Final answer:
The specimen from a tube with a light blue stopper is plasma for coagulation testing, with sodium citrate serving as an anticoagulant by chelating blood calcium.
Step-by-step explanation:
The specimen analyzed from a sample drawn in a tube with a light blue stopper, which contains sodium citrate as an anticoagulant, is most often plasma used for coagulation testing. The sodium citrate acts as an anticoagulant by chelating calcium in the blood, which is necessary for the coagulation process. When blood is drawn into a light blue stopper tube, it needs to be gently inverted to mix the anticoagulant and blood so that the sample does not clot before analysis. The blood is then typically centrifuged to separate the plasma, which is used in tests that monitor and diagnose disorders of blood clotting such as Prothrombin Time (PT) and International Normalized Ratio (INR).