adequate carbohydrate intake in the days before testing. Confirm preparation, relevant illness, and medication instructions before proceeding.

2. Collect the fasting specimen before the glucose drink and label it “fasting,” with its collection time.

3. Give the prescribed glucose dose according to the protocol. For a common adult, nonpregnant test, this is 75 g of glucose.

4. Start the timer as the protocol directs. In standard guidance, timing begins when the glucose drink is started. Collect each subsequent specimen at its exact ordered interval and label it with that interval and collection time.

5. Keep the patient under observation and follow the protocol regarding activity, food, smoking, and drink during the test. If the patient vomits, cannot finish the drink, or a collection is delayed or missed, document the event and contact the laboratory or ordering clinician for instructions. Do not silently substitute a different time point.

6. Process and transport specimens promptly using the required tube and handling procedure. Ensure every specimen in the series is identified and matched to its collection time.

Pregnancy protocols and quality checks

Pregnancy testing may use a Glucose challenge test as an initial screen, followed, if indicated, by a fasting OGTT with several timed samples. The challenge test and diagnostic OGTT are different procedures, and pregnancy protocols and interpretation thresholds differ. Follow the ordered protocol rather than applying a nonpregnant adult schedule.

Incorrect timing, an incomplete fast, a mislabeled tube, or delayed separation can make results unreliable and may require recollection or rescheduling. Maintain a clear record of glucose-drink administration and every draw, and consult the performing laboratory whenever test-specific instructions are unclear.