The aPTT measures the intrinsic pathway. Again, it's a historic term. We use citrated plasma (not serum!) Citrate chelates the calcium Hemolyzed or partially clotted samples are useless. We restore calcium to an aliquot of the plasma and Add an activating agent (silica, kaolin, elagic acid, among others) to serve as the trigger to activate Factor XII. Record time it takes for the sample to clot, typically 28 to 35 seconds depending on the activating agent. No equivalent of the INR exists here, although a reference range is always provided.