Dilution of plasma with Tris buffer increases measured catecholamines in plasma.

We investigated the effects of dilution of plasma samples on the measured concentrations of catecholamines. Diluting samples of human plasma 10-, 50-, and 100-fold with Tris buffer (100 mol/L, pH 8.6) improved analytical recovery of internal standards, suggesting that it decreases the commonly observed inhibition of methylation in radioenzymatic assays of catecholamines in plasma. However, the dilution is not associated with a proportional decrease in counted radioactivity. This extra amount of radioactivity, which is unlikely to be nonspecific in origin, accounts for a significant increase in the calculated catecholamine concentration. Tentatively, we suggest that Tris buffer releases both catecholamines and conjugated catecholamines bound to some unidentified low-molecular-mass component of plasma.