Plasma is blood that was never allowed to clot, and the chemical that stopped it stays in the product for good. Four anticoagulants cover almost every research plasma. Each works by a different mechanism and each leaves its own fingerprint on the assays you run afterwards. Picking the wrong one is the most common reason a plasma "doesn't work".
EDTA
Ethylenediaminetetraacetic acid binds calcium so tightly that the clotting cascade, which needs free calcium at several steps, can't get started. It also preserves cell morphology, which is why EDTA is the hematology standard for blood counts and smears, and it inhibits the metalloproteases that would otherwise chew up some analytes. The same chelation is its weakness. Any assay that needs divalent cations, including complement assays, many enzyme assays and every coagulation test, will be inhibited, and recalcifying EDTA plasma isn't practical. Use EDTA plasma for hematology, for DNA and RNA work, and for proteins and peptides that need protease protection.
Heparin
Heparin works further down the cascade. It potentiates antithrombin so that thrombin and factor Xa are neutralized as fast as they form. Calcium is left alone, which makes heparinized plasma the preferred matrix for clinical chemistry, electrolytes, hormones and most enzyme assays. Its limits are in molecular and coagulation work: heparin inhibits PCR polymerases, and it makes the plasma useless for clotting tests. Lithium heparin is used where sodium would interfere with an electrolyte measurement.
Sodium citrate
Citrate also binds calcium, but weakly and reversibly. At the standard 3.2% (0.109 M) it is used at a fixed 1:9 ratio to blood. Adding calcium back in the assay restarts the cascade, which is exactly what every coagulation test does, so citrated plasma is the required matrix for PT, aPTT, fibrinogen and factor assays, and for platelet function work where the platelets have to stay unactivated. It dilutes the plasma by about 10%, which you have to remember when comparing concentrations with serum.
ACD
Acid-citrate-dextrose combines citrate anticoagulation with dextrose and a lower pH that keep red cells metabolically alive. It comes from blood banking, where units have to survive weeks in a refrigerator. Choose it when the cells matter as much as the plasma: red-cell preparations, cell-based assays, and anything you'll store liquid for more than a few days.
So the choice usually makes itself: EDTA for hematology, nucleic acids and protease-sensitive proteins; heparin for chemistry, hormones and enzyme assays; sodium citrate for coagulation and platelet work; ACD when the cells need to stay viable or the plasma will be stored liquid. For complement or any other calcium-dependent assay, none of them is right. Use serum, or heparin with care.
We collect rabbit plasma into sodium citrate, EDTA (K2 and K3 EDTA are also listed), heparin and ACD, and we offer bovine, chicken and porcine plasma in the common anticoagulants, so each of the choices above maps onto a catalog item. For the other things on a plasma listing, sterility, hemolysis grade and storage, see key attributes of plasma products. If you need an anticoagulant or concentration that isn't listed, ask. It's set at collection, so it's a custom draw rather than a compromise.