REVIEW

A Practical Review of C-Peptide Testing in Diabetes

Emma Leighton . Christopher AR Sainsbury . Gregory C. Jones

Received: March 23, 2017 / Published online: May 8, 2017� The Author(s) 2017. This article is an open access publication

ABSTRACT

C-peptide is a widely used measure of pancreaticbeta cell function. It is produced in equimolaramounts to endogenous insulin but is excretedat a more constant rate over a longer time.Methods of estimation include urinary andunstimulated and stimulated serum sampling.Modern assays detect levels of c-peptide whichcan be used to guide diabetes diagnosis andmanagement. We explore the evidence behindthe various tests available. We recommend theglucagon stimulation c-peptide testing owing toits balance of sensitivity and practicality.C-peptide levels are associated with diabetestype and duration of disease. Specifically ac-peptide level of less than 0.2 nmol/l is associ-ated with a diagnosis of type 1 diabetes mellitus(T1DM). C-peptide level may correlate withmicrovascular and macrovascular complicationsand future use of insulin therapy, as well aslikely response to other individual therapies.We explore the potential uses of c-peptidemeasurement in clinical practice.

Keywords: C-peptide; Diabetes; Insulin; Type 1diabetes; Type 2 diabetes

INTRODUCTION

C-peptide is the part of proinsulin which iscleaved prior to co-secretion with insulin frompancreatic beta cells. Produced in equimolaramounts to endogenous insulin, it is not aproduct of therapeutically administered exoge-nous insulin and has been widely used as ameasure of insulin secretion. This review of theliterature will identify the main indications andrationale for c-peptide sampling in clinicalpractice and compare the available methods ofc-peptide testing. Overall, the aim is to offer arationale for the practical and pragmatic use ofc-peptide testing to guide clinical managementof individuals with diabetes.

This article is based on previously conductedstudies and does not involve any new studies ofhuman or animal subjects performed by any ofthe authors.

WHAT IS C-PEPTIDE AND WHYMIGHT IT BE USEFUL IN CLINICALPRACTICE?

C-peptide is a useful and widely used method ofassessing pancreatic beta cell function [1, 2].After cleavage of proinsulin, insulin and the

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E. Leighton � C. A. Sainsbury � G. C. Jones (&)Diabetes Department, Gartnavel General Hospital,Glasgow, UKe-mail: g.jones3@nhs.net

Diabetes Ther (2017) 8:475–487

DOI 10.1007/s13300-017-0265-4

31-amino-acid peptide c-peptide are producedin equal amounts [3, 4]. So why is c-peptidetesting preferable to insulin as a guide to betacell function? The degradation rate of c-peptidein the body is slower than that of insulin(half-life of 20–30 min, compared with thehalf-life of insulin of just 3–5 min), whichaffords a more stable test window of fluctuatingbeta cell response. In healthy individuals theplasma concentration of c-peptide in the fastingstate is 0.3–0.6 nmol/l, with a postprandialincrease to 1–3 nmol/l [4]. Half of all insulinsecreted by the pancreas is metabolized in theliver by first-pass metabolism, whereas c-peptidehas negligible hepatic clearance. C-peptide iscleared in the peripheral circulation at a con-stant rate, whereas insulin is cleared variablymaking direct measurement less consistent. Ininsulin-treated patients with diabetes, mea-surement of c-peptide also avoids the pitfall ofcross-reaction of assay between exogenous andendogenous insulin.

C-peptide is a cornerstone of the assessmentof non-diabetes-associated hypoglycemia andthe diagnosis of conditions such as insulinomaand factitious hypoglycemia but this area isbeyond the scope of this article.

Increasing evidence suggests that c-peptidemay also be useful in predicting future levels ofglycemic control, response to hypoglycemicagents, and risk of future diabetes complica-tions. We will examine the key methods ofsample collection for c-peptide determinationand advise on what is most reliable and practi-cal. Furthermore, we will summarize the clinicalrelevancy of c-peptide sampling through reviewof the evolving literature on this subject.

WHAT ARE THE POTENTIALPROBLEMS WITH C-PEPTIDEMEASUREMENT?

The majority of c-peptide is metabolized by thekidneys with 5–10% then excreted unchangedin the urine [1, 5]. This can make c-peptidemeasurement in individuals with chronic kid-ney disease inaccurate.

Modern ultrasensitive c-peptide assays areable to detect c-peptide values as low as

0.0015–0.0025 nmol/l [6]. It is also important tobe aware that cross-reactivity with proinsulinmust be less than 10%, which is generally thecase for modern assays [7]. The presence of largenumbers of anti-insulin antibodies that bindboth proinsulin and c-peptide can give a falselyhigh c-peptide reading. When consideringc-peptide estimation it is important to be awareof which assay your local laboratory uses and inparticular which method of collection theyroutinely process.

It should be noted that c-peptide will beexpressed in nanomoles per liter in this article, asopposed to picomoles per milliliter, picomolesper liter, or nanograms per milliliter, which areoften quoted in the literature (1 nmol/l = 1 pmol/ml = 1000 pmol/l= 3 ng/ml).

METHODS OF COLLECTIONOF C-PEPTIDE

Various methods of c-peptide estimation havebeen advocated. These have been summarizedin Table 1. Urinary c-peptide (UCP) is anon-invasive test, which can be performed in anoutpatient setting [8]. When collected in boricacid UCP is stable at room temperature for up to3 days. In patients with normal renal function,UCP quantity is reflective of 5–10% of totalc-peptide secreted by the pancreas [9]. The 24 hurinary c-peptide sample collection (24 h UCP)is a more time-consuming method, which isinconvenient for the patient, making it a lessattractive option than spot UCP. In subjectswith normal glucose tolerance urinary c-peptideto creatinine ratio (UCPCR) has been shown tocorrelate well with 24 h urinary c-peptide [5].This suggests that UCPCR might be a simple,reliable, and convenient method of estimatingc-peptide. Gender differences may arise, how-ever, owing to women having less muscle mass,resulting in reduced urine creatinine and higherUCPCR values than men [10]. In one studyUCPCR was found to significantly correlatewith fasting c-peptide in female subjects, withspot UCP being a better measure of beta cellfunction in male subjects [2].

Serum c-peptide has traditionally beenthought to be an inconvenient method as

476 Diabetes Ther (2017) 8:475–487

Table1

Methods

ofc-peptideevaluation

Cutoff

Positives

Negatives

Test

\0.2nm

ol/l

Sensitive,specific,quick,

reproducible,correlates

withdiagnosis

Nausea

Glucagonstim

ulationtest(G

ST)

\0.2nm

ol/l

Sensitive,specificreproducible,correlateswith

diagnosis

Tim

e-consum

ing,liquidmixed

mealsnotwidely

available

Mixed

mealtolerancetest(M

MTT)

\0.2nm

ol/l

Practicalifat

timeof

diagnosticOGTT

Tim

e-consum

ing,lim

ited

evidence

ofpredicting

beta

cellfunction

Oralglucosetolerancetest(O

GTT)

\0.2nm

ol/l

Sensitive

Tim

e-consum

ing,lim

ited

evidence

ofpredicting

beta

cellfunction

Tolbutamidetolerancetest(tCP)

\0.075nm

ol/l

Simple,quick,

correlates

withdiabetes

type

Insufficientto

detect

subtlerisesin

c-peptide

Fasting(fCP)

\0.2nm

ol/l

Easyto

perform,q

uick,sim

ple,correlates

with

diagnosis

Ifindeterm

inate,requires

confi

rmationwith

stim

ulationtesting

Random

non-fasting(rCP)

\0.2nm

ol/m

mol

Non-in

vasive,sim

ple,stablefor72

hin

boricacid,

correlates

withinsulin

deficiencyin

T2D

M

Inaccurate

inCKD,affectedby

gend

eras

aresult

ofdifferencesin

creatinine

concentration

Urinary

c-peptidecreatinine

ratio

(UCPC

R)

\0.2nm

ol/l

Someevidence

ofcorrelationwithbeta

cell

function

inmen

withT2D

M,stablefor72

hin

boricacid

Inaccurate

inCKD,lesssensitivethan

when

expressedas

ratioto

creatinine

(UCPC

R)

Urinary

c-peptide(U

CP)

\0.3nm

ol/l

Non-in

vasive,u

sefulin

detectinginsulin

deficiency,stablefor72

hin

boricacid

Inaccurate

inCKD,tim

e-consum

ing,requires

good

patientcompliance.Affectedby

variations

increatinine

24hurinarycollection(24hUCP)

T2D

Mtype

2diabetes

mellitus,C

KD

chronickidn

eydisease

Diabetes Ther (2017) 8:475–487 477

immediate lab analysis is required beforedegradation (when collected in serum gel orplain sample tubes). This is because c-peptide isa small linear peptide, which is susceptible toenzyme proteolytic cleavage [7]. Gel tubes aretherefore traditionally required to be trans-ported to the lab on ice, promptly centrifugedand separated, then stored in frozen conditionsunless lab analysis is possible at that center.However, c-peptide sample collection forc-peptide determination in whole blood inEDTA prepared tubes is stable at room temper-ature for up to 24 h [11].

Venous blood c-peptide levels can be mea-sured in the random, fasting, or stimulated state[12]. Random samples are taken at any timeduring the day without consideration of recentfood intake, whereas fasting samples are takenafter an 8- to 10-h fast. Stimulation methodsinclude using glucagon, intravenous/oral glu-cose, tolbutamide, sulfonylurea, and glu-cose-like peptide 1, amino acids, or a mixedmeal [7, 13–15].

Random non-fasting sampling (rCP) is thesimplest method allowing flexibility to test inan outpatient or inpatient setting. rCP has beenshown to correlate with fasting c-peptide (fCP)and post glucagon stimulation test (GST) sam-ples in subjects with well-defined type 1 or type2 diabetes [16]. Similarly rCP has been shown tocorrelate with 90 min mixed meal tolerance test(MMTT) c-peptide responses (r = 0.91,p\0.0001) [17].

Originally described in 1977, the GST is themost widely used of the provocation methodsand is described in Fig. 1 [18]. The MMTT hasbeen advocated as the ‘‘gold standard’’ of stim-ulation testing owing to excellent sensitivity indetecting residual insulin secretion [7, 19, 20].In the MMTT, a weight-based liquid meal, such

as Sustacal or Boost, is ingested over 5 min andtimed samples for c-peptide determination canbe taken 10 min prior to ingestion (t = -10), atbaseline (t = 0), and at 15, 30, 60, 90, and120 min. C-peptide can also be measured acrossa 75 g oral glucose tolerance test (OGTT) [21].C-peptide sampling as part of the OGTT hasbeen found to significantly correlate with insu-lin secretion in type 2 diabetes mellitus (T2DM),when samples are taken at 0, 30, 60, 90, and120 min (with the possibility of extending thisto include sampling at 150, 180, 240, and300 min) [22]. Adding c-peptide determinationto the protocol of an OGTT may therefore be apractical means of estimating beta cell functionwhen the test is already being performed fordiagnostic reasons, if further interval samplingis taken between 0 and 120 min.

Which method of c-peptide collection ismost clinically useful? GST has been shown tobe superior in sensitivity to use of glucose ortolbutamide as substrates, with a twofold highermean increase in c-peptide response [13]. Wesummarize the method of performing the GSTin Fig. 1. Further studies have demonstratedthat c-peptide levels following GST are a con-sistently sensitive measure of beta cell function,which may be associated with diabetes type andfuture use of insulin [9, 19, 23, 24]. In patientswith diabetes who were both insulin andnon-insulin treated, GST demonstrated a 29%rise in c-peptide compared to 19% rise post-prandially [24]. Further support of the clinicalutility of stimulated c-peptide comes from theAmerican Diabetes Association (ADA), whichsponsored a workshop evaluating the use ofc-peptide in assessment of beta cell function.They concluded that MMTT and GST were rec-ommended owing to their superior sensitivityand specificity [7].

Fig. 1 Glucagon-stimulated c-peptide test (GST)

478 Diabetes Ther (2017) 8:475–487

MMTT and GST are known to be sensitiveand reproducible tests of residual beta cellfunction in type 1 diabetes mellitus (T1DM),with peak responses seen at 90 and 6 min,respectively [19]. Two parallel randomized trialsby the TrialNet group and by the EuropeanC-peptide Trial (ECPT) found moderately supe-rior rises in peak response with the MMTT [19].Reproducibility and patient completion forboth tests were high. Although a majority ofpatients experienced nausea with the GST,which reduced overall patient satisfaction,patients older than 13 years preferred the GSTto the MMTT because of the shorter durationand ease of the test.

It is our view that serum c-peptide testingafter stimulation is superior to rCP, fCP, andurinary testing. Despite the fact that fasting orrandom non-fasting c-peptide measurements areeasier to perform, are less expensive, and canoften be carried out in a clinic setting, theirability to detect subtle levels of c-peptide is sig-nificantly limited. This difference could beimportant in T2DM patients who may beunnecessarily started on insulin therapy despitehaving adequate beta cell reserve. Applying acutoff value of less than 0.2 nmol/l to rCP testinghas been shown to correlate well with stimulatedc-peptide (r = 0.91) in insulin-treated subjectswith diabetes, but levels of rCP out of this rangerequire confirmation by another method [17].Likewise urinary testing, whether collected as aUCPCR or 24 h collection, has been shown to befar less sensitive and is inaccurate in individualswith any degree of renal impairment. Whilst in aresearch setting the MMTT may potentially bepreferable owing to its increased peak response,we feel that in day-to-day clinical practice a120 min test is too time-consuming and cum-bersome. The GST is preferable owing to theshort duration of the test leading to less patientinconvenience, coupled with good sensitivityand reproducibility of results.

DIAGNOSIS AND DIABETESCLASSIFICATION

C-peptide has been shown to denote endoge-nous insulin production and correlates with

type of disease, duration of diabetes, as well asage of diagnosis. The various practical applica-tions of c-peptide measurement are summarizedin Table 2.

In insulin-treated individuals, fCP less than0.2 nmol/l and GST of less than 0.32 nmol/lhave been found to correlate significantly withT1DM, with greater sensitivity and specificitythan urinary testing [9].

Table 2 Indications for c-peptide measurement

Diagnostic

To define T1DM

Criteria for acceptance for CSII

To determine whether T1DM or T2DM

Diagnostic test for MODY

Diagnostic test for LADA, in addition to antibody

testing

Prognostic

Marker of duration of diabetes

Lower levels are associated with microvascular

complication risk in T1DM

Associated with glycemic variability/HbA1C level

Lower levels are associated with greater hypoglycemia

risk

Therapeutic response

Lower baseline levels associated with increased need

for insulin

Lower baseline levels associated with shorter time to

insulin treatment

Higher levels present in patients who respond to

metformin and glibenclamide in combination

Higher levels associated with response to

thiazolidinediones

Correlates with reduction in HbA1C following

initiation of GLP-1 agonist therapy

CSII continuous subcutaneous insulin infusion, MODYmaturity-onset diabetes of the young, LADA latentautoimmune diabetes of adults

Diabetes Ther (2017) 8:475–487 479

The diabetes control and complications trial(DCCT) was the landmark study which helpedgenerate the targets we presently use for T1DM.Entry to the DCCT required individuals to haveinsulin-dependent diabetes mellitus of at least5 years’ duration with a baseline mixed mealstimulated c-peptide of less than 0.2 nmol/l[25]. Later in the study, entry criteria wereextended to include those with a baselinemixed meal stimulated c-peptide of up to0.5 nmol/l. The study determined that intensivetreatment with three or more insulin injectionsor continuous subcutaneous insulin infusion(CSII) therapy reduces the incidence ofmicrovascular complications and the later fol-low-up of the cohort showed a reduction inadverse cardiovascular outcomes [26, 27]. As therationale for intensive control with insulin inT1DM is based on data from the DCCT, it islogical that stimulated c-peptide is used as amethod of defining T1DM.

There is some evidence that c-peptide mayhave a role in the diagnosis of latent autoim-mune diabetes of adults (LADA), which can bemisdiagnosed as T2DM [28–30]. fCP is signifi-cantly lower in LADA compared with T2DM[30]. Whilst c-peptide sampling may thereforebe an effective initial screening tool for LADA,anti-GAD or anti-IA2 antibody measurementshould be considered to confirm diagnosis.

C-peptide concentration has been shown todecline over decades with duration of diabetes[6, 31–33]. DCCT data obtained at screening toenter the study showed that diabetes durationwas associated with c-peptide value; 48% ofindividuals with T1DM of up to 5 years’ dura-tion had a mixed meal stimulated c-peptide ofat least 0.2 nmol/l (corresponding with pre-served beta cell function), but only 8% of thosewith diabetes duration 5–15 years had a stimu-lated c-peptide of at least 0.2 nmol/l [7, 25].Recent cross-sectional studies confirmed thatc-peptide declines over time and is significantlyrelated to age of onset (p\0.0001) with ayounger age of onset (less than 10 years)resulting in a far more rapid c-peptide decline[6]. A higher percentage of detectable c-peptidehas been found in those with T1DM aged olderthan 18 years, compared to those younger than18 years [34].

Maturity-onset diabetes of the young(MODY) is a rarer, genetic form of diabetes,which can be misdiagnosed as T1DM [35].C-peptide has been proposed as a useful bio-marker in the detection of MODY prior togenetic testing. In MODY, whilst there isreduction in beta cell function, some insulinsecretion is retained compared to T1DM.UCPCR has been used as a tool to discriminatebetween two of the most common types ofMODY: HNF1A and HNF4A heterozygousmutations, and long-duration T1DM [36].UCPCR was found to be significantly lower insubjects with type 1 diabetes of greater than5 years’ duration, compared with subjects withHNF1A/4A MODY (p\0.0001).

The ‘‘Diabetes Diagnostics’’ app has beencreated by the University of Exeter diabetesresearch team as a convenient resource for thediagnosis of MODY and other types of diabeteson the basis of clinical criteria according tonational and international guidelines in addi-tion to c-peptide interpretation [37, 38].

C-peptide is a useful tool in the classificationof diabetes. It can help differentiate T1DM,T2DM, and MODY. C-peptide is associated withduration of disease as well as age of diagnosis.Whilst c-peptide is useful in classifying diabetesit must always be interpreted in clinical contextof disease duration, comorbidities, and familyhistory.

PREDICTION OF NEEDFOR INSULIN

There is limited evidence in the literature aboutwhether c-peptide can effectively predict whe-ther patients require insulin [23, 39]. An earlyprospective cohort study determined that apeak GST c-peptide of less than 0.6 nmol/l wasassociated with later treatment with insulin[23]. Further to this a retrospective cohort studyin a single diabetes outpatient center in Swedenfound that a median fasting c-peptide concen-tration at diagnosis was lower in patientsimmediately treated with insulin (0.24 nmol/l,range 0.10–1.54) compared with those managedinitially with diet with or without oral therapy(0.73 nmol/l, range 0.10–4.10) [39]. fCP of less

480 Diabetes Ther (2017) 8:475–487

than 0.25 nmol/l at diagnosis as an indepen-dent factor had 60% sensitivity and 96% speci-ficity for association with insulin treatment atfollow-up. Islet cell antibody (ICA) positivity incombination with low fCP was found to signif-icantly correlate with future insulin treatment.All such cohort studies are subject to potentialbias in that the c-peptide result may in itselfinfluence the decision to start insulin.

Time to insulin treatment may also be asso-ciated with c-peptide response [40]. In a retro-spective study of insulin-treated individualswith T2DM, time to insulin prescription inindividuals with an MMTT c-peptide of at most0.2 nmol/l (suggesting absolute insulin defi-ciency) was 2.5 (1.5–3) years compared to 6(3–10.75) years for those with a mixed mealstimulated c-peptide of greater than 0.2 nmol/l(p = 0.005).

The ability of c-peptide estimation to con-firm the appropriateness of intensive insulintreatment has been practically applied in somehealthcare systems such as being used as a cri-terion for determining if an individual withdiabetes is a suitable candidate for CSII therapy[41].

A stimulated c-peptide concentration of atmost 0.2 nmol/l may be used as a cutoff valuepredictive of poor beta cell reserve and likelyrequirement of insulin therapy, for whichintensive therapy has been shown to be effica-cious. Additionally, a fasting c-peptide valueless than 0.25 nmol/l, alone or especially if incombination with ICA positivity, is a useful testfor determining likely future insulin treatment.

PREDICTION OF RESPONSETO NON-INSULIN THERAPIESIN T2DM

There is variable evidence to support the use ofc-peptide to predict response to non-insulintreatments for T2DM.

Higher MMTT stimulated c-peptide has beenshown to be present in patients who respond tometformin and glibenclamide in combinationbut GST c-peptide did not predict response toglibenclamide alone [42, 43]. High fCP is asso-ciated with response to the thiazolidinediones,

rosiglitazone and pioglitazone, which is inkeeping with their action of reducing insulinresistance [44, 45]. Whilst cohort studies ofmixed DPP4 inhibitor use has shown that initialhigher fCP predicts reduction of HbA1c, thisassociation was not found in a retrospectivestudy of sitagliptin use in addition to met-formin or a sulfonylurea [46–48].

However, higher c-peptide levels do seem topredict response to GLP-1 agonists. Betterinsulin response to glucose is seen in thosepatients taking liraglutide with higher GSTc-peptide levels [49]. The clinical relevance ofthis finding is confirmed by studies showingthat fCP and post-meal UCPCR both predictreduction of HbA1c following initiation oftreatment with an GLP-1 agonist [50, 51].

In patients taking SGLT-2 inhibitors, c-pep-tide may be able to identify those patients whoare at risk of diabetic ketoacidosis. Unfortu-nately, low c-peptide is not a consistent findingin patients on SGLT-2 inhibitors who haveketoacidosis [52, 53].

CAN C-PEPTIDE PREDICT DIABETESCOMPLICATIONS?

An area of growing research is the question as towhether c-peptide can be used to predict dia-betes complications. This has been summarizedin Table 2.

Lower c-peptide values have been associatedwith poorer glycemic control and henceincreased HbA1c values [6, 54].

Retrospective analysis of subjects includedfor screening for the DCCT found a correlationbetween c-peptide and clinical outcomes ininsulin-dependent diabetes [26, 55]. Uniformlyin the ‘‘intensive’’ treatment group higher andsustained levels of c-peptide (i.e., greater than0.2 nmol/l consistently) were associated withreduced incidence of retinopathy andnephropathy (p\0.05). The risk reduction ofretinopathy between ‘‘non-responder’’ (c-pep-tide less than 0.2 nmol/l) and ‘‘respon-der’’(c-peptide 0.2–0.5 nmol/l) groups was 58%(26–76%, p = 0.0025), and for sustainedretinopathy the risk reduction was even higherat 79% (9–95%, p = 0.0360) [54]. For severe

Diabetes Ther (2017) 8:475–487 481

hypoglycemia the risk reduction between‘‘non-responders’’ and ‘‘responders’’ was 45%(38–52, p\0.0001). In another cross-sectionalanalysis, c-peptide less than 0.01 nmol/l wasfound to correlate with increased incidence ofnephropathy, neuropathy, retinopathy, andfoot ulcers (p = 0.03, odds ratio 3.1) [6].

Lower levels of c-peptide and decreased betacell function have been linked to greater levelsof glucose variability [20, 56]. As glucose vari-ability is known to be associated with increasedcomplications and mortality in patients withdiabetes it is possible that c-peptide may be apredictor of future outcomes independent ofHbA1c levels [57–59].

Whilst c-peptide levels may be associatedwith complications in diabetes through a gly-cemic mechanism it may also have directmolecular effects. C-peptide is generallythought of as a biologically inert peptide ofinterest only as a surrogate marker for insulinlevels but this may be overly simplistic. C-pep-tide has been shown in vitro to inhibitendothelial cell reactive oxygen species (ROS)formation in the presence of hyperglycemia[60, 61]. C-peptide also downregulates theexpression of several hyperglycemia-inducedadhesion molecules, including vascular cellularadhesion molecule 1 (VCAM1), reducingleukocyte adhesion to endothelial cell walls andpreventing the early stages of atherosclerosisplaque formation [4].

Some preliminary randomized controlledtrials demonstrate that giving subcutaneousc-peptide alongside insulin to individuals withT1DM may ameliorate microvascular compli-cations, such as albuminuria and autonomicnerve dysfunction [62, 63]. However thesestudies only included small cohort numbers andrequire further replication.

Higher levels of c-peptide have consistentlybeen shown to be associated with cardiovascu-lar and all-cause mortality in people withoutdiabetes [64–66]. This is presumably becauseraised c-peptide levels are a marker of insulinresistance and metabolic syndrome phenotype.Similar findings have been observed in some,but not all, observational studies in T2DM[67–70].

It seems that c-peptide is associated withincreasing microvascular complications whenlow and macrovascular complications whenhigh, which may make interpretation of resultsdifficult when attempting to predict outcomesin clinical practice.

GUIDELINES

Despite studies showing the potential impor-tance of c-peptide as a marker in the diagnosisand of outcomes in diabetes, current nationaland international guidelines do not advocate itsuse [71, 72].

According to the American Diabetes Associ-ation (ADA) guidelines, the role of c-peptide inthe diagnosis of diabetes is currently limited tounusual or ambiguous cases of T1 or T2DM [73].C-peptide is still felt to be a more important toolin research rather than in day-to-day clinicalpractice. An ADA-sponsored workshop in 2001concluded unanimously that c-peptide estima-tion be used as the most appropriate outcomemeasure for preservation of beta cell function[7]. In recent guidelines developed by theAmerican Association of Clinical Endocrinolo-gists (AACE) and American College ofEndocrinology (ACE), documenting the levelsof c-peptide is recommended when there isdoubt over the diagnosis T1 or T2DM [74]. Arecent systematic review supported the diag-nostic utility of c-peptide, recommending itsuse as the principal baseline measure of insulindeficiency [75].

Given the evidence, we would anticipatethat c-peptide estimation may in future have amore prominent role in guidelines relating todiagnosis and management of diabetes.

POTENTIAL FUTURE USES

Potential future uses of c-peptide are broadincluding aiding appropriate diagnosis, guidingtherapy choices, and predicting morbidity indiabetes. Stimulated c-peptide sampling is asensitive and specific test, which can helpdetermine type and duration of diabetes. Withmany previous studies including small numbers

482 Diabetes Ther (2017) 8:475–487

and using fasting c-peptide, future work couldinclude prospective design, larger patient pop-ulations, and use stimulated c-peptide as a moreaccurate parameter. Furthermore, evidence iscurrently limited with respect to predictingrarer forms of primary diabetes, such as MODYor LADA. In time, c-peptide may become ascreening tool to reduce need for autoantibodytests being performed in certain patients toconfirm or exclude a diagnosis. A lower c-pep-tide, specifically less than 0.2 nmol/l, can mostlikely predict requirement for insulin, but canvalues be used to predict likely time until insu-lin prescription? Similarly, can c-peptide sam-pling be used to guide the safe discontinuationof therapy? Lower c-peptide values have beenshown to correspond with increased incidenceof microvascular complications. It would beinteresting to determine whether c-peptideconcentrations are associated with increasedmacrovascular morbidity and mortality.

CONCLUSIONS

C-peptide is a useful indicator of beta cellfunction, allowing discrimination betweeninsulin-sufficient and insulin-deficient individ-uals with diabetes. Various methods of sam-pling are available, including a urinaryc-peptide to creatinine ratio, fasting serumc-peptide, and stimulated c-peptide. Owing tosensitivity, reproducibility, and convenience ofthe test, we would recommend glucagon stim-ulation testing in clinical practice. C-peptidehas been shown to correlate with diabetes type,duration of disease, and age of diagnosis.Interestingly c-peptide has been demonstratedto be associated with microvascular complica-tions. This requires further study but prelimi-nary evidence suggests that in future, alongsideHbA1C, sampling for c-peptide may be used asan integral diagnostic and monitoring tool.

ACKNOWLEDGEMENTS

No funding or sponsorship was received for thisstudy or publication of this article. The articleprocessing charges were funded by the authors.

All named authors meet the InternationalCommittee of Medical Journal Editors (ICMJE)criteria for authorship for this manuscript, takeresponsibility for the integrity of the work as awhole, and have given final approval for theversion to be published.

Compliance with Ethics Guidelines. Thisarticle is based on previously conducted studiesand does not involve any new studies of humanor animal subjects performed by any of theauthors.

Disclosures. The authors of this article,Emma Leighton, Christopher AR Sainsbury, andGregory C. Jones, have nothing to disclose.

Data Availability. Data sharing is notapplicable to this article as no datasets weregenerated or analyzed during the current study.

Open Access. This article is distributedunder the terms of the Creative CommonsAttribution-NonCommercial 4.0 InternationalLicense (http://creativecommons.org/licenses/by-nc/4.0/), which permits any non-commercial use, distribution, and reproductionin any medium, provided you give appropriatecredit to the original author(s) and the source,provide a link to the Creative Commons license,and indicate if changes were made.

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