* eDR (effective renin) = 0.13 x DR. It reflects the fact that the effectiveness of plasma renin is reduced by about 90% in patients taking an ACEI or ARB or aliskiren
Relationship of plasma renin activity (PRA) to direct plasma renin (DR) levels
Plasma renin activity (PRA) is used to estimate the capacity of the circulation to generate angiotensin II, the active hormone of the renin-angiotensin system. In fact PRA usually changes in relation to the circulating concentration of renin. However, PRA sometimes reflects changes in renin substrate (plasma angiotensinogen). This difference is usually not important since, unlike renin, plasma angiotensinogen rarely changes; moreover, when angiotensinogen does change it changes slowly (over days).
In contrast, the direct plasma renin test (DR) detects only the circulating concentration of the enzyme renin. If plasma angiotensinogen falls, DR usually rises to compensate for the reduced capacity of plasma to generate angiotensin; if plasma angiotensinogen rises, DR usually falls thereby maintaining a constant rate of angiotensin generation.
Under most circumstances, PRA and DR change together and in proportion to each other. Thus, for must purposes the two tests can be used interchangeably, as long as the difference in units is taken into account.