Abstract: A self-clearing extension set comprising a first shunt flow pathway for direct delivery of liquid from a source of pressurized liquid to a patient catheter and a second shunt flow pathway wherein injected fluid is cleared from an add-site to the patient catheter by selectively diverted flow from the source. Two methods and associated apparatus for injecting biologically active liquid and clearing the injected liquid through a smaller lumen of a dual lumen extension tube are disclosed. Two methods for occluding one flow pathway while providing a patent flow pathway through the other flow pathway are also disclosed. Each add-site sealingly accepts a syringe needle which is used to inject biologically active liquid into the second shunt flow pathway. Each add-site comprises a low geometric dead space such that no meaningful quantity of injected liquid remains in the add-site after the rest of the injected bolus of liquid has been cleared to the patient catheter.