Table 1.

Methods of c-peptide evaluation

Cutoff	Positives	Negatives	Test
<0.2 nmol/l	Sensitive, specific, quick, reproducible, correlates with diagnosis	Nausea	Glucagon stimulation test (GST)
<0.2 nmol/l	Sensitive, specific reproducible, correlates with diagnosis	Time-consuming, liquid mixed meals not widely available	Mixed meal tolerance test (MMTT)
<0.2 nmol/l	Practical if at time of diagnostic OGTT	Time-consuming, limited evidence of predicting beta cell function	Oral glucose tolerance test (OGTT)
<0.2 nmol/l	Sensitive	Time-consuming, limited evidence of predicting beta cell function	Tolbutamide tolerance test (tCP)
<0.075 nmol/l	Simple, quick, correlates with diabetes type	Insufficient to detect subtle rises in c-peptide	Fasting (fCP)
<0.2 nmol/l	Easy to perform, quick, simple, correlates with diagnosis	If indeterminate, requires confirmation with stimulation testing	Random non-fasting (rCP)
<0.2 nmol/mmol	Non-invasive, simple, stable for 72 h in boric acid, correlates with insulin deficiency in T2DM	Inaccurate in CKD, affected by gender as a result of differences in creatinine concentration	Urinary c-peptide creatinine ratio (UCPCR)
<0.2 nmol/l	Some evidence of correlation with beta cell function in men with T2DM, stable for 72 h in boric acid	Inaccurate in CKD, less sensitive than when expressed as ratio to creatinine (UCPCR)	Urinary c-peptide (UCP)
<0.3 nmol/l	Non-invasive, useful in detecting insulin deficiency, stable for 72 h in boric acid	Inaccurate in CKD, time-consuming, requires good patient compliance. Affected by variations in creatinine	24 h urinary collection (24 h UCP)

T2DM type 2 diabetes mellitus, CKD chronic kidney disease