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Factor Assays: How to Define Parallel Results

From Maria FrancoOrlando Health: Hi, George,  when running factor assays (1:10, 1:20, etc), is the agreement between results to r/o inhibitory activity all calculated from the original result (1:10)? Not sure what the most common or recommended practice is. I have heard to find the mean of all results and then try to calculate the deviation. Thanks, Maria.

Hello, Maria, and thank you for your question. The correct method is to compare the result from each dilution with the original (1:10) dilution. If a subsequent dilution, for instance, 1:20, 1:40, or 1:80 produces a result that exceeds the 1:10 dilution result by more than a set percentage, you may suspect the presence of an inhibitor. Many laboratory directors choose 10% as their limit, though I’ve seen presentations where 15% is recommended. This question is also addressed with a little more detail in our audio module, Factor Assays. Once you suspect an inhibitor, the next step is a Bethesda titer. If any of our participants has another approach, please comment below. Geo.

Added September 14: I spoke with Patti Tichenor and Laura Taylor in the University of Alabama at Birmingham Hospital special coagulation lab, and they reminded me that you should also watch for rising values as the dilution increases. For instance, if you see a result of 10% factor VIII in the 1:10 dilution,13% in the 1:20 dilution, 19% in the 1:40, and 24% in the 1:80, that is stronger evidence than say 10%, 13%, 14%, and 15%, even though the change from the 1:10 dilution to the 1:20 jumped by 30%.

Comments (3)
Factor Assays
Nov 6, 2012 11:11am

We use the increase in factor activity with dilution method.
We use the increase in factor activity with dilution method. In addition we created a visual aid: an excel chart where we plot the calibration curve vs the patient results on the same chart. If the slopes (by visual inspection) are parallel, then an inhibitor is unlikely. If the slopes are not parallel then an inhibitor might be suspected. There are some problems such as a very low (1%) factor level may look like an inhibitor when there is none because the slope will be very flat. The excel chart was developed to mimic an old MLA instrument chart.

Sep 17, 2012 3:02am

I thank you ALL!!

I thank you ALL!!


Herb Crown
Sep 13, 2012 12:45pm

Hi Maria and George. In addressing factor levels and determ
Hi Maria and George. In addressing factor levels and determining whether a inhibitor pattern exists or not, remember there are a number of inhibitors and we have to differentiate between them. The major inhibitors are lupus anticoagulants and heparin, and to a lesser extent factor inhibitors (rate of occurance not severity).

The factor results on a plasma contaminated with heparin will look very much like a patient who has a lupus anticoagulant (LA) with an increase in factor levels over dilution, but the 1:10 dilution rarely is less than 20% (in my experience). A thrombin time will help sort out the difference between a LA and heparin. On the other hand, a factor inhibitor will be significantly lower, ie 1-10% activity and will show increase over dilutions. A Bethesda assay will quantify the severity of the factor inhibitor.

Herb Crown
St. Louis University Hospital Coagulation Reference Laboratory

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