The Emerging Role of Copeptin

Abstract

Direct measurement of the nonapeptide vasopressin has been limited by analyte instability ex vivo and in vivo rapid degradation, low serum concentrations requiring a sensitive assay and inherent secretory pulsatility. Copeptin is a 39 amino acid glycopeptide cleavage product of vasopressin synthesis with high stability, providing a marker of vasopressin secretion. Copeptin measurement has applications in diagnosis of diabetes insipidus and other diseases with altered vasopressin secretion. This review summarises our current understanding of serum copeptin measurement in diabetes insipidus and possible future applications of copeptin assays. As vasopressin is a stress hormone, there is emerging evidence on the use of copeptin for diagnosis and prognostication of disorders such as syndrome of inappropriate anti-diuretic hormone secretion, diabetes mellitus, critical illness, stroke, cardiovascular disease, respiratory disease, renal disease and thermal stress. Copeptin concentration measurement is likely to improve the diagnostic reliability of diabetes insipidus and, as a marker of stress, may have diagnostic or prognostic utility in specific clinical circumstances. Further studies are needed to determine if goal-directed therapy using plasma copeptin concentrations may improve patient outcomes.

Introduction

Vasopressin (also known as antidiuretic hormone (ADH) or arginine vasopressin (AVP)) has been well described as an important hormone regulating fluid homeostasis and vascular tone. It is synthesised in the supraoptic and paraventricular nuclei of the hypothalamus. Hypothalamic vasopressinergic magnocellular neurons project from the supraoptic nucleus to the posterior pituitary, utilising neurophysins. Vasopressin is then secreted into the circulation for the purpose of osmotic regulation via renal arginine vasopressin 2 (AVP2) receptors which lead to the membrane localisation of aquaporin-2 (Aq2) channels in distal renal tubules enhancing water reabsorption from tubular fluid into the circulation. Vasopressin also contributes to vascular tone via arginine vasopressin 1a (AVP1a) receptors. Parvocellular neurons in the paraventricular nucleus secrete vasopressin into the hypothalamo-hypophyseal portal circulation where it acts synergistically via arginine vasopressin 1b (AVP1b) receptors to stimulate pituitary adrenocorticotrophic hormone (ACTH) and ultimately adrenal cortisol secretion.^1–3 Hence, vasopressin is secreted into the general and hypophyseal portal circulations in response to stress. Vasopressin also has a role in promoting platelet aggregation and hence haemostasis.⁴ Copeptin is a 39-amino acid glycosylated peptide with a leucine-rich core. It is derived from pre-provasopressin together with (AVP) and neurophysin II (Figure). In contrast to vasopressin, the physiologic function of copeptin is not known. Copeptin was first described in 1972 by Holwerda⁵ and is co-synthesised in response to increasing osmolality with vasopressin and co-secreted in equimolar concentrations into the circulation.⁶ Vasopressin is a nonapeptide with a short half-life of approximately 24 min,⁷ and there are many limitations in direct vasopressin measurement assays including the rapid in vitro degradation of vasopressin and secretory pulsatility, complicating interpretation.⁸ Copeptin’s in vitro stability, stable serum levels due to its long circulating disappearance half-life by virtue of its glycosylation, and its length allowing more epitopes for raising antibodies for immunoassay development, make it an ideal surrogate biomarker for vasopressin release. In Australia, copeptin is measured with a chemiluminescence sandwich immunoassay (BRAHMS Copeptin proAVP) using two polyclonal antibodies to amino acids 132–164 of preprovasopressin.⁹ This review summarises the emerging role of copeptin in the evaluation of various conditions.

Figure.

Co-secretion of copeptin and vasopressin.

For reasons that are not clear, men have higher plasma copeptin levels under normo-osmotic conditions but no sex difference was seen after hypertonic saline stimulation.¹⁰ Copeptin levels do not increase with age and do not display circadian variation.^11,12 Stress, defined as a threat to homeostasis, stimulates copeptin/vasopressin release, and this phenomenon allows copeptin to be used a stress biomarker.^13–16

Copeptin and Diabetes Insipidus

Diabetes insipidus (DI) is a disorder characterised by polyuria and polydipsia. There are three types of DI: central DI where there is a deficiency in the secretion of vasopressin; nephrogenic DI where there is normal vasopressin secretion but renal resistance to its water retaining effect; and gestational DI due to the breakdown of endogenous vasopressin by placental vasopressinase. Determining the type of DI and distinguishing this condition from the differential diagnosis of primary polydipsia is crucial, as management of these conditions is different and an inappropriate management strategy may cause harm. Several tests have been proposed to evaluate polyuria and polydipsia.

Nephrogenic DI cases were excluded from this review but it is known that copeptin levels in nephrogenic DI are markedly elevated.¹⁷ Nephrogenic DI is much less common than central DI and studies on prevalence are limited but it is estimated that for X-linked nephrogenic diabetes, the prevalence is approximately 1 in 250,000 individuals.¹⁸ An underlying cause for nephrogenic DI is often clinically apparent, such as a family history of genetic nephrogenic DI, such as an X-linked AVP2 receptor mutation or rarer aquaporin-2 channel mutations in children.¹⁹ In adults, prolonged lithium administration, hypokalaemia, hypercalcaemia or the post-obstructive phenomenon after clearing a renal outflow tract blockage is generally readily apparent. Desmopressin is not used in treatment of nephrogenic DI.

Indirect Water Deprivation Test

The indirect water deprivation test measures the maximal urine concentration during a prolonged period of abstinence from oral liquids and serial measurements of urine concentration following administration of desmopressin – a synthetic form of arginine vasopressin which provides an indirect marker of vasopressin activity.²⁰ This has been the diagnostic standard for over 50 years. Urine osmolality is measured hourly while fluids are withheld and once the urine osmolality becomes constant, blood is drawn to evaluate plasma osmolality and desmopressin or vasopressin is administered. Urinary osmolality is then re-assessed 60 min after. In some instances, this test may take up to 16–18 h to complete due to limited renal concentrating ability related to high urine output and reduced renal medullary tonicity limiting the response to circulating vasopressin.²¹ Complete central DI is diagnosed in patients who have a maximal urine osmolality of less than 300 mOsm/kg and an increase in urine osmolality of more than 50% after administration of desmopressin. In partial central DI, the maximal urine osmolality is between 300 and 800 mOsm/kg and the increase in urine osmolality is between 9% and 50% after desmopressin. In primary polydipsia, the maximal urine osmolality is between 300 and 800 mOsm/kg and the increase in urine osmolality is less than 9% after desmopressin.²⁰ The direct measurement of vasopressin following osmotic stimulation improved the diagnostic accuracy of the water deprivation test for central vs primary polydipsia²² but the lack of a reliable serum vasopressin assay severely limited its use.

Copeptin as a Surrogate Marker of Vasopressin

The measurement of copeptin in addition to the indirect water deprivation test was investigated.²³ Patients with complete central DI had lower concentrations of plasma copeptin at the end of the study compared to patients with primary polydipsia as determined using the ‘gold standard’ of clinical assessment, review of investigations and evaluation of the response to a trial of desmopressin. This was the first study to investigate the use of plasma copeptin to diagnose patients presenting with polyuria and polydipsia and found that the ratio of the change in copeptin concentration for the duration of the test and serum sodium concentration at the end of the test, was accurate in distinguishing between partial central DI and primary polydipsia (sensitivity 86% and specificity 100%) using a specific index cut-off.²³ However, this approach was equally as time-consuming as the indirect water deprivation test so a follow up study²⁴ evaluated the accuracy of a ‘stimulated’ copeptin concentration. Plasma copeptin concentration was measured after plasma sodium concentration increased to greater than 147 mmol/L either from water deprivation or in cases where water deprivation did not raise sodium concentrations to this threshold after 5 hours, a 3% saline infusion (hypertonic) at 0.1 mL/kg/min was administered until plasma sodium exceeded 147 mmol/L. Plasma copeptin concentrations measured when plasma sodium concentrations exceeded 147 mmol/L were greater in primary polydipsia compared with partial central DI with a sensitivity and specificity of 94% using a cut-off of >4.9 pmol/L. This study provided the basis to investigate diagnosing DI without having a patient undergo a water deprivation test. A further study was done to evaluate the accuracy of stimulating copeptin with hypertonic saline alone compared with the indirect water deprivation test.²⁰ An initial 250 mL bolus of 3% saline was administered and then continued at a rate of 0.15 mL per kilogram per minute. Blood samples were taken every 30 min to measure sodium, osmolality, urea and glucose until a target sodium level of 150 mmol/L was obtained. Thereafter, a final blood sample was taken and plasma copeptin concentration measured. Patients were then given water orally within 30 min followed by a 500 mL infusion of 5% glucose within 40–60 min after the patient had received water. Plasma sodium levels were measured again 1 hour after the start of the glucose infusion to ensure it is within the normal range before the patient was discharged. Similar to the previous trial, a plasma copeptin concentration cut-off of greater than 4.9 pmol/L suggested the diagnosis of primary polydipsia rather than central DI. Compared with the reference ‘gold standard’ diagnosis by the clinical assessment of two independent endocrinologists who reviewed results of the water deprivation test, laboratory data, imaging and therapeutic response at the three month follow up, stimulated copeptin concentration measurement had a superior diagnostic accuracy compared with the water deprivation test (95.2% vs 73.3%) with a sensitivity of 82.6% and specificity of 100% in distinguishing primary polydipsia from partial central DI.

Another method of stimulating copeptin included the use of arginine which compared a smaller cohort of 52 patients.²⁵ Copeptin secretion was stimulated by 0.5 g/kg of intravenous L-arginine hydrochloride over 30 min and copeptin concentrations were measured at frequent intervals following the infusion for 120 min. Using a cut-off of 3.8 pmol/mmol at 60 min, diagnostic accuracy was reported to be 93% using pooled data from two cohorts.

Comparison of the Indirect Water Deprivation Test and Stimulated Copeptin Tests

The stimulated copeptin tests have some advantages over the indirect water deprivation test and the same copeptin assays are used in Australia. Early studies show that the stimulated copeptin tests, particularly the hypertonic saline stimulation test, result in better diagnostic accuracy compared with the traditional water deprivation test. Hypertonic saline reliably invokes hyperosmolality which is more physiologically relevant to vasopressin function under day-to-day ambient conditions than the hypovolaemia invoked by water deprivation. In addition, the water deprivation test can be time consuming compared with the stimulated copeptin tests. Compared with the indirect water deprivation test, the copeptin stimulation test was preferred by 62% of the study participants and adverse effects were similar between both tests.²⁶

Advantages of arginine stimulated copeptin²⁵ are that it is better tolerated, of a shorter duration, has similar diagnostic accuracy to the hypertonic saline stimulation test and could potentially be used in children, for whom use of hypertonic saline for stimulation is contraindicated. There are no head-to-head comparison studies of hypertonic saline and arginine tests to date. The Table summarises the advantages and disadvantages of the three tests.

Table.

Summary of the advantages and disadvantages of diagnostic tests for Diabetes Insipidus.

	Advantages	Disadvantages
Indirect water deprivation test	Current gold standard Most extensively tested Lower diagnostic accuracy	Time consuming Not well tolerated
Hypertonic saline stimulated copeptin test	High diagnostic accuracy Better tolerated than water deprivation test	Not suitable for children
Arginine stimulated copeptin test	High diagnostic accuracy Best tolerated Shortest duration	Small trial sample size

The distinction between primary polydipsia and central DI is crucial. Central DI is treated with hormone replacement using a vasopressin analogue DDAVP that is modified to reduce the oxytocic and vasoconstrictive properties of vasopressin and increase its circulating half-life.²⁷ DDAVP was FDA approved in 1978 and replaced earlier use of biologically sourced pituitary extract such as pitressin. If used in a patient with primary polydipsia such as habitual drinking, severe hyponatraemia may ensue with a risk of cerebral oedema and death.

A strong association between low copeptin levels and post-operative DI following pituitary surgery has also been identified.²⁸ Central DI occurs in at least 20% of pituitary surgery cases, but further studies are required to evaluate the potential role of copeptin for early goal-directed management. At present, postoperative DI is managed on clinical grounds, taking into account urine output, urine osmolarity or specific gravity at the bedside and, on conscious patients, an assessment of thirst. It is often transient and desmopressin is given on the basis of these clinical parameters.

Copeptin and Syndrome of Inappropriate Secretion of Anti-Diuretic Hormone

Hyponatraemia is common in hospitalised patients and frequently a result of a condition known as syndrome of inappropriate secretion of anti-diuretic hormone (SIADH). In response to stress, particularly nausea or pain, vasopressin is secreted into the general and hypophyseal portal circulations resulting in a combination of water retention and secondary sodium loss. Differentiating between SIADH and other causes of hyponatraemia can be challenging as concurrent disease or treatment can complicate the diagnostic work up. Therefore, the use of copeptin concentrations as a diagnostic aid has been explored.

In a prospective, multicentre, observational study of 298 patients with hyponatraemia and hypo-osmolarity, low plasma copeptin (<3.9 pmol/L) were predictive of primary polydipsia with high specificity (91%) and high plasma copeptin (>84 pmol/L) were predictive of hypovolaemic hyponatraemia with high specificity (90%).²⁹ Sensitivity for both these associations was limited and apart from these specific conditions, copeptin had limited utility in differentiating between SIADH and other causes of hyponatraemia. The diagnostic utility of copeptin may be improved when combined with other parameters such as the urinary sodium concentration (U-Na). In a prospective, observational study of 106 patients, the copeptin to U-Na ratio was able to accurately differentiate between volume depleted and normovolaemic (e.g. SIADH) causes of hyponatraemia but unable to distinguish diuretic induced hyponatraemia.³⁰ Given there are often multiple factors contributing to SIADH, the use of copeptin to subclassify SIADH and tailor management approaches is an interesting concept that needs to be evaluated further.³¹

Copeptin and Critical Illness

In the critical care setting, a prospective, observational study of 218 patients showed that high copeptin concentrations on admission to ICU was a predictor of short and long-term mortality.³² Inflammatory cytokines, key mediators of the stress response, such as IL-1, TNF-α stimulate vasopressin secretion and plasma copeptin is elevated in sepsis compared to patients with infections without systemic inflammation.³³ Further supporting this relationship is a prospective, observational study of 50 critically ill patients that found an association between elevated plasma copeptin levels and advanced vasodilatory shock due to sepsis or systemic inflammatory response syndrome.³⁴ These studies raise the possible use of copeptin for risk stratification of inpatients to determine who may need higher acuity care but further studies are needed to evaluate the effectiveness of this approach.

Copeptin and Diabetes Mellitus

A cross-sectional population study of 4742 patients found new onset diabetes mellitus to be associated with increasing plasma copeptin concentrations. Patients who subsequently developed diabetes mellitus over the 12.6 year follow up period had a 28% higher mean copeptin level.³⁵ Women with polycystic ovary syndrome were found to have a 31% higher mean copeptin levels compared with healthy controls.³⁶ The elevation in copeptin may be due to the low-grade chronic inflammation and elevated circulating inflammatory cytokines in metabolic syndrome.

Plasma copeptin levels have also been positively associated with major cardiovascular outcomes in type 2 diabetes mellitus³⁷ as well as adverse renal outcomes. In a cohort study of 1328 patients, baseline copeptin levels were positively associated with an elevation in urinary albumin-creatinine ratio.³⁸ Similarly, a higher baseline copeptin concentration was associated with a lower estimated glomerular filtration rate.³⁸ Another retrospective study showed a significant association between baseline copeptin and progression to stage 3 chronic kidney disease in newly diagnosed diabetes.³⁹ Whether more aggressive blood pressure, albuminuria or glycaemic targets could prevent progression to chronic kidney disease in patients with higher copeptin levels at diagnosis is an important question that needs prospective trials to evaluate. Therefore, there is emerging evidence that copeptin may have a role in predicting which patients may be predisposed to complications of diabetes mellitus.

Copeptin and Stroke

The usefulness of copeptin to differentiate between ischaemic stroke, transient ischaemic attack and stroke “mimics” such as delirium, complex migraine, epilepsy or vestibular neuronitis was explored in a pilot study of 45 adults.⁴⁰ Median plasma copeptin concentration within 4.5 h of the onset of symptoms was approximately twice as high in patients with ischaemic stroke compared to transient ischaemic attacks. Using a plasma copeptin concentration cut-off of >16 pmol/L resulted in a sensitivity of 80% and specificity of 44% in confirming ischaemic stroke. Seven patients in this cohort had a diagnosis in the stroke mimic category which had a large interquartile range (7.57–255 pmol/L) and as such, the diagnostic utility of copeptin in distinguishing stroke from stroke mimics could not be determined from this study. Copeptin, however, may be useful in differentiating ischaemic stroke from transient ischaemic attacks particularly if combined with other risk stratification scores such as the ABCD2 score.⁴¹ A higher plasma copeptin concentration also predicts stroke severity on admission, mortality and stroke recurrence.^42,43 Poorer outcome among stroke patients with higher plasma copeptin may imply a role of vasopressin in brain oedema and neuronal injury.⁴³ While the precise mechanism is not fully understood, blocking vasopressin receptors in mice models or using vasopressin deficient mice appears to attenuate brain oedema after ischaemia and trauma.^44–46 Therefore, copeptin concentration has the potential to be used in stroke risk stratification but as it is elevated in a wide range of conditions, it will likely need to be used in combination with other parameters.

Copeptin and Cardiovascular Disease

Most studies have shown that copeptin is positively associated with blood pressure. A suggested mechanism for this relationship is that activation of the renin-angiotensin-aldosterone system (RAAS) stimulates release of vasopressin. This mechanism may also be ‘bi-directional’ as vasoconstriction and tubular retention of sodium from vasopressin may contribute to blood pressure elevation.⁴⁷

Copeptin in combination with troponin measurements may increase the detection rate of acute coronary syndromes at admission⁴⁸ and enable more accurate exclusion of acute myocardial infarction.^49–51 It has also been shown that elevations in copeptin occur even before CK-MB and troponin T levels have risen.⁵² There has been suggestions that copeptin may have a role in rapid diagnosis, triaging of patients presenting with symptoms suggestive of acute coronary syndromes and determining whether invasive management is indicated.⁵³ It has also been suggested that copeptin may help in the prediction of major adverse cardiovascular events in patients with symptomatic coronary artery disease.⁵⁴ However, non-specific stressors may limit the utility of copeptin in differentiating chest pain aetiologies. For example, although acute aortic syndromes have been associated with an elevated plasma copeptin concentration, this is similarly seen in important differential diagnoses such as acute coronary syndromes and pneumonia.⁵⁵ While the specificity of copeptin concentrations may be insufficient to accurately ‘rule in’ acute coronary syndromes, it may be able to more effectively ‘rule out’ acute coronary syndromes when used in combination with troponin concentration measurements. Combined copeptin and troponin concentration measurements compared with measurement of troponin concentration alone has a 98% negative predictive value (vs 96%) and 92% sensitivity (vs 81%) but lower specificity.⁵¹

Copeptin has also been investigated for its role in prognosticating heart failure. Copeptin is a strong biomarker for mortality and morbidity in patients with heart failure after acute myocardial infarction and the predictive value of copeptin is even stronger than BNP or NT-proBNP.⁵⁶ A possible factor for copeptin’s potential utility is that unlike BNP, copeptin does not vary with age.

Copeptin and Respiratory Disease

Pulmonary disease has long been associated with hyponatraemia and elevated vasopressin of central origin or ectopic production from neuroendocrine tumours, such as small cell lung cancer. Elevated plasma copeptin on admission have been associated with increased severity and mortality of community acquired pneumonia.^57–62 Multiple studies have also shown that elevated copeptin levels has been associated with increased severity of ventilator-associated pneumonia and is an independent predictor of mortality.^63,64 Plasma copeptin concentrations have been incorporated into a risk index that predicts mortality in chronic obstructive pulmonary disease.⁶⁵ In a small study with 28 paediatric cystic fibrosis patients, higher plasma copeptin concentrations was associated with worsening symptoms of severity and more significant radiologic changes during pulmonary exacerbations of cystic fibrosis.⁶⁶ Raised copeptin in severe respiratory disease may be due to inflammatory cytokines, the known effect of hypoxia and hypercapnia on vasopressin secretion, or other lung-central vasopressin release pathways.⁶⁷

The potential use of copeptin as a prognostic or diagnostic tool in various lung diseases requires further investigation.

Copeptin and Kidney Disease

Copeptin has a negative correlation with glomerular filtration rate (GFR) and a positive correlation with albuminuria.⁴⁷ The exact mechanisms are not known but there are two hypotheses. Firstly, given copeptin is cleared by renal excretion, a decrease in renal function will result with an increase in copeptin levels. Secondly, as renal function declines, there may be a loss of the ability to concentrate water, disruption of water homeostasis and activation of the renin-angiotensin-aldosterone system. Arguments against these hypotheses include the observation that copeptin levels increase before estimated GFR decreases and a study that showed no change in copeptin levels following a reduction in GFR from the donation of a kidney in healthy donors.⁶⁸ Therefore, it is likely that GFR alone is not the only determinant of copeptin levels.

Cross-sectional analysis has shown a correlation between copeptin and disease severity in autosomal-dominant polycystic kidney disease (ADPKD).⁶⁹ The vasopressin antagonist Tolvaptan has been shown to have a renoprotective effect in ADPKD and further studies are needed to determine if copeptin may be a marker of treatment efficacy.⁷⁰

Other Roles for Copeptin

Copeptin has been investigated in the evaluation of heat-related illnesses. One study found greater elevations in plasma copeptin levels in volunteers who had undergone heat-related stress⁷¹ further supporting the role of copeptin as a stress biomarker.

Conclusion

There has been emerging interest in the role of copeptin as a surrogate for vasopressin for numerous conditions. Hypertonic saline stimulated copeptin concentrations appear to provide improved diagnostic accuracy over conventional water deprivation studies in the largest study in the field. Copeptin is a potential biomarker for increased stress and inflammation. Given the widespread range of conditions that cause an elevation in copeptin, there may be limitations in its use as a diagnostic tool, however, when combined with other clinical parameters, the diagnostic accuracy may be improved. Copeptin is a prognostic marker in various diseases and while there are only early observational data for the majority of conditions, evidence for the use of copeptin in diagnosis of central DI and diagnosis of acute coronary syndromes is accumulating and has the potential to make changes to clinical practice. Further studies are required to determine whether management plans using copeptin for risk stratification or goal-directed therapy for other conditions could improve patient outcomes.