29, 36
A normal activity-to-antigen ratio does NOT exclude acquired von Willebrand syndrome on ECMO. If the question is being asked because a patient is bleeding, request multimer analysis by name. The direction is supported by mechanism: a ratio cannot see a size distribution.
The conventional von Willebrand activity-to-antigen ratio (cut-off 0.7)
Abstract, International Journal of Laboratory Hematology 2024;46(S2):3-121 supplement. DOI 10.1111/ijlh.14396
Direct multimer analysis by immunofixation electrophoresis, adjudicated independently by two specialists from the gel
Conference abstract only; paediatric; single centre; 28 specimens. [VERIFICATION REQUIRED: whether published in full.]
24 of 28 specimens were classified as acquired von Willebrand syndrome on the gel. ALL 24 had a high-molecular-weight multimer fraction below 40 percent, but only 46 percent had an activity-to-antigen ratio below 0.7. Among controls, 92 percent had a normal multimer fraction and 89 percent a normal ratio. Multimers below 40 percent gave sensitivity 100 percent, specificity 75 percent, overall accuracy 96 percent. Authors: the activity-to-antigen ratio cannot be a screening test for acquired von Willebrand syndrome.
28 specimens from children on ECMO, plus controls