Bar charts of patients' PF4/polyanion OD values over time as measured using the Stago assay and based on whether they have received PEX. Median PF4/polyanion OD values for patients who received PEX over time (n = 9) (A) compared with age- and sex-matched control patients who did not receive PEX (n = 10) (B). The tails represent interquartile ranges. The positivity cutoff for the Stago assay was an OD of 0.238. Wilcoxon matched-pairs signed-rank tests were used to determine statistically significant differences between mean PF4/polyanion OD values at different time points, with P < .05 considered to be significant. ns, not significant.