Objective: Several studies have advocated the use of a random plasma aldosterone/plasma renin activity ratio (ARR) as a sensitive screening test for primary aldosteronism (PA) in patients with hypertension. However, both body position and salt intake influence plasma aldosterone and plasma renin activity levels and it is unknown how these conditions affect the diagnostic power of this test. We examined prospectively the effect of both conditions on this ratio in patients with PA and in patients with essential hypertension (EH).Design/Methods: In 19 patients with PA (14 adenomas and 5 hyperplasia) and in 10 subjects with EH, plasma aldosterone levels and plasma renin activity were measured both after a 6 day saltless diet and after 6 g salt diet and after 18 g salt diet, both after overnight supine (OS) and after 4 h of ambulation (AMB). The diagnosis of PA was based on an increased aldosterone secretion rate (isotope dilution) of > 220 nmol/day after the 18 g salt diet. As cut-off level of the ARR for calculation of the positive and negative predictive values (PPV, NPV), the mean + 2SD in the EH patients was used.Results: The PA/PRA ratio was higher in the PA than in the EH patients in all conditions. During saltless diet, the PPV's were 92 and 100% after OS and after AMB, respectively, whereas the NPV's were 83 and 69%. During the 6 g salt diet, the PPV's were 90 and 83% after OS and after AMB, respectively, whereas the NPV's were 71 and 60%. During the 18 g salt diet, the PPV's were 94 and 91% after OS and after AMB, whereas the NPV's were 69 and 50%. The diagnostic accuracy value was highest during the saltless diet after OS (88%) and lowest at the 6 g salt diet after AMB (65%).Conclusion: The results of this study indicate that the diagnostic accuracy of the PA/PRA ratio is limited in both positions and during all 3 levels of salt intake. In particular, a normal ARR test result does not exclude the diagnosis of PA in at least 17% of the patients.