Tuesday, January 6, 2009
Mixing Studies-To correct or not correct: that is the question
Ah yes, one of the simplest test in coagulation that can give a clinician a tremendous amount of information is a mixing study.
However, simple and coagulation in the same sentence is like an oxymoron. There is nothing simple when it comes to coagulation.
Okay, back to the mixing study- Here we have a test that can tell the clinician quickly is there is a factor deficiency or an inhibitor. But like any test, in particular coagulation tests, if it is not performed correctly, your answer can affect every aspect of patient care. So how do we give a good answer- lets discuss-
First- what is a mixing study and how does it work- We know that if a mixing study corrects it represents a factor deficiency, that is the factor in the pooled normal plasma (PNP) replaces the factor that is missing in the patients sample. But, if there is not a correction, it suggests there is something present that is "inhibiting" the correction. Seems simple enough- well it is anything but!
A normal PT or APTT tells the clinician that there is at least 30-40% of factor levels present. Any less than that and a patient may bleed-
So what are we doing, we are taking pooled normal plasma (PNP) and adding 1 part of PNP to 1 part of patient plasma ( which represent a 1:2 dilution). Very simple, but there are a few things that all labs need to be aware of-
You should NOT use a control to mix with the patient plasma. A control contains buffers and stabilizers- and has a different matrix than fresh frozen PNP. That is what should be used to mix with the patient's plasma.
All of the factor levels should be known- okay so why is that important- well lets say we have a PNP with a factor VIII level of 60%- (text book levels of normals for factors are 50-150%) We make the 1:1 mix, the 60% now becomes 30%- this may NOT be enough to correct a factor VIII deficiency, so it gives the appearance of an inhibitor. That is why it is important to make sure you have sufficient levels of factors in your PNP.
One of the biggest unanswered questions is: Is your reagent sensitive to all the factors, that is will your PT or APTT be prolonged when factor levels reach 30-40%? Some reagents are insensitive to certain factors, which means you can have a level <30% deficient =" %" mix =" 30" mix =" 50" formula=" %" correction =" Patient">75% correction = Factor Deficiency
APTT < correction =" Inhibitor"> 75% correction = Factor Deficiency
PT <70% correction = Inhibitor
(Cheng, S, American Journal of Clinical Pathology, 2002; 117.)
Rosner Index: Looks at the Clotting time of the Patient =A
Clotting time of the 1;1 mix =B
Clotting time of PNP =C
INDEX = B - C
High Index suggests an inhibitor
Low Index suggests a factor deficiency
Cutoff's must be determined in each lab- Most common cutoff is 15.
Conclusion: So how do we tell if there is a correction? You need to use all of the available clinical information and compare to patient results. Make sure that all testing that is done in the laboratory is consistent and have good standard operating procedures. At present this is the best way to ensure standardization within your laboratory- hope this helps!
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