© 2008 European Society of Cardiology
A clinician's experience of using the Cardiac Reader NT-proBNP point-of-care assay in a clinical setting
Department of Cardiology, Heart Center, University Hospital of Linköping SE-581 85 Linköping, Sweden
* Corresponding author. Tel.: +46 13 22 20 00. E-mail address: urban.alehagen{at}ihs.liu.se (U. Alehagen).
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Abstract |
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 Top Abstract 1. Introduction2. Methods3. Results4. Discussion5. ConclusionReferences |
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The evaluation of natriuretic peptides has become increasingly valuable in a clinical setting, where information is often needed promptly.
Objectives: To compare the usefulness of the recently released Roche Cardiac Reader® NT-proBNP assay against the Roche Elecsys® NT-proBNP laboratory system in a clinical setting.
Design and results: Blood samples from 440 patients, who were either admitted with acute coronary syndromes or worsening heart failure, or who were heart failure outpatients, were evaluated. The relation between the two assays was analysed and the diagnostic concordance calculated. A good correlation was found between the assays (r = 0.96, 95% CI: 0.94–0.97) with a diagnostic concordance of 93%. A separate analysis was performed in the range where most clinical decisions are made (60–3000ng/L), with a diagnostic concordance of 88%. The usefulness in a clinical setting where time is important was high.
Conclusion: The Roche Cardiac Reader® NT-proBNP assay has been evaluated in a clinical setting. The point-of-care method shows good results, although with a restricted analytical range, compared with the reference.
Key Words: Natriuretic peptides • Point-of-care • Heart failure
Received March 11, 2007; Revised November 16, 2007; Accepted January 9, 2008
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1. Introduction |
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TopAbstract1. Introduction2. Methods3. Results4. Discussion5. ConclusionReferences |
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The use of cardiac natriuretic peptides has become an increasing part of the clinical routine in handling patients with heart failure. There is extensive information in the literature about the relation between decreased cardiac systolic function and increased plasma concentration of B-type natriuretic peptides (BNP) and the N-terminal fragment of proBNP (NT-proBNP) [1]. Similarly, patients with an impaired diastolic function have increased plasma concentration of BNP and NT-proBNP (pseudonormal or restrictive filling pattern) [2]. The utility of BNP and NT-proBNP in treating patients with dyspnoea is well documented [3]. Moreover, there is very intriguing information concerning the ability to use peptide levels to guide heart failure treatment [4]. Recent data indicate that ischaemia "per se" could result in increased plasma concentration of BNP and NT-proBNP [5,6]. It has also been shown that a high NT-proBNP plasma concentration is highly predictive of 1-year mortality in patients with non-ST-elevation acute coronary syndrome [7].
Therefore, there is an increasing need to evaluate these natriuretic peptides in routine clinical practice. However, the use of point-of-care (POC) systems in general is not uncontroversial in light of the necessary quality assessments. This is well illustrated by a report which evaluated nine POC devices for monitoring anticoagulation, and in which substantial differences were found between the assays and the laboratory reference method [8].
The Cardiac Reader® system, a commercial assay for NT-proBNP that can be used as a POC system, has recently been developed by Roche Diagnostics. This POC system has been evaluated in a laboratory environment [9]; however, there is currently no data from a clinical setting. Furthermore, it is not certain that the POC system will give the same results in a clinical setting compared with the laboratory standard, as pointed out by Kost et al. [10]. Therefore, the usefulness of the reported method in a clinical setting is uncertain.
The aim of this study was therefore to evaluate the usefulness of Cardiac Reader® POC NT-proBNP assay in a routine clinical setting, and to compare it with the Elecsys NT-proBNP clinical laboratory assay.
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2. Methods |
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TopAbstract1. Introduction2. Methods3. Results4. Discussion5. ConclusionReferences |
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From November 2005 to February 2006, and from June 2007 to August 2007, patients with acute coronary syndromes admitted to the Coronary Care Unit (CCU), patients admitted to the cardiology ward because of documented heart failure that had deteriorated, and outpatients at the Department of Cardiology, Heart Center, Linköping University Hospital, Sweden, were consecutively included in the evaluation. Blood samples were drawn with the patients at rest in supine position. Samples of whole blood were analysed on a Cardiac Reader® situated in the CCU. Plasma samples with lithium heparin as anticoagulant were sent to the Laboratory of Clinical Chemistry at Linköping University Hospital for analysis using the Elecsys NT-proBNP assay.
The Ethics Committee of the University Hospital of Linköping approved the study protocol.
2.1. Assays
2.1.1. Laboratory system
The laboratory method used to measure NT-proBNP was an electrochemiluminiscence immunoassay utilizing two polyclonal antibodies directed against amino acids 1-21 and 39-50 (Elecsys 2010, Roche Diagnostics, Mannheim, Germany), first described by Karl et al. [11]. The analytical range was 5-35.000 ng/L according to the Roche package insert. The total interassay coefficient of variation (CV) was 4.8% at the level of 217 ng/L (n=70) and 2.1% at the level of 4261 ng/L obtained at our laboratory. All blood samples were analysed within 1 h after being drawn. The NT-proBNP assay using the Elecsys analytical platform has been extensively evaluated in the literature [12].
2.1.2. Point-of-care system
The POC system used was the Cardiac Reader® (Roche Diagnostics, Mannheim, Germany), utilising one monoclonal and one polyclonal antibody directed against amino acids 27-31 and 39-50. The monoclonal antibody was gold-labelled, and the polyclonal antibody was biotinylated, forming a sandwich structure. This consists of the gold-labelled antibody, the NT-proBNP molecule, and finally the biotinylated antibody. The described complex is attached to a streptavidin molecule, and all this is coupled to the base of the test strip. The colour intensity of the resulting sandwich complex is measured by an optical system in the Cardiac Reader®. The total analysis time is 12 min. This POC assay uses 150 
L of heparinised venous whole blood for the analysis. The stated analytical range was <60-3000 ng/L. Total CV was 12.8% at the level of 163 ng/L and 8.6% at the level of 1166 ng/L (n=13) during the study. All blood samples were analysed within 30 min after being drawn.
2.2. Statistics
Descriptive data are presented as percentages or means. In the case of continuous variables, analyses have been performed using Student's unpaired two-sided T-test, whereas for discrete variables, chi-square test was used. In the correlation analysis, Cardiac Reader NT-proBNP assay values <60 ng/L were replaced with 60 ng/L, and >3000 ng/L with 3000 ng/L. The same transformation was made for the Elecsys NT-proBNP assay.
In the analyses, regressions of the comparison of methods were performed using Deming, Passing and Bablok regressions [13], whereas the differences in methods are presented as bias plots according to Bland-Altman [14].
Data were analysed using commercially available statistical analysis software packages (Statistica v 7.1, Statsoft Inc, Tulsa, OK, USA; Analyse-it v 1.63, Analyse-it Software Ltd, Leeds, UK).
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3. Results |
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TopAbstract1. Introduction2. Methods3. Results4. Discussion5. ConclusionReferences |
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Four hundred and forty two patients agreed to participate in the study. The majority of the patients had been admitted to the CCU with acute coronary syndromes (287 out of 442). Data for two patients were excluded due to a mix-up of samples. Therefore, blood samples from 440 patients (males/females: 251/189; mean age: 65 years) were evaluated in the study. We did not exclude any blood samples due to "outliers", results from all samples are therefore presented. The basal characteristics of the study population are presented in Table 1a and 1b
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A scatter plot illustrating the relationship between the Cardiac Reader assay and Elecsys NT-proBNP assay is presented in Fig. 1. From the scatter plot, a good relation was found between the Cardiac Reader and the Elecsys assays in the range of evaluation (r=0.96, 95% CI: 0.94-0.97). There are some discrepancies in the values obtained between the two assays, especially in the upper range. However, the discrepancies are spread equally on both sides of the regression line, and the deviation of the regression line from the identity line is only slight. A statistical analysis of the plasma concentrations of the two assays dividing the analytical range for the Cardiac Reader into two parts (60-1000 ng/L and 1000-3000 ng/L) showed a correlation of 0.91 and 0.87, respectively.
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A Bland-Altman plot of the regression shows excellent agreement between the methods used (Fig. 2). It is possible to identify a couple of outliers, but we chose not to exclude them in the calculations.
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The Cardiac Reader NT-proBNP assay provides restricted information on plasma concentrations. Values below 60 ng/L are indicated as <60 ng/L, and values higher than 3000 ng/L as >3000 ng/L. An analysis of the diagnostic concordance was performed in the three analytical ranges (<60, 60-3000, and >3000 ng/L) in order to evaluate the impact of this restriction. The diagnostic concordance in this study setting was found to be 93%, as presented in Table 2.
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In addition, an analysis of sensitivity and specificity in the three evaluation ranges of the Cardiac Reader (<60, 60-3000, and >3000 ng/L) was performed. In the analytical range <60 ng/L, the sensitivity was 82% and the specificity was 99%, rendering a positive predictive value of 82% and a negative predictive value of 99%. In the analytical range 60-3000 ng/L, a sensitivity of 97% and a specificity of 83% were found, rendering a positive predictive value of 95% and a negative predictive value of 89%. Finally, in the analytical range >3000 ng/L, a sensitivity of 82% and a specificity of 99% were found, rendering a positive predictive value of 97% and a negative predictive value of 92%, when compared with the Elecsys assay used as a reference.
During the study, quality control was performed on a daily basis, using the internal quality control of the instrument, and on a weekly basis (Roche CARDIAC Control proBNP Level Low and High; Roche Diagnostics). The analysis showed a CV of 12.8% in the lower range and of 8.6% in the higher range.
Haemoglobin (range 66-161 g/L), haematocrit (range 0.20-0.77) and triglycerides (range 0.5-8.1 nmol/L) were also measured in all 440 blood samples. No significant interference could be found using correlation analysis.
Depending on the situation, different NT-proBNP cut-off values may be needed in the clinical decision process. We have previously shown this using the Elecsys NT-proBNP assay [15]. The upper limit for reference values for healthy elderly patients was found to be <540 ng/L, whereas based on cardiovascular mortality a steep increase in risk was noted if the plasma concentration was >1700 ng/L, used here as the decision limit [15]. This analytical range is covered by the Cardiac Reader system (60-3000 ng/L), as shown in Table 3). In this evaluation the diagnostic concordance was 88%. Different cut-off values could also be required if the method is used in an environment in which sensitivity is given priority, such as in the emergency room, compared with a heart failure clinic, where specificity might be prioritised instead.
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As a part of the analysis of a potential high concentration hook effect, all plasma concentrations >4000 ng/L (range 4001->35000 ng/L) obtained by the Elecsys system were checked against the Cardiac Reader. In all the samples, the Cardiac Reader displayed >3000 ng/L. Hence no high concentration hook effect could be found.
Moreover, an analysis of potential differences in plasma concentrations between the two assays when analysing males and females in the restricted range that Cardiac Reader NT-proBNP assay could evaluate did not show any significant differences between the two assays (Table 4).
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In 120 patients, duplicate samples of NT-proBNP were analysed on the Cardiac Reader system (Fig. 3). A Bland-Altman plot of the differences between the results is shown in Fig. 4. The analysis indicates good precision for the Cardiac Reader POC system. We also wanted to compare the POC system with the usual laboratory system in terms of costs. The cost comparison is shown in Table 5. The analysis shows that the POC system is cost-effective. However, we have not added the cost for the nurse inserting the blood sample into the Cardiac Reader, which takes about 2 min.
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4. Discussion |
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TopAbstract1. Introduction2. Methods3. Results4. Discussion5. ConclusionReferences |
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This evaluation demonstrates for the first time that the POC method of testing NT-proBNP is useful in a clinical setting in patients with ACS or HF. We have shown that the method is sufficiently robust, permitting health-care personnel on duty to perform the analyses without unacceptable quality impairments; and that the assay is cost-effective compared with the laboratory reference system for the analysis of NT-proBNP. However, the use of POC methods raises certain interesting issues that need to be discussed.
In the emergency room, the CCU and the heart failure outpatient clinic, where time is important, there is a need for fast, reliable information about the plasma concentration of natriuretic peptides. For patients with dyspnoea the positive consequences of having a POC system for BNP in the emergency room are well documented [3]. These reports show that the availability of this POC analysis influences the correct diagnosis of patients in a hospital facility, where time is an important dimension of quality of care. It is, therefore, reasonable to believe that the results can be extrapolated to NT-proBNP when analysed through a POC system.
In the CCU, the evaluation of NT-proBNP in patients with acute coronary syndromes might provide additional prognostic information that could help the clinician to decide on further therapeutic action [16]. This has also been shown by other authors [17].
In the clinical setting there is a considerable difference between obtaining test results in 12 min rather than waiting for a couple of hours, which is the reality for many clinicians even if the sample is marked as an emergency test. The clinical consequence of this might be to enable a decision on whether coronary angiography is appropriate or not in patients for whom the objective facts are not overwhelming. It is also important to have a useful instrument at hand that predicts the level of risk of cardiovascular death [18]. The practical consequence of using the POC system in our CCU, was that the cardiologists on duty felt that the information given by the POC system within the limited time was a valuable addition to the cardiac biomarkers used in routine practice, such as troponin-I, for determining whether a patient should be referred to angiography or not.
An important issue when measuring natriuretic peptides in routine clinical practice is that the same epitopes and the same reference values must be evaluated for each peptide both in the POC system and in the hospital laboratory. Otherwise there will be confusion in the interpretation of values obtained from the two systems as the patient is moved from the emergency room or CCU to the ordinary ward. For this reason, the different cut-off values for the different assays have to be incorporated into the interpretation of results [19]. Roche Diagnostics have produced a POC method that analyses the same peptide (NT-proBNP), uses the same epitopes in the analysis, and has the same reference values as the Elecsys 2010 multi-analysing system, which is used in many hospital laboratories. The results obtained from the POC system can therefore be directly compared with the results obtained from the Elecsys multi-analysing platform. In our study, we wanted to evaluate the usefulness of the Cardiac Reader NT-proBNP assay in comparison with the Elecsys platform in a realistic clinical setting, in which the Cardiac Reader was operated by the nurses on duty at the CCU, and not by analytical technicians, as is often the case in technical evaluations of laboratory techniques [9]. The nurses at the CCU who performed the analyses on the Cardiac Reader found it easy to learn and to handle without any obvious impairment of the results obtained.
In our evaluation, the Cardiac Reader correlated well with the Elecsys method used as a reference, for measuring NT-proBNP. The precision was higher in the reference system, but as illustrated in the Bland-Altman analysis, the divergences are acceptable for use in routine clinical practice. The regression analyses show good correlation between the methods used.
No gender related differences were found using the Cardiac Reader NT-proBNP assay. However, reports in the literature indicate increased plasma concentrations of NT-proBNP in healthy females compared with healthy males [20]. One possible explanation for the lack of gender differences in our study population might be that we were testing a diseased population in which the influence of disease may have overwhelmed any gender-specific differences.
Our results show that the POC system provides values in the range within which most blood samples fall. In routine clinical practice, it is certainly a disadvantage not to have reliable information about the entire plasma concentration range, as is provided by the laboratory system (5-34,000 ng/L). However, for the clinician involved in everyday clinical practice, patients with greatly increased NT-proBNP concentrations (>3000 ng/L) usually do not present as a diagnostic problem. Although, the clinician may feel more comfortable knowing the exact plasma concentration, the information given by the Cardiac Reader (i.e. >3000 ng/L) indicates a greatly increased risk of cardiovascular mortality [21].
For samples with plasma concentration <60 ng/L the fact that the exact numerical value is not known is unlikely to influence the clinical decision. The fact that the concentration is very low is usually sufficient. However, patients with a moderately increased peptide concentration are those that can present as diagnostic and therapeutic problems. In patients with acute coronary syndrome, an increased peptide concentration might strengthen the decision to perform angiography because patients are at risk. The Cardiac Reader provides information in this important plasma concentration range (60-3000 ng/L). In a recent publication, Januzzi et al. specified decision limits for patients with acute heart failure derived from a large pooled multicenter study [22]. The Cardiac Reader POC system is able to present all but one of the cut-off values used by Januzzi.
In discussing the advantages and disadvantages of a POC system, one argument is the wider CV values that are documented. For the Cardiac Reader NT-proBNP assay, the CVs were 12.8 and 8.6%, respectively, in the lower and higher control levels tested. These coefficients of variation are wider than for the laboratory system, but are not unacceptable for the analyses performed. Kupchak et al. reported the influence of various widths of CV values on the analysis of BNP when analysing the AUC in a ROC analysis. They used a dataset from a clinical setting, but with theoretically different coefficients of variation. The surprising result was that the width of CV values had little effect on the ROC analysis measuring the AUC for the method [23]. Therefore, when comparing different methods using a ROC analysis, a wider CV might be acceptable. Thus we found that the Cardiac Reader NT-proBNP POC system was useful for the clinician in certain areas in which limited time is a reality.
The interesting, and most important, issue is whether the POC method analyses the variable that the manufacturer claims it does. This aspect is discussed by Hawkridge et al. in a study of patients with high levels of BNP32 as indicated by the POC assay. When analysed by high precision mass spectrometry, they did not detect any BNP32 at all [24]. In the case of the present POC assay, no such studies have been conducted to our knowledge, but the same antibodies and the same epitopes are used in the laboratory reference method, which makes them easier to compare.
Cost aspects are also an important issue, as the use of cardiac natriuretic peptides in routine clinical practice is likely to increase in the future. Ongoing studies are currently evaluating the effect of treatment guided by cardiac natriuretic peptide measurements on mortality and hospitalisation for heart failure patients. If these studies show a positive effect on outcome, the resulting increased demand for rapid evaluation of plasma concentrations of cardiac natriuretic peptides is likely to increase the use of POC systems in heart failure outpatient clinics. Our basal cost analysis of the two systems supports the use of the NT-proBNP POC system.
4.1. Limitations
The present report should be regarded as a clinician's experience of using the POC system in a clinical setting. Hence, more extensive information on the imprecision of the system evaluated is lacking. In a recently published multicenter study, Zugck et al. performed an excellent evaluation of this new POC system which showed good analytical precision of the system and a lack of interference from some of the major substances that may influence the peptide concentration obtained [9].
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5. Conclusion |
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TopAbstract1. Introduction2. Methods3. Results4. Discussion5. ConclusionReferences |
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In conclusion, a comparison with the Elecsys NT-proBNP assay shows that the Cardiac Reader NT-proBNP assay may be used in a clinical setting with excellent results. The usefulness of the POC method is high in this setting. Our results suggest that the limitations of wider CV values, and the limited analytical range of the Cardiac Reader, do not interfere with the clinical assessment made, and might instead contribute important information due to the short processing time required.
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Acknowledgements |
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The study was supported by grants from The Linköping University Research Foundation CIRC, and was supported by a research grant from Roche Diagnostics Sweden. We would like to thank Mrs. Britt-Marie Ödlund, Roche Diagnostics Sweden, for advice.
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