Protocol - Oral Glucose Tolerance Test

Description

Participants are asked to fast for at least 9 hours (generally overnight) prior to the Oral Glucose Tolerance Test (OGTT). Participants are asked a series of exclusionary questions and then have an initial blood draw to determine Fasting Plasma Glucose levels. Participants are next asked to drink a calibrated dose (generally 75 grams/10 ounces) of a dextrose solution (e.g., Trutol®) and have a second venipuncture 2 hours (plus or minus 15 minutes) after the first venipuncture. The plasma is separated from the blood via centrifugation, and glucose concentration is determined via the hexokinase method.

Specific Instructions

The Oral Glucose Tolerance Test (OGTT) includes a Fasting Plasma Glucose assay as the first step. Therefore, performing the OGTT and the Fasting Plasma Glucose Assay for Diabetes Screening together will result collecting in the Fasting Plasma Glucose assay twice. The Diabetes Working Group (WG) notes that an alternative to the single 2-hour post-load value is to establish a catheter and obtain samples for Serum Glucose concentrations at 0 minutes, 30 minutes, 60 minutes, 90 minutes, and 120 minutes. There are also protocols for more rapid sampling of Serum Glucose concentrations. In addition, the Diabetes WG notes that there are also protocols to collect samples for Serum Insulin concentrations at 0 minutes, 30 minutes, 60 minutes, 90 minutes, and 120 minutes and that this could be done at the same time as the OGTT. Such protocols would give extra specificity and information but would increase the burden on both the participant and investigator.

Note from the Diabetes WG: Blood should be collected in an appropriate 10 mL EDTA tube. Invert the tube three to four times to mix, store on ice, and centrifuge within 30 minutes. Processing should yield ~4.5 mL of plasma, which can then be separated into several 0.5 mL aliquots for multiple different tests.

Note from the Diabetes WG: The Diabetes WG recommends that the investigator record whether the blood was drawn and whether the full amount was obtained.

Note from the Diabetes WG: Plasma should be stored at -80°C until testing and shipped on dry ice to prevent thawing.

Note from the Expert Review Panel (ERP): A recent febrile illness may impact the results of an OGTT. Consideration can be given to excluding a sample person with a history of a febrile illness in the 3 days prior to the planned OGTT. The investigator should record the reason a person is excluded from the OGTT.

Note from the ERP: Plasma or Serum

Collection of blood samples for the measurement of analytes requires a general determination of whether to use serum or plasma for the assay and also a determination of the type of collection tube to be obtained. For example, if plasma is to be used, a determination needs to be made as to whether EDTA- or heparin-containing collection tubes are used. While comparable values are obtained for many analytes from either serum or plasma, there may be situations where differences are more pronounced and serum- or plasma-specific norms will be needed for references. The protocol presented here uses EDTA plasma. At times, it may be possible to collect both, but other considerations, such as participant burden, may be the deciding factor. It is important to match assay type with sample type. Some automated devices may preclude the use of EDTA plasma, for example, while others may be optimized for it. Investigators should choose methods of collection that match the methods of analysis. This will best be done by communicating with the laboratory where the proposed assays will be performed. They will become an important partner with you in assuring that there is compatibility from collection to assays to interpretation and reporting of levels and results.

Availability

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Protocol

The following is a summary version of the full National Health and Nutrition Examination Survey 2007-2008 protocol.

The full National Health and Nutrition Examination Survey 2007-2008 Oral Glucose Tolerance Test Procedures can be found here: 2007-2008 NHANES Oral Glucose Tolerance Test Manual.

Exclusion Criteria:

Persons will be excluded from this component if they:

• Report that they are taking oral medications for diabetes;

• Report that they are taking insulin;

• Report that they are pregnant;

• Report that they have hemophilia;

• Report that they have received cancer chemotherapy in the last 3 weeks; and

• Report that they have not fasted at least 9 hours.

SP = Sample Person.

1. Did you eat or drink anything other than plain water after [Insert time at 9 hours prior to sample collection] last night?

[ ] Yes

[ ] No

[ ] Refused

[ ] Don’t Know

If the answer is "No," then he or she has met the 9-hour fast. If the answer is "Yes," "Don’t know," or "Refused," then the actual fasting time is unknown. The SP is excluded from the Oral Glucose Tolerance Test if the 9-hour fast is not met and will not be met with 1 hour and 40 minutes left in the session.

Confirmation Question:

2. Have you had any of the following since {insert time from 1 here}?

Coffee or tea with cream and sugar? [Include milk or non-dairy creamers.]