The authors used an ELISA antibody test developed at Mount Sinai (sensitivity 92.5%, specificity 100%), defining positive cases as those with the spike protein titer greater or equal to 1:80. Titer categories were defined as low (1:80 and 1:160), moderate (1:320), and high (1:960 and >=1:2880). 27% were low, 22% moderate, and 61% were high. The authors recalled 121 plasma donors that had been screened at a mean 30-day post symptom onset and compared the overall titers at mean 82-day post symptom onset.