Abdul Mannan: The Most Important Tube in the FXIII Assay
Abdul Mannan, Consultant Haematologist at Betsi Cadwaladr University Health Board, shared a post on LinkedIn:
”The most important result in this FXIII assay came from a tube designed to show nothing.
It was not a failed test. It was the blank.
In an ammonia-release FXIII assay, the first result is the total signal. But total signal can include background signal that is not due to FXIII.
The blank asks a simple question: what does the assay read when the actual FXIII reaction is not taking place?
Then the laboratory can:
- Measure the total signal
- Measure the blank
- Subtract the background signal
- Report the FXIII-specific result
In a published case of autoimmune FXIII deficiency, activity appeared to be 18.5 percent before blank correction. After correction, it was less than 1 percent in the same ammonia-release assay.
The patient’s biology had not changed. The laboratory had separated the genuine reaction from the background signal.
That is why laboratory–clinical correlation matters. If the bleeding does not fit the number, do not simply repeat the number louder. Ask how the assay works, what its blank measures, and whether the result has been corrected.
What is the laboratory control you think trainees most often overlook?”

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