C-Peptide – plasma

Diagnostic Use

C-peptide represents the connecting peptide which is cleaved from proinsulin in the production of endogenous insulin. As such, the presence of C-peptide indicates endogenous production of insulin, whereas C-peptide is absent if only exogenous insulin is administered. For accurate interpretation in appropriate clinical settings, plasma C-peptide should be paired with laboratory plasma glucose (± insulin) testing. Depending on the clinical question, either fasting or non-fasting specimens may be appropriate.

In patients with diabetes, C-peptide may be of value in the assessment of residual pancreatic function in insulin treated diabetics, differentiating between types of diabetes (eg in presence of high or normal glucose, C-peptide is typically low or low-normal in Type 1 diabetes, but typically high in MODY or Type 2 diabetes).

In patients with hypoglycaemia, high C-peptide may be associated with insulinoma, dumping syndrome or sulphonylurea use, whereas low C-peptide is an appropriate physiological response representing less release of endogenous insulin. A 72 hour fast or mixed meal test may be performed in certain settings to provoke hypoglycaemia for this purpose and aid clinical diagnosis if it is not captured opportunistically.

It is suggested that an endocrinologist be consulted for further advice regarding interpretation.

Department

Endocrinology Lab

Delphic Registration Code

CPEP

Turnaround Time

7 days

Test Code

7650

Specimen Type

• Plasma

Specimen Requirements

Tube/Container Type:

Preferred

• EDTA (Lavender Cap)

Preferred if West Coast

• SST II (Gold Cap)

Acceptable

• Serum (Red Cap)
• SST II (Gold Cap)
• Lithium Heparin - Non-gel (Green Cap)
• Lithium Heparin - Gel (Light Green Cap)

Preferred Sample Volume (ml)

2

No of Samples Required

1

Additional Specimen Information

C-peptide requests should be paired with a plasma glucose collected at the same time to ensure accurate interpretation. This may be either fasting or non-fasting, depending on the clinical question.

Paired C-peptide and glucose (±insulin) may be useful to differentiate between different types of diabetes. For this purpose either fasting or non-fasting samples are acceptable.

Paired C-peptide and glucose (±insulin) may also be useful to differentiate between different causes of hypoglycaemia, with fasting status depending on the situation where hypoglycaemia is present.

Avoid haemolysis.
Transport to lab on water/ice slurry if haemolysis likely.

Serum or Plasma Stability:
Ambient 4 hrs
Chilled (2-8deg) 24 hrs
Frozen 30 days

Aliquot Instructions

Sample Type

• Plasma

Minimum Volume (ml)

0.5

Transport to Lab

Chilled (2 - 8 degrees Celsius)
If overnight - Frozen

Additional Referral/Aliquot Information

West Coast: If specimen is unable to be separated, take blood in SST and send chilled to Greymouth lab. At Greymouth Lab, send serum/plasma frozen to CHL.

Reference Intervals

350 – 750 pmol/L (fasting, BMI <25)

Test Method

Roche Cobas

Non-Funded Test Cost

$47.25 (Exclusive of GST)

Billing Code

7650

Measure glucose at the same time.

At room temperature, C-peptide is sensitive to hemolysis, and the rate of C-peptide loss increases with the degree of hemolysis. For example, at a level of 0.8g Hb/L, 10% of C-peptide is lost after 24 hours at room temperature.

C-peptide in plasma is not stable long-term at -20o. Loss is of the order of 2% per month. Stable at -40o or lower.

Contact Information

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