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Factor IX Assay Unexpected Results

Mike Meyer asks, “we run three dilutions for all of our factor testing. Occasionally, we see results in which upon higher dilutions, the factor value decreases after adjusting for the dilution. For instance, I had a case today in which the FIX values were 0.64, 0.55, and 0.43 U/mL for the 1:10, 1:20, and 1:40 dilutions [values corrected for the dilution effect], the opposite effect seen with inhibitors. Calibration curve seems fine.” He adds, “the patient’s FVIII level was very high [about 3.60 U/mL]. We possibly thought that the higher dilutions would bring the FVIII level back closer to a more normal range. The high concentration of FVIII in the first dilution may be accelerating the PTT clotting time yielding a falsely higher FIX value. Please comment.”

George sent this question to colleague Bob Gosselin. Here is George’s response based on Bob’s analysis: The high factor VIII levels would shorten the PTT as you suggest. As you dilute the factor VIII levels, the PTT would lengthen. Bob suggests assaying factor IX using a chromogenic factor IX assay if you have access to a chromogenic kit.

We’d like to get comments from others who have had experience with this situation.

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