[Measurement of low levels of plasma renin activity. A methodological improvement]

The present method to measure plasma renin activity is cumbersome and imprecise, factors that limit its clinical application.

To assess the importance of blood sampling conditions and the usefulness of increasing incubation time to measure plasma renin activity at low levels.

Twenty hypertensive patients, 14 female, aged 14 to 76 years old, were studied. Two blood samples were obtained after a 10 min rest in the sitting position and after a 30 min rest in supine position. One blood sample of each condition was sent to the laboratory at room temperature and the other sample was sent refrigerated. Angiotensin I concentration was determined after 3 h of enzymatic incubation at 37 degrees C and, in subjects with an activity of less than 1 ng/ml/h, after 18 h of incubation.

No significant differences in plasma renin activity were observed between the samples obtained with different rest times or different transportation methods. In people with low plasma renin activity, the 18 h enzymatic incubation reduced the lower detection from 0.3 to 0.014 ng/ml/h and the coefficient of variation from 14.4 to 3.2%.

A simplified blood sampling method does not change plasma renin activity values, and the longer enzymatic incubation in people with low plasma renin activity improves both the sensitivity and accuracy of the determination.