Acts as a blood coagulant in vivo and anticoagulant in vitro:

• A. Sodium tartrate
• B. Sodium citrate
• C. Sodium phosphate
• D. Sodium thiosulfate

Answer: (B)

Coagulation depends on calcium; removing calcium prevents clot formation. Sodium citrate binds calcium ions, forming calcium citrate, which removes free calcium from plasma. This stops the coagulation cascade in vitro, so blood collected in citrate-containing tubes remains fluid for testing. In vivo, citrate is rapidly metabolized (mainly to bicarbonate) and does not promote clotting; its effect there is not to induce coagulation. Among the options, sodium citrate is the classic substance used to prevent clotting in blood samples outside the body, making it the best fit for an anticoagulant in vitro. The other salts do not provide the same reliable calcium-chelating anticoagulant effect used in standard lab practice.