Any measurement related to a morphological or physiological parameter, such as the amount or composition, of interleukins in blood, the fluid that circulates through the heart, arteries, capillaries and veins carrying nutrients and oxygen to the body tissues and metabolites away from them. Interleukins comprise a group of multifunctional cytokines that are produced by a variety of lymphoid and nonlymphoid cells and whose effects occur at least partly within the lymphopoietic system.