Rigidity Index (RI) of erythrocytes incubated in vitro with variable HA concentrations, and reversibility of HA effect. In vitro effect of several.hyaluronic acid (HA) concentrations on rigidity index (RI). Each sample was fractioned in 5 aliquots (1 ml). The first sample (control; n = 6) was additioned with 1 ml of neutralized elution solution and the other ones with 1 ml of purified HA in raising concentrations, yielding the following final nominal concentrations (μg/ml): [HA1] = 50; [HA2] = 87; [HA3] = 109; [HA4] = 190 (n = 6). As can be seen, after two washings, RI returns to control values. Data: median, C.I.95%: confidence interval. (n = 6). RI: Kruskal Wallis' test: H = 27.87; p < 0.0001. Post hoc tests were performed with Mann-Whitney's U between groups, p < 0.05. RI after wash: Kruskal Wallis' test n.s.