Detection of T antigen on VWF in plasma. (A) Serial dilutions of plasma were added to microtiter wells coated with anti-VWF antibodies, and PNA binding was determined as described in the legend of Figure 1. Plotted is absorbance at 490 nm versus concentration of VWF in diluted samples for NPP (X) and a representative sample (▪). The slope found for NPP was set at 100%. The slope for this particular sample was calculated to be 177%. Relative PNA binding was determined for plasma samples from healthy volunteers (n = 111; B) and samples that were randomly provided by our diagnostic laboratory (n = 66; C). (C) The dotted lines indicate VWF values of 0.5 and 1.5 U/mL. The drawn solid lines were obtained by fitting the data in a model for linear regression (Pearson rank = −0.43; B) or a model describing a single exponential decay (r² = 0.57; C). (D) Data shown in panel C were subdivided into 3 groups based on VWF antigen levels that were less than 0.5 U/mL (n = 15), between 0.5 and 1.5 U/mL (n = 32), and higher than 1.5 U/mL (n = 19). Data represent the mean of 2 independent determinations, and variance between measurements was 16% or less. Differences between groups were tested with the unpaired 2-tailed t test, with the Welch correction applied when necessary. Bars indicate mean values.