What’s the difference between the thrombin time and fibrinogen assay?
Posted Dec 27 2010 4:48am
Q. I was wondering if you could help clarify two coagulation tests. A few of my classmates have been trying to figure out the difference between the thrombin time and fibrinogen assay. I’ve looked in Robbins and on the internet for clarification but haven’t found anything that has helped me.
I understand that the thrombin time evaluates the ability of fibrinogen to be converted to fibrin under the influence of thrombin and you perform this test by adding thrombin to citrated plasma. Where I get a little confused is how that is different from the fibrinogen assay.
We were taught that the fibrinogen assay will tell you how much fibrinogen you have based on adding thrombin to diluted citrated plasma. I am wondering how one test tells you about the ability of fibrinogen to be converted to fibrin and the other test tells you about the concentration of fibrinogen when it seems like both tests are performed using the same procedure.
Is there a difference between these tests? And how do you decide if it is a problem with the conversion of fibrinogen as opposed to a problem with the concentration of fibrinogen?
A. Great questions! First, you’re exactly right about the way the thrombin time works: add thrombin to plasma, measure the time until a clot forms. Here’s a nice summary of the thrombin time .
The fibrinogen assay your professor was referring to is the the Clauss assay, which is one of a number of different methods of assaying fibrinogen. Here’s a summary of the different fibrinogen assays:
1. Clauss assay To do this assay, you need some reference plasma with a known concentration of fibrinogen. Typically, you’d purchase this from a company. You take this reference sample and dilute it a bunch of times (so you have, for example, four tubes with 1:5, 1:10, 1:15 and 1:20 dilutions). Then you run a thrombin time on each sample, and draw a curve. The greater the amount of fibrinogen in the sample, of course, the shorter the thrombin time will be. A line is drawn at the 1:10 dilution mark to represent “normal” (because when you run your patient’s sample, you’ll dilute it 1:10).
Then, you take your patient sample (diluting it 1:10) and perform a thrombin time on it. Depending on where your patient’s thrombin time falls on the curve, you can extrapolate how much fibrinogen must be in the sample (assuming the fibrinogen itself is working properly!). You need to do these dilutions in your own lab, with your own reagents, because the thrombin time results can vary from lab to lab, and you wouldn’t want to just use someone else’s curve – that might not be comparable to the curve you get in your lab. Here’s a nice summary of the Clauss assay .
2. Fibrinogen antigen assays These assays look for the amount of fibrinogen by using anti-fibrinogen antibodies. These tests do not measure how functional the fibrinogen is, just how much of it there is around. You can detect the amount of fibrinogen antigen using radial immunodiffusion, enzyme-linked immunosorbent assay (ELISA), or electrophoresis.
3. Clottable protein assay In this method, you form a clot using the patient sample and some thrombin, and then you dissolve it using some special reagents and do spectrophotometry on the clot mixture, measuring the amount of protein present. Most of the protein present in the clot consists of fibrin – so the protein level in the clot is pretty much equivalent to the fibrinogen level in the blood. This is a labor- and time-intensive process, and it isn’t done much.
4. PT-based assay This is really similar to the Clauss assay – only you use a prothrombin time instead of a TT. It’s not recommended for routine lab use because the results can vary quite a bit from lab to lab. Also, under certain conditions (like liver disease, DIC, renal disease, dysfibrinogenemia, and in patients on anticoagulants or thrombolytic therapy, which pretty much includes everybody!), the values are higher than they are using the Clauss assay.
Finally, you asked about how to find out if the patient has a qualitative problem with fibrinogen (meaning there is a problem with the conversion of fibrinogen to fibrin) as opposed to a quantitative problem with fibrinogen (meaning that there is too much or too little fibrinogen around). To do that, you’d need to do both a functional assay (like the Clauss assay) and an antigen-based assay (like an ELISA for fibrinogen antigen).