Sample Collection and Processing for the Assay of Plasma Renin Activity

Plasma renin activity was measured in samples collected in lithium heparin, sodium heparin, and ethylenediaminetetraacetate. The samples were incubated under two widely used sets of conditions to generate angiotensin I, and the results were compared. Under certain incubation conditions, the absence of ethylenediaminetetraacetate from the sample tube produced low plasma renin activity values. This was obviated by using an alternative incubation medium and was not, therefore, due to direct inhibition of renin by heparin. These lower values were not seen in samples from patients receiving captopril. This suggests that the discrepant values were due to some angiotensin I conversion to angiotensin II and lower homologues in the incubates. Sodium heparin samples compared closely with lithium heparin samples, indicating that the presence of lithium is not significant. These results show that, provided the incubation conditions are suitably modified, samples collected into lithium or sodium heparin can be used for measurement of plasma renin activity.