Reference Values for Plasma B-Type Natriuretic Peptide in the First Days of Life

The clinical relevance of B-type natriuretic peptide (BNP)1 and the amino terminal fragment of its prohormone (NT-proBNP) as biomarkers in pediatric heart disease have recently been shown (1)(2). BNP and NT-proBNP concentrations are dependent on age and sex, at least in adults(3). Because commercial methods are affected differently by the presence in plasma of several peptides derived from the degradation of intact prohormone and BNP(3), little if any agreement exists among reported reference intervals, especially those used for infants during the first days of extrauterine life(1). Data are scarce regarding the reference values for BNP and NT-proBNP in infancy (1)(2)(4). Recently, Nir et al.(2) summarized NT-proBNP concentrations measured with an electrochemiluminescence immunoassay method in 690 healthy individuals (47% males) with ages ranging from birth to 18 years, including 127 newborns in the first week of life (43 in the first 2 days). The NT-proBNP concentrations were highest in the first days of life, and then showed a marked decline. Concentrations in males and females differed only for children age 10–14 years. Large reference value studies for BNP for newborns and infants have not been reported. To assess the reference values for BNP in the first days of life, we measured the peptide concentrations in residual EDTA plasma from blood samples (0.2–0.4 mL) collected from a superficial venous vessel of the heel in 126 apparently healthy newborns (females 57, males 69) throughout the first 7 days of extrauterine life. We measured peptide concentrations by using a fully automated immunoassay platform (Triage BNP reagents, Access Immunoassay Systems, REF 98200, Beckman Coulter). Moreover, we obtained EDTA plasma samples from 33 healthy infants (females 15, males 18) with ages ranging from 1–6 months. Blood was collected from healthy newborns and infants who underwent routine screening for genetic disorders (for newborns) or an endocrine work-up (for infants). All newborns were delivered at term (between 35 and 42 weeks of pregnancy) and presented at birth with body weight ranging from 2.5–4.1 kg and with an Apgar score ≥8; 24.6% of deliveries were by cesarean section. In infants, clinical examination results excluded the presence of acute illness (i.e., infections or electrolyte imbalance), and laboratory test results were within the reference intervals. We obtained parental informed consent for all newborn and infant study participants. To minimize degradation of plasma BNP (3), immediately after withdrawal of blood samples we put them into disposable polypropylene tubes containing EDTA (1 g/L of plasma). Plasma samples were rapidly separated by centrifugation (2400g) for 15 min at 4 °C and then assayed as soon as possible (no more than 4 h after blood withdrawal). The analytical characteristics and performance of the Access immunoassay method used in this study for the measurement of BNP were previously evaluated in our laboratory(5). Plasma BNP concentrations were highest in the first 2 days of life (1-way ANOVA after logarithmic transformation of original data, P < 0.0001), with a progressive decline in the next days and weeks (first 2 days: mean 282.4 ng/L, SD 169.3 ng/L, median 243.5 ng/L, 90th percentile 468.7 ng/L, n = 66; third and fourth day: mean 193.0 ng/L, SD 141.9 ng/L, median 169.0 ng/L, 90th percentile 348.0 ng/L, n = 40, P < 0.0230 by Scheffè test after 1-way ANOVA compared to the 2 first days; from the 5th to the 7th day: mean 102.6 ng/L, SD 168.2 ng/L, median 45.0 ng/L, 90th percentile 259.5 ng/L, n = 20, P < 0.0001 compared to the 2 first days; from 1–6 months: mean 21.5 ng/L, SD 10.0 ng/L, median 23.0 ng/L, 90th percentile 37.2 ng/L, n = 33, P < 0.0001 compared to the 2 first days) (Fig. 1 ). BNP values were not sex dependent (males: mean 179.7 ng/L, SD 168.0 ng/L, median 147.0 ng/L, n = 87; females: mean 187.3 ng/L, ng/L, SD 184.1 ng/L, median 155.5 ng/L, n = 72; P = 0.9909 by t-test after logarithmic transformation of original data). There was no difference in BNP values in infants who were born with vaginal vs cesarean deliveries. Trend and distribution of BNP values obtained for the 126 newborns during the first 7 days of life. The trend is indicated by the continuous line, which was assessed by smooth spline analysis, by using the statistical software JMP for Macintosh OS X (JMP Sales SAS). Our data for BNP, the biologically active hormone, are in agreement with the results reported by Nir et al. (2) for NT-proBNP, which is an inactive peptide with a longer plasma half-life. Both BNP and NT-proBNP plasma concentrations are highest in the first 2 days of life, with a progressive decrease in the next days and weeks. Soldin et al.(4) reported obtaining higher values when they measured BNP by a point-of-care testing method in EDTA whole blood samples during the first month of life. It is well known that BNP and NT-proBNP values are strongly method dependent(3); consequently, clinicians should take into account the method used for the measurement of BNP and NT-proBNP when comparing our values to those found in their own laboratory. Author Contributions:All authors confirmed they have contributed to the intellectual content of this paper and have met the following 3 requirements: (a) significant contributions to the conception and design, acquisition of data, or analysis and interpretation of data; (b) drafting or revising the article for intellectual content; and (c) final approval of the published article. Authors’ Disclosures of Potential Conflicts of Interest:No authors declared any potential conflicts of interest. Role of Sponsor: The funding organizations played no role in the design of study, choice of enrolled patients, review and interpretation of data, or preparation or approval of manuscript.