Renin activity is a measure of the levels of renin, an enzyme produced by the kidneys, in the bloodstream. Renin plays a key role in the regulation of blood pressure by controlling the renin-angiotensin-aldosterone system, which helps to maintain fluid and electrolyte balance in the body. Elevated renin activity can indicate a possible underlying health condition, such as hypertension or kidney disease. Measuring renin activity can help healthcare providers diagnose and monitor these conditions, as well as guide treatment decisions.