Naren, a first year Med student, has a question - why use 3.8% sodium citrate solution as an anticoagulant - why not a higher (or lower) concentration. Here is an attempt at answering it.
Sodium citrate is used as a anticoagulant because of its ability cause chelate calcium ions which as you know are essential for the clotting of blood. Sodium citrate is technically, Sodium tricitrate (Na3C6H5O72H20). The citrate ions are responsible for chelating calcium ions. Higher the concentration of the sodium citrate solution used, better the anti-coagulation effect. However, with increasing concentration, the osmolarity of the anti-coagulant will increase and the blood cells collected in it will be subjected the effects of this osmolarity. Ideally, the concentration of the anti-coagulant should be equivalent to an osmalarity of about 300 mOsm, which would be iso-osomolar to that of plasma and therefore would have minimum osmolar effects on RBCs. At higher concentrations, the hyperosmolar anticoagulant solution would cause shrinkage of RBCs due to exosmosis and therefore would affect measurements such as packed cell volume and related blood indices.
Two forms of sodium citrate preparations are commonly used for anticoagulantion - 3.8% and 3.2% solutions. Let us see what is the osmolarity of these sodium citrate solutions.
3.8% solution = 3.8g/100L = 38g/L
Molecular weight of sodium citrate = 294g
38g/L = (38/294)Mol/L = 0.129 Mol/L = 129mMol/L
Now each molecule of sodium citrate gives three Na+ ions and one citrate ion in solution = total 3 osmotically active particles
129 mMol/L of sodium citrate would therefore have a osmalarity of 129 x 4 = 516 mOsm/L
3.8% sodium citrate provides good anticoagulation but is a little hypersomolar compared to plasma and therefore may have efects on RBC size.
3.2% sodium citrate solution has a osmolarity of 436 mOsm/L (calculation similar to that shown above).
This provides sufficient anticoagulation (although weaker than the 3.8% solution) and its osmolarity is closer to that of plasma (300mOsm/L) and would have less effects on RBC volume.
So the concentration of sodium citrate used is a balance between having enough to provide effective anticoagulation but not too much to prevent hyperosmotic RBC size changes.
