['The N-terminal fragment of cardiac-derived pro-B-type natriuretic peptide is a glycosylated polypeptide. It is unknown whether N-terminal pro-atrial natriuretic peptide (proANP) fragments are also covalently modified. We therefore evaluated the clinical performance of 2 distinctly different proANP assays on clinical outcome.', 'We examined 474 elderly patients with symptoms of heart failure presenting in a primary healthcare setting. Samples were analyzed with an automated immunoluminometric midregion proANP (MR-proANP) assay and a new processing-independent assay (PIA) developed in our laboratory. The results were compared with Bland-Altman plots, and clinical performance was assessed by generating ROC curves for different clinical outcomes.', 'Despite linear regression results indicating a good correlation (r = 0.85; P < 0.0001), the PIA measured considerably more proANP than the MR-proANP assay (mean difference, 663 pmol/L; SD, 478 pmol/L). In contrast, the clinical performances of the 2 assays [as assessed by the area under the ROC curve (AUC)] in detecting left ventricular dysfunction were similar [proANP PIA, 0.71 (95% CI, 0.63-0.79); MR-proANP assay, 0.74 (95% CI, 0.66-0.81); P = 0.32]. The prognostic ability to report cardiovascular mortality during a 10-year follow-up revealed AUC values of 0.66 (95% CI, 0.60-0.71) for the proANP PIA and 0.69 (95% CI, 0.63-0.74) for the MR-proANP assay (P = 0.08, for comparing the 2 assays).'] Does n-terminal pro-atrial natriuretic peptide measurement in plasma suggest covalent modification?