The use of natriuretic peptide testing has dramatically risen recently, in recognition of the value of these blood tests for assisting in the evaluation of patients with suspected heart failure (HF). B-type natriuretic peptide (BNP) is formed as a consequence of production of a 108-amino-acid precursor peptide that is synthesised within the cardiomyocyte; this 'pro-BNP108' is subsequently cleaved at the time of release of the peptide by the cardiomyocyte into two fragments: the 32-amino-acid BNP and the 76-amino-acid amino-terminal portion (NT-proBNP). BNP has numerous biologic effects in the body, including induction of diuresis and natriuresis, vasodilation and downregulation of the renin-angiotensin-aldosterone system. These effects bespeak of the potential usefulness of measurement of BNP or NT-proBNP in disease states such as HF.
The first US Food and Drug Administration (FDA)- approved BNP assay became available in 2001. Subsequently, automated assays for NT-proBNP received approval in 2002 and, since then, testing for the natriuretic peptides has been shown to be exceptionally useful for numerous clinical applications, including the evaluation of the patient complaining of dyspnea, as elevated concentrations of these markers have been shown to be present in patients with acute HF, a diagnosis that is typically difficult to secure using standard clinical methods.
Since the earliest experiences with BNP and NT-proBNP, much has been learned about the utility of these tests for evaluation of patients with suspected or proven HF. This review will provide a brief comparison of the methods for measuring BNP and NT-proBNP, and then will focus on the recent lessons learned, in particular regarding the great utility of NT-proBNP for evaluating and managing the patient with suspected or proven HF.
Comparing BNP with NT-proBNP
Analytical Differences
As would be expected, the analytical methods for testing both BNP and NT-proBNP differ considerably, and as such it is not surprising that considerable differences between the two markers exist with respect to optimal methods for their testing. It has been shown that recoverable levels of BNP fall rapidly after phlebotomy,1 which likely reflects on-going activity of neutral endopeptidases in the blood sample or activation of the kallikrein system in the tube within which the blood sample is collected.2 Indeed, a recent study using exquisitely accurate mass spectral techniques actually demonstrated that among HF patients with extremely high concentrations of 'BNP32' (as assessed by a point-of-care immunoassay for the marker), no detectable intact BNP actually existed in the samples.3
This disturbing finding raises the concern that what is actually measured by BNP methods is a mixture of varying chain lengths of BNP, as well as potentially other cross-reactive species from the natriuretic peptide family of markers. In contrast, NT-proBNP is remarkably stable after release,1 and the methods for its measurement are highly precise - another major problem for several methods of BNP measurement.
Clinical Differences
In general, the clinical information that is gained by measurement of either BNP or NT-proBNP is largely similar; results of head-to-head comparisons of the markers demonstrate similar utilities in unselected populations of patients with suspected HF.4-6 Some noteworthy differences between BNP and NT-proBNP do exist, however.
In head-to-head comparisons, while both BNP and NT-proBNP had similar sensitivity and specificity for evaluation of symptomatic patients, Mueller and colleagues demonstrated that NT-proBNP had superior sensitivity to BNP for the detection of those patients with asymptomatic left ventricular (LV) dysfunction.5 This has serious ramifications, as the detection of patients with asymptomatic or early forms of HF has recently been emphasised by the American Heart Association (AHA).7 Furthermore, recent data suggest that up to 21% of symptomatic patients with established chronic HF may demonstrate BNP values below 100pg/ml,8 currently the only cut-off identified for use for BNP (which was based on data from acutely dyspneic patients in the emergency department (ED) setting). Indeed, a source of concern is that no optimal cut-off for use of BNP in office-based evaluation has been established for the identification of asymptomatic structural heart disease or longitudinal evaluation of patients with established HF. For NT-proBNP, clinical studies demonstrate significant utility of a cut-off of 125pg/ml for patients <75 years of age and 450pg/ml for patients >75 years of age for the outpatient evaluation of patients with suspected HF (see Table 1).
Another important category to consider is that of 'non-systolic' HF or 'diastolic' HF; these patients account for up to half of all patients presenting with acute HF and have a high rate of morbidity and mortality, including recurrent hospital presentations for destabilised HF. Thus, natriuretic peptide testing would be expected to play an important role in the evaluation and management of such patients.
Both BNP and NT-proBNP are significantly lower in patients with 'non-systolic' HF,9,10 likely reflective of the influence of ejection fraction on the concentrations of both markers. Importantly, in comparative studies, O'Donoghue and colleagues10 demonstrated that BNP was falsely negative in 20% of subjects with 'non-systolic' HF, compared with only 9% for NT-proBNP, suggesting superiority of NT-proBNP in this category (see Table 2).
NT-proBNP in Acute Patient Evaluation
Lessons learned from recent studies of NT-proBNP for acute patient evaluation have demonstrated numerous utilities for NT-proBNP testing in the ED setting, including the evaluation of dyspnoea, applications in diagnoses other than acute HF, such as evaluation of acute coronary syndrome (ACS) and pulmonary embolism (PE) and triage and management of acute HF.
With respect to evaluation of acute dyspnoea, several studies now demonstrate the value of NT-proBNP for correctly identifying or excluding acute HF. The most definitive of analyses were the ProBNP Investigation of Dyspnea in the Emergency Department (PRIDE)11 and International Collaboration of NT-proBNP (ICON)12 studies. The PRIDE study was a prospective blinded trial of 600 subjects presenting with complaints of dyspnoea; the study demonstrated that NT-proBNP testing was highly accurate for identifying or excluding acute HF in these subjects, and further showed that NT-proBNP testing was superior to the clinical assessment of the managing physician at the time of evaluation for identification of HF. The results of PRIDE do argue that the best method for evaluation of patients using natriuretic peptide testing was to combine the results of NT-proBNP with clinical judgement, a finding that echoes that of the Breathing Not Properly (BNP) Multinational Study of BNP.13
An important topic that is frequently discussed is the optimal method for applying NT-proBNP (and, for that matter, BNP) testing for the evaluation of the dyspneic patient. Both NT-proBNP and BNP have been approved for the exclusion of HF, given their high negative predictive value (NPV); however, physicians also wish to apply these assays for correctly identifying acute HF, which requires a cut-off with high positive predictive value (PPV). It is well-recognized that several factors may influence the PPV of both BNP and NT-proBNP, most notably age.
An important observation from PRIDE and ICON is that a single cut-off of 300pg/ml was excellent for its NPV-based exclusion of acute HF in dyspnoeic patients. For identifying acute HF, a single cut-off delivered similar PPV to that for BNP, but application of age stratification for optimally 'ruling in' acute HF was superior on many levels: it preserved sensitivity and specificity compared with a single cut-off and increased PPV to nearly 90%, a significant improvement over a single-cut-off strategy, which further reduced the 'grey zone' for NT-proBNP testing (i.e. a result below the cut-offs for excluding and confirming the diagnosis of HF) (see Table 1).11,12 Furthermore, age stratification obviates the need to further adjust optimal NT-proBNP cut-offs for renal function, as has been suggested for BNP. Indeed, for evaluation of the patient with renal disease, in the PRIDE study, NT-proBNP was highly sensitive and specific for the diagnosis of acute HF across all ranges of renal function and delivered powerful prognostic data for these subjects.14
Among patients presenting to the ED with ACS or PE (both diagnoses frequently accompanied by dyspnea), NT-proBNP testing has been shown to be of value for risk stratification; among patients with ACS, NT-proBNP was recently shown to be the strongest predictor of mortality following presentation - a stronger predictor than serum troponin testing or renal function15 (see Figure 1). The use of NT-proBNP plus troponin testing may further add clarity as to the optimal patient in whom early efforts towards revascularisation may be applied,16 and may be most useful when tested serially over the first 24 hours following presentation, in order to identify graded levels of ACS risk.17 With respect to utility in PE, measurement of NT-proBNP has been demonstrated to be useful for the prediction of a complicated course following presentation,18 presumably on the basis of identifying those patients with right ventricular (RV) dysfunction secondary to massive embolism; indeed, recent data suggest that NT-proBNP may be useful for identifying those most likely to have right heart dysfunction,19 and thus may be useful for determining optimal triage to more timely or delayed echocardiographic evaluation.
Patient Triage and Management
Any discussion of management on the basis of a biomarker concentration must not only consider the diagnostic utility of the marker, but also consider the usefulness of the marker for predicting adverse consequences of the disease state it identifies. In addition to the data regarding the diagnostic value of NT-proBNP,11,12 data strongly support the value of NT-proBNP testing for the prediction of hazard following presentation with dyspnea.20 Among patients in the PRIDE study with dyspnoea, an NT-proBNP concentration >986pg/ml was the strongest predictor of death to one year (see Figure 2); this risk was independent of a diagnosis of acute HF, which demonstrates the usefulness of the marker for risk stratification in diagnoses other than HF (such as ACS and PE). Given the usefulness of NT-proBNP testing with respect to diagnostic and prognostic evaluation, algorithms for use of NT-proBNP have now been published and validated.21 These algorithms incorporate clinical and biochemical testing in a logical manner to optimise the PPV of NT-proBNP, while reminding the clinician of other factors to consider with respect to correct interpretation of the results of testing for NT-proBNP.
A common question is whether serial testing for NT-proBNP is necessary once the diagnosis of acute HF has been secured; it would seem that, at minimum, a baseline and pre-discharge assessment of NT-proBNP may be superior to a baseline measurement. In work by Bettencourt and colleagues, those patients who did not demonstrate a fall in their NT-proBNP following treatment of HF were the most likely to suffer adverse consequences after hospitalisation for acute HF.22 Indeed, the addition of an objective measure such as NT-proBNP testing for pre-discharge 'clearance' is a welcome advance as improvement may be difficult to ascertain based on clinical factors; for those patients in whom a fall of NT-proBNP is not observed, it may be very well prudent to maintain the patient in the hospital and intensify their therapy.