Wednesday, 24 October 2007

What is plasma? What is serum?

Plasma is blood minus the blood cells (red blood cells, white blood cells and platelets). It is therefore largely water, but contains numerous other things such as proteins (especially albumin), glucose, lipoproteins, amino acids, hormones, ions (e.g. sodium, potassium, etc.) and carbon dioxide, to name but a few.

Serum is the name we give to plasma that has had the clotting factors removed. You'd usually achieve this by allowing the sample to clot first, thus using up the coagulation factors.

Since allowing the blood to clot also traps all the blood cells, most chemistry tests of the blood are serum tests, not plasma tests. It's just a lot easier to get at - once the blood has clotted, the remaining fluid will be almost entirely serum. (You obviously want to measure the plasma for things like clotting factors, though, so these samples have an anticoagulant in them to keep the blood from clotting.)


  1. why is all the clotting factors absent in serum.the clot is made by,the absence of clotting factor 1 can be understood.but the other clotting factors only help to activate the steps in blood coagulation.cloting factors are protein.serum contain can we explain the absence of ALL clotting factors in serum?

  2. Hi has,

    The best thing I can come up with, for now, is as follows.

    When the blood clots, coagulation factors are activated and the exponential process ends up with fibrinogen being cleaved to form "sticky" fibrin. This much we all agree on.

    However, this process does not continue indefinitely - a clot formed in one of your vessels doesn't continue to propagate and propagate, until ALL the blood in your body is coagulated. There are numerous reasons for this, but one of the major ones is that coagulation factors are degraded by the body's anticoagulation system.

    For instance, a substance called Protein C degrades activated factors V and VIII. In this way, many of the factors are removed after clotting.

    I'm not entirely happy with this answer, but I'm afraid I can't find anything more substantial. If you have other ideas, or other sources, do share - I'm always keen to learn!