APLN and chronic kidney disease: Because we were unable to detect endogenous apelin peptides in the plasma of 6 healthy volunteers and 4 CKD patients during the initial experiments, we determined apelin peptide recovery by adding a fixed concentration of the synthetic peptides both at the beginning (before adding magnetic beads) and in an identical sample at the end (directly into the eluent after adsorption from the magnetic beads) of sample preparation.