Author + information
- Received March 4, 2002
- Accepted May 31, 2002
- Published online September 4, 2002.
- Margaret M Redfield, MD*,* (, )
- Richard J Rodeheffer, MD*,
- Steven J Jacobsen, MD, PhD†,
- Douglas W Mahoney, MS‡,
- Kent R Bailey, PhD‡ and
- John C Burnett Jr, MD*
- ↵*Reprint requests and correspondence:
Dr. Margaret M. Redfield, Guggenheim 9, Mayo Clinic, 200 First Street, Southwest, Rochester, Minnesota 55905, USA.
Objectives We wished to examine the effects of age and gender on plasma brain natriuretic peptide (BNP) concentration in a population-based study.
Background Measurement of BNP concentration is approved for use in the diagnosis of heart failure and may aid in the detection of left ventricular dysfunction. Although BNP is approved for clinical use, there are few data regarding the range of BNP observed in persons without cardiovascular disease or cardiac dysfunction. These data are essential for the interpretation of BNP.
Methods In 2,042 randomly selected residents of Olmsted County, Minnesota, >44 years old, BNP (Shionogi and Biosite assays), Doppler echocardiography, and medical record review were performed. A normal subset of subjects (n = 767) in sinus rhythm without cardiovascular, renal, or pulmonary disease or diabetes; on no cardiovascular medications; and with normal systolic, diastolic, and valvular function was identified.
Results Within the normal subset, the distribution of BNP differed by age, gender, and assay system. With both assays, BNP increased significantly with age and was significantly higher in women than men, leading to age-, gender-, and assay-specific reference ranges. Receiver operating characteristic analysis for the ability of BNP to detect an ejection fraction ≤40% was performed in each age/gender stratum in the entire cohort (n = 2,042) and confirmed that discriminatory values for BNP for detection of reduced ejection fraction were higher in women and older persons and were different between the two assays.
Conclusions Interpretation of BNP should include consideration of age-, gender-, and assay-specific partition values.
Brain natriuretic peptide (BNP) is a member of the family of genetically distinct natriuretic peptides synthesized and released by cardiomyocytes in response to increased transmural wall stress (1). Although this response is critical for cardiorenal regulation, the increase in plasma BNP concentration has diagnostic implications as well. Indeed, BNP may be useful for the detection of asymptomatic left ventricular (LV) dysfunction (2,3) and the diagnosis (4,5) and management (6) of congestive heart failure (CHF). Whereas most previous studies have utilized an immunoradiometric assay (Shionogi), recent Food and Drug Administration approval of a point-of-care assay (Biosite Triage) for use in the diagnosis of CHF has made the possibility of using BNP in clinical practice a reality.
The range of BNP observed in subjects without cardiovascular disease or cardiac dysfunction has not been well established. Although a few studies suggest that age and gender may influence circulating natriuretic peptide levels (7–9), the magnitude of these effects and their potential importance in the interpretation of BNP remains unclear. Therefore, we measured BNP with two commercially available assays in a population-based cohort. In subjects without cardiovascular disease or cardiac dysfunction (n = 767), we assessed the influence of the assay used, age, and gender on BNP. We evaluated the influence of clinical factors, renal function, and cardiac structural parameters on BNP, as age/gender-related changes in these factors could mediate associations between BNP and age/gender. In the entire cohort (n = 2,042), we determined whether the BNP that identified subjects having systolic dysfunction (ejection fraction [EF] ≤40%) with optimal sensitivity and specificity varied with age, gender, and assay.
This study was approved by the Mayo Institutional Review Board.
The characteristics of the Olmsted County, Minnesota, population and the unique resources for population-based epidemiologic research in Olmsted County have been previously described (10,11).
Population sampling, subject recruitment, and enrollment
A random sample of the population >44 years was invited to participate. A sampling fraction of 7% was applied within each of the gender and age (five-year) specific strata. Of the 4,203 subjects invited, 2,042 (47%) participated. Analysis of the medical records of 500 nonparticipants revealed no clinically significant differences between participants and nonparticipants. Subjects gave written consent and underwent echocardiography and phlebotomy.
Of the 2,042 participants, 1,020 had no history of cardiovascular, renal, or pulmonary disease and were on no cardiovascular medications. Of these, 75 were excluded because of an abnormal two-dimensional (2-D) echocardiogram, 64 were excluded by an inability to characterize diastolic function, and 114 had abnormal diastolic function. Thus, 767 subjects had normal EF (>50%), no wall motion abnormalities, normal diastolic function, no valve disease, and normal sinus rhythm; these make up the normal subgroup. The criteria used to establish the normal subgroup resulted in small numbers in the oldest (age 75 to 83) normal subgroup, where there were 18 women and two men. In this age group, the majority of excluded subjects were excluded because of the presence of known cardiovascular disease (72% of women excluded, 84% of men excluded) and a minority were excluded purely on the basis of diastolic dysfunction (10% of women excluded, 7% of men excluded).
Medical record review
All Olmsted County care providers have maintained a unified medical record, which is indexed by the Rochester Epidemiology Project. Each subject’s medical record was reviewed by trained nurse chart abstractors using established criteria for hypertension (12), myocardial infarction (13), and congestive heart failure (10). Clinical diagnoses of coronary artery disease, obstructive lung disease, renal failure, and diabetes were recorded. Medication questionnaires were completed by each subject.
All echocardiograms were performed by one of three registered diagnostic cardiac sonographers with the same echocardiographic instrument (HP-2500) according to protocol and interpreted by a single echocardiologist (M. M. R.). Two-dimensional and color Doppler imaging were performed to screen for valvular stenosis or regurgitation.
In each subject, measurement of EF by M-mode (modified Quinnones formula), quantitative 2-D (BiPlane Simpsons) and semiquantitative 2-D (visual estimate) methods was attempted as previously described (14–17). For analysis in the normal subgroup, subjects had normal EF (>40%) by all three methods. For analysis in the entire population, the visual estimation of EF was used.
Pulsed-wave Doppler examination of mitral (before and with Valsalva maneuver) and pulmonary venous inflow as well as Doppler tissue imaging of the mitral annulus was performed in each subject. Diastolic function was categorized as normal, impaired relaxation without evidence of increased filling pressures (“impaired relaxation”), impaired relaxation associated with moderate elevation of filling pressures (“pseudonormal filling”), and advanced reduction in compliance (“restrictive filling”), as previously described (18–21). Decreases in the ratio of the mitral early to late filling velocities (E/A ratio) indicate a reduction in early diastolic filling related to impairment of LV relaxation. We defined impaired relaxation as an E/A ratio ≤0.75, reflecting the average lower confidence interval for the E/A ratio in subjects aged >45 years in a previous normal cohort (22). Doppler indices used to discriminate pseudonormal or restrictive from normal are independent of age.
Left ventricular dimension and mass (M-mode) and left atrial volume (2-D) were indexed to body surface area.
Blood for BNP was collected in the fasting state in ethylenediaminetetraacetic acid-treated tubes and placed on ice. After centrifugation at 2,500 rpm and 3°C, the plasma was stored at −80°C.
Plasma BNP was determined by immunoradiometric assay (nonextracted) using antibody to human BNP (Shionogi Co. Ltd., Tokyo, Japan), as previously described (23). The intra-assay (within-day) and interassay (total) coefficients of variation were both 8% and were consistent across concentrations of BNP.
One-milliliter aliquots of frozen plasma were shipped to Biosite Diagnostics (San Diego, California), and the nonextracted samples were batch-analyzed by a fluorescence immunoassay (5,24). The within-day coefficient of variation (9.4% to 15.2%) and total coefficient of variation (10.1% to 16.2%) increased from low to high BNP.
Because the variability of BNP increased with its mean level, the natural log transformation was used in the regression analyses to satisfy modeling assumptions. Nomograms based on age and gender were established using the least-squares regression fit of log-transformed BNP with age and gender as predictor variables. From the fitted model, the 5th, 25th, 50th, 75th, and 95th percentiles were estimated and back-transformed to the natural scale. An interaction term with age and gender was also evaluated to determine if the association of age with BNP differed between the genders.
Unadjusted and adjusted associations of BNP were evaluated using Spearman’s correlation coefficient and linear least-squares regression, respectively. For adjusted associations, a model with age and gender was fit and, in turn, variables were added to see if there was significant residual association that could be explained by these characteristics.
The ability of BNP to detect an EF ≤40% was evaluated using receiver operating characteristic (ROC) curves within age/gender-specific strata. The optimal discriminatory value was identified as the BNP value that had a combined sensitivity/specificity at the smallest distance to 100%/100%, respectively (23).
The association of BNP with age and gender in the normal subgroup (n = 767)
The distribution of the normal subgroup by age/gender and corresponding BNP is shown in Table 1. Adjusting for age, BNP was higher in women than men and increased with age within each gender. Plasma BNP was 32% higher in women than men (confidence interval [CI] = 15% to 51%, p < 0.001) by Shionogi assay and 80% higher by Biosite assay (CI = 50% to 116%, p < 0.001). Nomograms for BNP as a function of age, gender, and assay are reported (Fig. 1). The age/gender-specific reference ranges (5th and 95th percentile) for each assay as derived from the regression fit are provided in Table 2. The association between gender and BNP remained similar across all age ranges for both assays (p > 0.90).
We sought to determine if age/gender-related changes in BNP were due to age- or gender-related changes in clinical factors, renal function, or cardiac structural parameters. Table 3displays the variables considered as potentially associated with BNP. Several of these parameters were associated with BNP but were also associated with age and/or gender, as indicated in Table 3. Only age, gender, and left atrial volume index were independently associated with BNP (Table 4). Within each gender and assay group, BNP increased according to left atrial volume index.
To further evaluate the association between gender and BNP, we determined whether BNP was different in the presence or absence of hormone replacement therapy (HRT). As measured by the Biosite assay, there was a relationship between HRT and BNP among women. Of the 388 women in the normal subgroup, 186 (48%) were taking HRT. Controlling for age, BNP was 21% higher in women on HRT than in women not on HRT (CI 6% to 40%, p < 0.005) (Fig. 2). Plasma BNP was 99% higher in women on HRT than in men (CI 75% to 125%, p < 0.005) and was 64% higher in women not on HRT than in men (CI 44% to 85%, p < 0.005). Similar trends were observed with the Shionogi assay, but the difference in BNP between women with or without HRT was not significant (data not shown).
Discriminatory values of BNP for the detection of systolic dysfunction in the entire cohort (n = 2,042)
Figure 3 displays the ROC curves for the detection of an EF ≤40% within each age stratum by gender and assay. Forty subjects (eight women) had an EF <40%. On each curve, both the BNP representing the upper (95th percentile) reference limit, as derived from the regression fit, and the BNP representing the optimal point on the ROC curve are indicated. The values for the BNP at the upper reference limit and at the optimal point on the ROC curve, and the corresponding sensitivity and specificity for the detection of an EF ≤40%, are shown in Table 5. The areas under the ROC curves were similar within each age/gender stratum for each assay. However, in general the optimal discriminatory value for BNP increases with age and is higher in women. Only eight female subjects had an EF ≤40% and this significantly limited the power to demonstrate age/gender differences in the optimal discriminatory value for BNP. The available data in women are included to demonstrate the effect on specificity of age/gender-specific partition values derived from the reference population.
In this population-based study, BNP increased with age and was higher in women than men in a subgroup of subjects without known cardiovascular disease or detectable structural heart disease. These associations were not explained by age- or gender-related changes in blood pressure, renal function, or cardiac structure. The association of female gender and BNP appears to be in part related to estrogen status, as BNP levels were higher in women using HRT. Although the absolute value of BNP varied between the two assays, the associations with age and gender were consistent between assays. In addition, the BNP with optimal sensitivity and specificity for the detection of systolic dysfunction in the entire population increased with age and was higher in women, underscoring the clinical relevance of the relationship among age, gender, and BNP.
Plasma BNP in the normal subgroup
The mechanisms whereby age influences BNP are unclear. Previous studies have reported that levels of natriuretic peptides are higher in the elderly, but did not exclude those with altered cardiac structure or function (7,9). These investigators suggested that age-related changes in diastolic dysfunction influence BNP. However, in the current study subjects with diastolic dysfunction and the diseases that cause it were excluded. Although age-related alterations in renal function could alter BNP, the effect of age was independent of renal function. The atria and ventricles are sites of BNP production, and age-related changes in cardiac size could influence BNP. However, the effect of age and gender on BNP was independent of atrial volume, LV dimension, and LV mass. These data would suggest that as yet undefined alterations in BNP production, secretion, or degradation occur with age, and further studies are needed to elucidate the mechanism(s) responsible for this effect. An alternative explanation is that BNP increases in response to age-related alterations in cardiac structure or function that are not detectable by current techniques. Such a mechanism remains speculative and not pertinent to current diagnostic use of BNP, as the prognostic implications of BNP in the absence of detectable structural heart disease have not been defined.
The effect of gender on BNP was also remarkable. This effect was independent of other factors, observed with both assay systems, and substantial. As gender-related differences in endothelin and angiotensin-converting enzyme activity have been reported to be associated with hormonal status (25,26), we investigated the effect of HRT status on BNP and found evidence of a relationship between HRT and BNP. Although preliminary, these data suggest that BNP production may be sensitive to estrogen regulation and represent an area for further study.
After adjusting for age and gender, we found a modest but significant relationship between BNP and left atrial volume index in the normal subgroup. Thus, in the normal state, plasma BNP may be related to the size of the gland that produces it. Although the relationship between BNP and LV mass was not significant after adjustment for age and gender, production of BNP by ventricular myocytes is clearly increased in the presence of hypertrophy. Thus, a stronger relationship between plasma BNP and LV mass or left atrial volume may be apparent if subjects with cardiovascular disease are included.
Implications for use of BNP as a diagnostic test
Previous studies have reported the sensitivity and specificity of BNP for the detection of CHF or LV dysfunction (2–5). These studies derived a single abnormal (partition) value for BNP by ROC analysis and have not examined the use of age- or gender-specific partition values. For the Biosite assay, values of 75 to 80 pg/ml have been reported as partition values for detection of ventricular dysfunction (with and without symptoms) or diagnosis of CHF (5,24). For the Shionogi assay, values of 17.9 to 79 pg/ml have been reported for detection of ventricular dysfunction or CHF in highly variable populations (2,8,23,27,28). Although we found that the area under the ROC curves was similar among the age/gender strata, the optimal partition value for the detection of systolic dysfunction differed by assay, age, and gender.
These data suggest that age-, gender-, and assay-specific partition values should be used to interpret BNP. The ultimate partition values used may also depend on the abnormality being screened for (CHF vs. asymptomatic LV dysfunction), its severity, the implications of false positives or negatives and, importantly, on the population being studied. The operating characteristics of diagnostic tests may vary according to the population to which they are applied (11). Even though patients with severe CHF have very high BNP, age/gender effects still need to be considered when choosing a partition value. A previous study that utilized the Biosite assay to measure BNP reported that a BNP of 80 pg/ml is diagnostic of CHF in symptomatic patients presenting for urgent care (5). Although the average BNP value in patients with CHF was very high, and the performance of the test was outstanding, the study population was young (mean age 63 years) and predominately men (94%). In the current study, a value of 80 pg/ml would be abnormal in most male age groups (Table 2, Fig. 1) but would be well within the 95th percentile for women. A higher partition value may be needed in women or older patients where use of age- and gender-specific partition values may enhance the predictive characteristics of the test, particularly avoiding specificity problems in the elderly.
The population of Olmsted County, Minnesota, is primarily white, and thus we are unable to investigate potential influence of ethnicity on BNP. Our study was confined to subjects >44 years of age.
In this population-based cohort, we found that BNP increases with age and is higher in women among subjects without cardiovascular disease or cardiac dysfunction. The magnitude of the effects of age and gender on BNP in the normal subgroup suggested that both parameters need to be considered when interpreting BNP, and indeed, the optimal discriminatory value of BNP for the detection of systolic dysfunction in the population was higher in women and older persons. Although the ultimate partition value for BNP used in clinical practice will be influenced by a number of variables, these data suggest that age-, gender-, and assay-specific values will be needed.
The authors are indebted to the study sonographers, Trudy Wellik, Mary Wenzel, and Joan Lusk; and the data analysts, Lynn Urban and Matthew Johnson for their technical expertise.
☆ Gottlieb C. Friesinger II, MD, acted as guest editor for this article.
- brain natriuretic peptide
- congestive heart failure
- confidence interval
- early-to-late filling velocities
- ejection fraction
- hormone replacement therapy
- left ventricular
- receiver operating characteristic
- Received March 4, 2002.
- Accepted May 31, 2002.
- American College of Cardiology Foundation
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