You are required to draw blood from a patient and to keep it in a...

Clotting of collected blood can be prevented by - coating test tubes with silicon (which produce non wettable surface similar in its smoothness to endothelial lining of blood vessels). adding chelating agents (includes trisodium citrate, sodium oxalate and sodium EDTA) which remove calcium which is important for blood coagulation, and prevent blood clotting. - adding heparin, most powerful anticoagulant which acts indirectly by activating plasma antithrombin III. Heparin is effective both in vivo and in vitro. Whereas the option $\mathrm{a}, \mathrm{b}$ and $\mathrm{d}$ are effective in vitro. Heparinized blood is not suitable for blood counts (as it alters the shape of RBC's and WBC's which affects blood testing), Fragility testing and complement fixation tests. Hence (c) is the correct answer.