SARS-CoV-2 nAb titers were determined in IVIG and human plasma samples that were used undiluted, or prediluted with cell culture medium 1:4, 1:5, 1:10, or 1:20 depending on sample amount available, and then serially diluted in 2-fold steps. Equal volumes of sample dilutions were mixed with virus stock at 103.0 tissue culture infectious doses 50% per milliliter (TCID50/mL) SARS-CoV-2 (strain BavPat1/2020, kindly provided by C. Drosten and V. Corman, Charité Berlin, Germany) and incubated for 150 minutes ± 15 minutes, before titration on Vero cells (Cat. no. 84113001, European Collection of Authenticated Cell Cultures, Porton Down, Salisbury, UK) in 8-fold replicates per dilution. The virus-induced cytopathic effect was determined after 5–7 days of incubation. The reciprocal sample dilution resulting in 50% virus neutralization (NT50) was determined using the Spearman-Kärber formula, and the calculated neutralization titer for 50% of the wells reported as 1:X. The detection limits were as follows: <1:0.8 for undiluted, <1:3.1 for 1:4 prediluted, <1:3.9 for 1:5 prediluted, <1:7.7 for 1:10 prediluted, and <1:15.4 for 1:20 prediluted IVIG.