The confounding effects of age, gender, serum creatinine, and electrolyte concentrations on plasma B-type natriuretic peptide concentrations in critically ill patients*

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Abstract
To investigate the confounding effects of age, gender, serum creatinine, and electrolyte concentrations on plasma B-type natriuretic peptide (BNP) concentrations in critically ill patients. A prospective cross-sectional study. A 20-bed general intensive care unit of a tertiary referral hospital. Patients were 121 patients admitted to the intensive care unit over a period of 9 wks. Intravenous blood was collected for BNP measurements, and cardiac investigations including echocardiography were carried out for every patient on admission. The mean BNP concentration was 201 ± 317 pg/mL (n = 121). Thirty-five patients (28.9%), identified to have cardiac abnormalities, exhibited higher BNP concentrations than those without cardiac abnormalities (518 ± 394 vs. 60 ± 98 pg/mL, p < .001). The females exhibited higher concentrations of BNP than males in the noncardiac abnormality group (96 ± 132 pg/mL, n = 39 vs. 31 ± 38 pg/mL, n = 47, p = .016). BNP correlated significantly with age (r2 = .19) and creatinine (r2 = .084). The latter correlation became insignificant when patients with cardiac abnormality were excluded. No correlation was found between serum Na+ and K+ concentrations with BNP. Multivariate analyses demonstrated that the presence of cardiac abnormalities accounted for nearly 50% of the BNP variation. Addition of age and gender improved R2 to 60%. The contribution of creatinine was found to be insignificant. There was no association between BNP concentrations and serum Na+ and K+ concentrations. Logistic analysis confirmed that BNP is the strongest predictor for cardiac abnormalities in the critically ill patients. The current study demonstrated that plasma BNP concentrations increased with age and were higher in females than in males. Although the presence of cardiac disease was the most important determinant for BNP variations, age and gender also contributed significantly. The results suggest that age and gender need to be taken into account in the interpretation of BNP concentrations in critically ill patients.