Variation in historical VWF testing for Zimmerman Program subjects. (A-B) Comparison of the historical (A) and study entry (B) VWF:Ag (circles) and VWF:RCo (triangles) for all subjects enrolled with a diagnosis of type 1 VWD. The insets show the comparison of VWF:Ag on the x-axis and VWF:RCo on the y-axis for historical laboratory values (A) and study entry laboratory values (B). The correlation is much lower for historical values and improved for study entry values, as expected, given that all study testing was performed in the same laboratory and all testing was performed on the same sample for each subject. However, there still remain issues with the lower limit of the ristocetin cofactor assay, as seen by the number of VWF:RCo values at or below the lower limit of detection. hist, historical.