Course of RBC OF in passively immunized and control cats. RBC OF (y-axis, as percentage (w/v) NaCl concentration with 50% hemolysis) of individual cats in group A (A) and B (B) at day 0 and at days 21, 100 and 179 pi and mean (±SD) RBC OF in group A and B (C) are shown. All cats were subcutaneously inoculated with Mhf at day 0. Significant differences between groups are indicated with asterisks (pMWU < 0.05). The reference interval for RBC OF based on measurements in nine healthy cats is shaded grey (50% hemolysis in NaCl 0.50–0.57% w/v) . RBC OF significantly changed during the course of infection in cats in group A (pF = 0.0004, significantly higher levels at days 21 and 100 pi compared with day 0 pre-inoculation, pD < 0.05), but not in cats in group B.