Normal C-peptide levels for a fasting test are generally considered to be anything between 0.5 nanograms (ng) per millileter (ml) and 3 ng/ml, although people who do not have diabetes may occasionally stray out of this range. The following is a range of C-peptide values in people without diabetes, as compiled by Endocrine Sciences, Inc., a California-based laboratory that conducts the test. It should be noted that, in some cases, subjects fell below the normal range of C-peptide values, but were still not found to have diabetes. The range of values may also vary according to what lab your health care practitioner uses.

Children (< 15 years old) 8:00 a.m. fasting: 0.4 to 2.2 ng/ml

Adults 8:00 a.m. fasting: 0.4 to 2.1 ng/ml

Two hours postprandial (after a meal): 1.2 to 3.4 ng/ml

Two hours post glucose load: 2.0 to 4.5 ng/ml

Although anything less than these numbers is generally an indicator of type 1 diabetes, values within the normal range can mean different things.

"Type 2s with insulin resistance could actually be making more insulin than a non-diabetic slim person," says Richard Bernstein, MD, FACE, FACN, CWS, of the Diabetes Center in Mamaroneck, New York. Values on the lower end may also indicate a honeymoon phase of type 1, when insulin production is slowing down but has not yet ground to a halt.

Bernstein also points out that even in type 1s, a positive C-peptide test should be a source of optimism.

"Of all my patients, I only have two who don't make any C-peptide, and I'm one of them," Bernstein says. He says this proves that most type 1s still produce at least some insulin and raises the possibility that therapies like beta-cell regeneration may eventually restore normal BG levels.