Protocol - Fasting C-peptide - Assay

Description

This protocol provides instructions for drawing, processing, and storing blood according to the National Health and Nutrition Examination Survey (NHANES) methods. As there are no standard assays for C‑peptide, the protocol also provides basic guidelines to aid comparability among different studies.

Specific Instructions

Although all three are valuable stand-alone measures and all should be collected if resources are available, the Diabetes Working Group (WG) recommends that investigators prioritize Insulin first, C‑peptide second, and Proinsulin third. This is because results from the Insulin bioassay can be combined with results from Fasting Plasma Glucose bioassay to derive the homeostatic model assessment (HOMA) of insulin resistance and pancreatic beta cell function. The Diabetes WG notes that while it is not the preferred method, the C-peptide assay can be performed on individuals who have not met the fasting requirements.

Note from the Diabetes WG: The investigator should record the reason a sample person is excluded from the blood draw, whether the subject is fasting, and the number of hours since the last meal.

Note from the Diabetes WG: Blood should be collected in an appropriate 5 mL or 10 mL red-top tube.

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: Serum should be stored at -80°C until testing and shipped on dry ice to prevent thawing.

Note from the Expert Review Panel: 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 serum is to be used, a determination needs to be made as to whether red-top or serum gel separator 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 red-top/serum separator tubes. 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 serum, 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. The laboratory 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

Available

Download PDF

Protocol

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

Exclusion Criteria

Persons will be excluded from this component if they:

• 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.

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.]