Abstract: Disclosed is a composition that synergistically prevents proteolysis or modification of peptides in sampled biological fluids using sulfonyl fluoride family protease inhibitors at high concentrations combined with at least one additional protease inhibitor of a different type, preferably a broad spectrum protease inhibitor, and a chelator. A preferred embodiment uses AEBSF at 10 mM, Benzamidine at 20 mM and EDTA as the chelator. The disclosed composition may be combined with other protease inhibitors to further modulate its specificity, for instance to additionally target acidic proteases. Additional protease inhibitors, reducing agents, stabilizers and buffering agents may be combined with the disclosed compositions in devices for sampling or testing biological fluids for levels of peptides of interest, or methods therefore. The disclosed devices, compositions and methods are of particular use in sampling and testing for the level of natriuretic peptides.