OBJECTIVES The aims of this study were to explore the relationship of baseline levels of natriuretic peptides (NPs) with outcomes and to test for an interaction between baseline levels of NPs and the effects spironolactone. BACKGROUND Plasma NPs are considered to be helpful in the diagnosis of heart failure (HF) with preserved ejection fraction (HFpEF), and elevated levels are associated with adverse outcomes. Levels of NPs higher than certain cutoffs are often used as inclusion criteria in clinical trials of HFpEF to increase the likelihood that patients have HF and to select patients at higher risk for events. Whether treatments have a differential effect on outcomes across the spectrum of NP levels is unclear. METHODS The TOPCAT (Treatment of Preserved Cardiac Function Heart Failure With an Aldosterone Antagonist Trial) trial randomized patients with HFpEF and either prior hospitalization for HF or elevated natriuretic peptide levels (B-type NP [BNP] >= 100pg/ml or N-terminal proBNP >= 360 pg/ml) to spironolactone or placebo. Baseline BNP (n = 430) or N-terminal proBNP (n = 257) levels were available in 687 patients enrolled from the Americas in the elevated-NP stratum of TOPCAT. RESULTS Higher levels of NPs were independently associated with an increased risk for TOPCAT's primary endpoint of cardiovascular mortality, aborted cardiac arrest, or hospitalization for HF when analyzed either continuously or grouped by terciles, adjusting for region of enrollment, age, sex, atrial fibrillation, diabetes, renal function, body mass index, and heart rate. There was a significant interaction between the effect of spironolactone and baseline NP terciles for the primary outcome (p = 0.017), with greater benefit of the drug in the lower compared with higher NP terciles. CONCLUSIONS Similar to the effects of irbesartan in the I-PRESERVE (Irbesartan in Heart Failure With Preserved Ejection Fraction) trial, a greater benefit of spironolactone was observed in the group with lower levels of NPs and overall risk in TOPCAT. Elevated NPs in HFpEF identify patients at higher risk for events but who may be less responsive to treatment. The mechanism of this apparent interaction between disease severity and response to therapy requires further exploration. (C) 2017 Published by Elsevier on behalf of the American College of Cardiology Foundation.