The REFER (REFer for EchocaRdiogram) protocol: a prospective validation of a clinical decision rule, NT-proBNP, or their combination, in the diagnosis of heart failure in primary c... oleh Tait Lynda, Roalfe Andrea K, Mant Jonathan, Cowie Martin R, Deeks Jonathan J, Iles Rachel, Barton Pelham M, Taylor Clare J, Derit Marites, Hobbs FD