Prognostic value of pro-adrenomedullin and copeptin in acute infective endocarditis

Background Infective endocarditis (IE) is a life-threatening disease whose prognosis is often difficult to predict based on clinical data. Biomarkers have been shown to favorably affect disease management in a number of cardiac disorders. Aims of this retrospective study were to assess the prognostic role of procalcitonin (PCT), pro-adrenomedullin (pro-ADM) and copeptin in IE and their relation with disease characteristics and the traditional biomarker C-reactive protein (CRP). Methods We studied 196 patients with definite IE. Clinical, laboratory and echocardiography parameters were analyzed, with a focus on co-morbidities. PCT, pro-ADM and copeptin were measured on stored plasma samples obtained on admission during the acute phase of the disease. Results Pro-ADM and copeptin were significantly higher in older patients and associated with prior chronic kidney disease. Pro-ADM was an independent predictor of hospital mortality (OR 3.29 [95%C.I. 1.04–11.5]; p = 0.042) whilst copeptin independently predicted 1-year mortality (OR 2.55 [95%C.I. 1.18–5.54]; p = 0.017). A high PCT value was strictly tied with S. aureus etiology (p = 0.001). CRP was the only biomarker associated with embolic events (p = 0.003). Conclusions Different biomarkers correlate with distinct IE outcomes. Pro-ADM and copeptin may signal a worse prognosis of IE on admission to the hospital and could be used to identify patients who need more aggressive treatment. CRP remains a low-cost marker of embolic risk. A high PCT value should suggest S. aureus etiology. Supplementary Information The online version contains supplementary material available at 10.1186/s12879-020-05655-7.


Background
Infective endocarditis (IE) is a life-threatening disease with a mortality essentially unmodified over the last decades, despite of the improvement in medical and surgical care [1,2]. The use of biomarkers able to predict the severity of this disease as well as the short-and mid-term prognosis could be very useful to tailor the therapeutic approach.
C-reactive protein (CRP) has long been the key marker of inflammatory, thrombotic and infectious diseases, although showing suboptimal specificity [3]. Recently, other molecules have been studied as biomarkers of bacterial infection and/or short term prognosis. Among them, procalcitonin (PCT), pro-adrenomedullin (pro-ADM) and copeptin seem to be the most promising based on existing evidence.
PCT, a calcitonin precursor synthetized by almost all human tissues, is considered a valid marker of the host response to acute bacterial infection, based on its synthesis and kinetics [4][5][6]. In addition, PCT often prompts initiation of antimicrobial therapy and guides antibiotic treatment de-escalation and/or discontinuation [7][8][9], also in critically ill patients [10]. PCT has been recently designated as novel biomarker for cardiologists, helping to discriminate the possible infectious origin of conditions such as dyspnea, congestion, valve disease, and acute coronary syndromes [11].
Pro-ADM is a stable precursor of ADM, a vasodilatator and natriuretic peptide, secreted by endothelial cells and vascular smooth muscle cells. It shows elevated levels in sepsis and is considered a marker of mortality in septic and nonseptic shock [12][13][14][15].
Copeptin, a precursor of pre-pro-vasopressin, is an established biomarker in different conditions of cardiovascular injury, including myocardial infarction, heart failure and stroke [16][17][18][19]. More recently, copeptin levels have been studied in sepsis and septic shock in children and adults [20,21]. Higher copeptin levels on admission to the hospital were present in non survivors and, consequently, considered as a negative prognostic marker in adults with sepsis [21].
PCT, proADM and copeptin have different specificity and sensitivity in the course of infectious syndromes and, taken together, could improve the diagnostic and follow-up pathways of patients with IE. While the role of PCT in IE has been the subject of a few studies, and remains controversial [22][23][24], no data are available at present on the dynamics and possible utility of pro-ADM and copeptin in IE.
Accordingly, the aim of the present study was to assess whether PCT, proADM and copeptin have a prognostic value in IE and may predict IE outcome. Moreover, we studied the relation of these biomarkers with IE clinical features, microbial etiology, other inflammation/infection markers, i.e. CRP, as well as liver and kidney function.

Study design
This was a retrospective study conducted at the Unit of Infectious & Transplant Medicine, Monaldi Hospital, University of Campania "Luigi Vanvitelli", involving patients with a diagnosis of IE admitted between 2007 and 2019 and for whom a plasma sample obtained in the acute phase of the disease was available. IE diagnosis was done according to existing criteria over time (modified Duke criteria until 2014 and ESC criteria from 2015 on) [25,26]. All patients hospitalized at our Unit with diagnosis of IE undergo blood sampling. In the period 2007-2019 we observed in our Hospital, a regional referral center for IE, 525 IE cases. We included in this study all adult patients (n = 196) who were admitted to our Unit with a recent diagnosis of IE and without need of emergent surgery. They were referred from other Departments of our hospital (4 Cardiology units), other hospitals and after observation as outpatients. Most referred patients (n = 327) were excluded, although we had a sample available, because this sample was a later sample during the course of the disease. No patient with shock or septic shock was included, as these are routinely admitted to the Intensive Care Unit (ICU), not our Unit.
The study was approved by the Ethics Committee of the University of Campania "Luigi Vanvitelli" and AORN Ospedali dei Colli. All patients gave their written informed consent to blood sampling and the anonymous use of their clinical data.

Patients included
Detailed data of patients were available as part of a standardized protocol of IE evaluation in use at our Unit, which includes a baseline clinical evaluation, particularly clinical history, physical examination, body mass index (BMI), chest X-ray, abdominal ultrasound scan and laboratory analyses (including CRP, creatinine, urea, glycemia, blood count, INR). According to the protocol, a trans-thoracic echocardiogram (TTE) was performed in all patients within 72 h of admission, followed by a trans-esophageal echocardiogram (TEE) where needed. Detailed information about IE characteristics (on native, prosthetic or cardiac implantable electronic device [CIED]), endocardial vegetations (number, size and position) and isolated causative pathogens were also collected. Embolic events, defined as acute complications causing overt clinical manifestations [27], and their characteristics (location, extension, complications) were also recorded.
Chronic heart failure (CHF), chronic kidney disease (CKD), liver disease and diabetes mellitus were considered as the principal co-morbidities. The Charlson comorbidity index (CCI) was also calculated for each patient.
PCT, pro-ADM and copeptin were measured in included patients on plasma samples, as detailed below.
The obtained values of these biomarkers were analysed in relation with clinical (age, sex, embolism, involved valves, co-morbidities) and laboratory (CRP, white blood cells, creatinine, glycemia, alanine transferase (ALT), isolated microorganism) parameters. The estimated glomerular filtration rate (eGFR) was calculated by Modification of Diet in Renal Disease (MDRD) formula. In addition, PCT, pro-ADM and copeptin plasma levels were studied in relation to short term (at hospital discharge) and long term (at 1 year from the IE diagnosis) mortality.

Biochemical assays
Blood was collected in EDTA tubes (Greiner Bio-One, Kremsmünster, Austria), cooled at 4°C and centrifuged; plasma samples were stored at − 80°C at the time of IE diagnosis, for subsequent use.
Other laboratory parameters were obtained by routine methods used in our Hospital central laboratory, including C-reactive protein (cut-off for positivity 0.3 mg/dL).

Statistical analysis
Numerical data are presented as median with range, whilst categorical/nominal data as number and percentage. The Mann-Whitney U test was used to assess statistical significance of the differences between numerical groups of variables and the Fisher's exact test for nominal variables. Logistic regression analysis of independent predictors of hospital mortality and 1-year mortality was performed by block entering in the model all variables significantly associated with each of these outcomes on the univariate analysis. Correlation between numerical variables was assessed by Spearman's coefficient. The biomarkers pro-ADM, copeptin, PCT and CRP have been studied altogether for the first time in IE and no specific cut-off correlated to short and long term outcome of the disease was available. Using as positivity cut-offs the biomarker values indicated on the manufacturer instruction manual as reported in the methods section, most patients had positive values (pro ADM 185 vs 11, PCT 169 vs 27, copeptin 177 vs 19, CRP 187 vs 9). Accordingly, we evaluated each biomarker using as cutoff its median value observed in the overall study group.
To assess the predictive performance of biomarker levels on IE outcome, we calculated the area under the Receiver Operating Characteristic (ROC) curve, entering in-hospital mortality as the state variable. The significance level was set at 5% and all tests were 2-tailed. All analyses were performed using the statistical software for Windows SPSS 20 (SPSS, Inc., Chicago, Illinois, USA).

Results
One-hundred and ninety-six patients were included. Median age was 62.3 years and 71% were males. General characteristics of patients are shown in Table 1.
IE occurred more frequently on the aortic valve and on native valves, and Staphylococci and Streptococci were the most frequent causative pathogens isolated. Inhospital mortality rate was 12.3%, 1-year mortality rate was 30% and there were 7 patients (3.5%) lost to followup.
Median levels of CRP, pro-ADM, PCT and copeptin are shown in Table 1. Strong correlations among the different biomarkers were observed (see Additional Table 1). The strongest linear correlation was observed between pro-ADM and copeptin (r = 0.629; p < 0.001). All four biomarkers correlated with D-dimer levels and, except CRP, with both NT-pro-BNP and creatinine, suggesting a relation with cardiac dysfunction and a strong influence of a declining renal function on biomarker levels. No significant difference was observed in copeptin levels according to gender (12.1 pg/ml in males vs 15.0 pg/ml in females; p = 0.96). None of the study patients had any sign of diabetes insipidus or the syndrome of inappropriate antidiuretic hormone.

Biomarkers and IE mortality
Biomarker levels were evaluated in relation to major clinical features and hospital mortality of IE patients ( Table 2). At univariate analysis, pro-ADM and copeptin were significantly higher in older patients. In-hospital mortality was significantly associated with higher levels of pro-ADM, PCT and copeptin, while 1 year mortality was significantly associated only with higher levels of pro-ADM and copeptin (Table 2).
We subsequently performed a multivariate logistic regression analysis to identify variables independently associated to IE outcomes. As shown in Table 3, panel A, only pro-ADM was an independent predictor of in hospital mortality (OR 3.29 [95%C.I. 1.04-11.5]; p = 0.042) ( Table 3). In contrast, higher copeptin levels were independently associated to 1 year mortality (OR 2.55 [95%C.I. 1.18-5.54]; p = 0.017), while pro-ADM showed At ROC curve analysis, pro-ADM and copeptin appeared to best predict in-hospital mortality (areas under the ROC curves: 0.784 and 0.731, respectively) compared with PCT and CRP (Fig. 1).

Biomarkers and IE clinical features
Embolic events occurred in 77 patients (39.3%). CRP was the only biomarker associated with embolic events (p = 0.003) ( Table 2). Most patients (151, 77%) had an indication for cardiac surgery at completion of diagnostic assessment, but this outcome was not related to any of the studied biomarkers (Table 2). Of these 151 patients, 128 (84.7%) finally underwent surgery, but actual surgical treatment was not associated with any biomarker level ( Table 2).
Pro-ADM levels were higher in patients with left-sided IE compared to those with right-sided IE (p = 0.001; see Additional Table 2a), while no differences were observed for any biomarker in relation to the actual subtype of infection (native valve vs prosthetic valve vs CIED, see Additional Table 2b).
To evaluate association with IE etiology, we analyzed biomarker levels according to IE causative pathogen ( Fig. 2). Interestingly, higher levels of pro-ADM were observed in patients with infection due to Enterococci and Staphylococci (Fig. 2a), whereas Staphylococcus aureus cases had the highest PCT levels (Fig. 2b). S. aureus IE cases also had higher CRP levels (Fig. 2d). The enterococcal etiology was associated to a significantly higher copeptin level (Fig. 2c). Streptococcal IE was characterized by the lowest biomarker levels (Figs. 2a-d2). Table 4 summarizes the relationship between biomarkers and co-morbidities. Liver disease and diabetes mellitus did not influence biomarker plasma concentration (Table 4). High levels of pro-ADM were significantly influenced by prior CHF and CKD while high levels of copeptin were associated with CKD only. CRP was also higher in the presence of CHF (p = 0.05). As shown in Additional Fig. 1, the CCI showed a direct correlation with pro-ADM and copeptin, but not CRP or PCT.

Biomarkers and IE patient co-morbidities
We also assessed whether an IE-related kidney dysfunction influenced biomarker levels. As shown in Table  4, all biomarkers were significantly higher in patients with a baseline eGFR < 60 ml/min, except CRP.    Comorbidities were also associated with clinical outcomes (Additional Table 3). In particular, kidney disease and diabetes were associated with hospital mortality, whereas heart failure and kidney disease were related with 1 year mortality. Also, the CCI was significantly as-

Discussion
Biomarkers able to rate severity and predict outcome of IE could be useful in the management of the disease, possibly allowing to tailor the therapeutic approach. In this study, we evaluated the significance of PCT, proADM and copeptin levels in IE and their relation with a traditional biomarker such as CRP. Furthermore, we analysed their prognostic value to predict IE outcomes. Our data suggest that in the setting of IE biomarkers have a substantially distinct profile and signal different conditions.

Biomarkers and prognosis of IE
To the best of our knowledge, this is the first study evaluating pro-ADM and copeptin in IE. As previously observed in other infectious syndromes [10,13,18,19], pro-ADM and copeptin were strong and independent predictors of hospital and 1 year mortality, respectively, in IE. Pro-ADM levels showed a nearly 80% sensitivity for hospital death. In contrast, PCT did not predict IE mortality.

Role of PCT and CRP in IE
PCT and CRP were tied to other disease features, including a staphylococcal etiology for PCT and the occurrence of embolic events for CRP. Our data fill the knowledge gap on the role of PCT in IE, where controversial findings emerged in previous studies. PCT previously appeared a useful biomarker for the diagnosis of IE, and also showed a correlation with specific bacterial etiology and disease prognosis [22][23][24]. However, other studies failed to demonstrate superiority of PCT over CRP in predicting IE [23]. In our study, PCT showed a pattern similar, but not overlapping with CRP, but none had an independent predictive role for IE mortality. Only higher CRP concentrations confirmed to be associated with embolic events [27], a serious complication in IE.

Biomarkers and IE etiology
Our results on the relation between biomarkers and IE etiology deserve comment. Different IE etiology may be associated with diverse risk factors, disease course, treatment response and time of referral. It was therefore interesting to observe how copeptin and pro-ADM, strongly increased in older patients, were associated with enterococcal IE, that typically occurs with increasing incidence in the multi-morbid elderly [28]. In agreement with other studies [22][23][24], we found that PCT levels were highest in infections due to S. aureus or gram negative bacteria.

Biomarkers and co-morbidities
As many patients with IE have co-morbidities, it was interesting to evaluate their effect on/relation with biomarker levels. Pro-ADM and copeptin were higher in IE patients with a history of CKD and CHF, confirming the association of these biomarkers with organ failure [10,13,18,19]. Moreover, pro-ADM and copeptin correlated with both NT-pro-BNP and creatinine, suggesting a relation with acute, IE-associated cardiac dysfunction, and also a strong influence of renal dysfunction on biomarker levels.
As mentioned, older age translated into higher pro-ADM and copeptin, and this could influence their effect on mortality. However, these biomarkers predicted mortality (either hospital or 1 year) independent of age.

Study limitations
This was a single center, retrospective study and determination of the biomarkers pro-ADM, PCT and copeptin was performed on stored samples at a single time-point. The study sample was relatively small, although homogeneously managed according to existing guidelines.
Previous conflicting data suggested a key role for PCT in the assessment of antimicrobial therapy effectiveness in infectious syndromes other than IE [7][8][9]. Unfortunately, we could not analyze the on treatment dynamics of PCT to predict therapeutic response in this study.
Finally, we were unable to confirm our findings in a prospective validation cohort.

Conclusions
Pro-ADM and copeptin may signal a worse prognosis of IE on admission to the hospital and provide information different from usual inflammatory biomarkers, such as CRP and PCT. CRP remains a low-cost marker of embolic risk and a tool to monitor treatment response. A high PCT value should suggest S. aureus etiology. A higher pro-ADM and copeptin level may instead identify IE patients for whom a more aggressive therapeutic approach could be warranted.