C-Peptide, 24-Hour Urine

CPT: 84681
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Test Details

Synonyms

  • Connecting Peptide
  • Insulin C-Peptide
  • Proinsulin C-Peptide

Use

Urine C-peptide is measured when a continuous assessment of β-cell function is desired or frequent blood sampling is not practical (eg, in children).1 C-peptide excretion in urine has been used to assess pancreatic function in gestational diabetes and in patients with unstable glycemic control in insulin-dependent diabetes mellitus (IDDM).2,3

Methodology

Electrochemiluminescence immunoassay (ECLIA)

Reference Interval

17.2−181.0 μg/24 hours

Additional Information

C-peptide is a single-chain 31-amino acid (AA 33-63) connecting (C) polypeptide with a molecular weight of approximately 3021 daltons.1,5 In the process of biosynthesis of insulin, the C-peptide is formed as a byproduct together with insulin by the proteolytic cleavage of the precursor molecule proinsulin, stored in secretory granules in the Golgi complex of the pancreatic β-cells. Proinsulin, in turn, was cleaved from preproinsulin.1,6

C-peptide fulfills an important function in the assembly of the two-chain insulin (α- and β-chain) structure and the formation of the two disulfide bonds within the proinsulin molecule. Insulin and C-peptide are secreted in equimolar amounts and released into circulation via the portal vein.2 As half of the insulin, but almost none of the C-peptide, is extracted in the liver, C-peptide has a longer half-life (about 35 minutes) than insulin; 5 to 10 times higher concentration of C-peptide persists in the peripheral circulation, and these levels fluctuate less than insulin.1,2,6

The liver does not extract C-peptide, which is removed from the circulation by the kidneys and degraded, with a fraction excreted unchanged in the urine. The concentration in urine is about 20- to 50-fold higher than in serum. C-peptide concentrations are, therefore, elevated in renal disease.1,5,6

Specimen Requirements

Specimen

Urine (24-hour), frozen

Volume

10 mL

Minimum Volume

1 mL

Container

Plastic urine container

Collection

Instruct the patient to void at 8 AM and discard the specimen. Then collect all urine including the final specimen voided at the end of the 24-hour collection period (ie, 8 AM the next morning) in a 24-hour urine collection container. pH should be 4 to 7.

Storage Instructions

Freeze

Stability Requirements

Temperature

Period

Room temperature

1 day

Refrigerated

14 days

Frozen

14 days

Freeze/thaw cycles

Stable x3

Causes for Rejection

pH <3 or pH >8

Clinical Information

Special Instructions

This test may exhibit interference when sample is collected from a person who is consuming a supplement with a high dose of biotin (also termed as vitamin B7 or B8, vitamin H, or coenzyme R). It is recommended to ask all patients who may be indicated for this test about biotin supplementation. Patients should be cautioned to stop biotin consumption at least 72 hours prior to the collection of a sample.

Footnotes

1. Sacks DB. Carbohydrates. In: Burtis CA, Ashwood ER, eds. Tietz Textbook of Clinical Chemistry. 3rd ed. Philadelphia, Pa: WB Saunders Co;1999: 750-808.
2. Lunell NO, Persson B, Devarajan LV, et al. Urinary C-peptide in the neonate correlates both to maternal glucose tolerance and to fetal size at birth. Am J Perinatol. 1988 Apr; 5(2):144-145. 3348860
3. Cha T, Tahara Y, Ikegami H, et al. Urinary C-peptide as an index of unstable glycemic control in insulin-dependent diabetes mellitus (IDDM). Diabetes Res Clin Pract. 1991 Sep; 13(3):181-187. 1959481
4. C-Peptide on Elecsys 1010/2010 and Modular Analytics E170, [package insert ] 2007-06, V4. Indianapolis, Ind: Roche Diagnostics; 2007.
5. Clark PM. Assays for insulin, proinsulin(s) and C-peptide. Ann Clin Biochem. 1999; 36(Pt 5):541-564. 10505204
6. Thomas L. Insulin, C-peptide, proinsulin. Clinical Laboratory Diagnostics. 1st ed. Washington, DC: AACC Press;1998:149-150.

LOINC® Map

Order Code Order Code Name Order Loinc Result Code Result Code Name UofM Result LOINC
003236 C-Peptide, Urine 003237 C-Peptide, Urine ng/mL 1987-7
003236 C-Peptide, Urine 003238 C-Peptide,Urine 24hr ug/24 hr 27944-8

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