Methods:We recruited 43 patients with stable chronic heart failure living in Curaçao (22 males, 21 females; median age, 63 years; range, 20 -86 years; New York Heart Association classes I-III). Samples were collected for within-day CV i (n ‫؍‬ 6; every 2 h starting at 0800), day-to-day CV i (n ‫؍‬ 5; samples collected between 0800 and 1000 on 5 consecutive days), and week-to-week CV i (n ‫؍‬ 6; samples collected between 0800 and 1000 on the same day of the week for 6 consecutive weeks). NT-proBNP (Roche) and BNP (Abbott) were measured by immunoassay. Results: Median (range) concentrations were 134 (0 -1630) ng/L (BNP) and 570 (17-5048) ng/L (NT-proBNP). Analytical variation, week-to-week CV i , and reference change values were 8.4%, 40%, and 113% (BNP), and 3.0%, 35%, and 98% (NT-proBNP). Week-to week CV i s were inversely related to median BNP concentrations. Week-to week CV i s for BNP were 44% (BNP <350 ng/L)