WST-1 cell viability assay in patients with aHUS. (A) Similarly increased percentages of nonviable TF1 cells among aHUS patients in different disease states (acute phase, eculizumab treatment, and remission) compared with TTP patients. (B) Effects of serial dilution of eculizumab-treated serum on cell viability (1:1, 1:2, 1:4, 1:8, and 1:16). All aHUS patients showed no increase in killing at 1:1 and 1:16 dilutions, 2 of the 3 aHUS patients had no increase in killing at 1:2, and all 3 aHUS patients showed increased killing at 1:4 and 1:8 dilutions. Serum from a PNH patient treated with eculizumab (indicated by gray symbols) showed no increase in killing at any dilution. Ecu-aHUS, aHUS patient treated with eculizumab.