In ELISAs, the median (range) antibody titer (in RU) in the three groups was AMD patients, 196 (0–1495); age-matched control subjects, 316 (0–1743); and blood donor control subjects, 121 (0–3104) (
Fig. 1; Kruskal-Wallis Test
P < 0.001). Pairwise comparisons by Mann-Whitney test showed that all three groups were significantly different from one another (AMD patients versus blood donor control subjects,
P < 0.05; AMD patients versus age-matched control subjects,
P < 0.01; age-matched control subjects versus blood donor control subjects,
P < 0.001). In the three groups, there was no significant difference in median antibody titer between the men and women (AMD patients, male 190 vs. female 206; age-matched control subjects, male 317 vs. female 316; blood donor control subjects, male 123 vs. female 108). The relationship between autoantibody titer and age is shown as a composite scattergram (
Fig. 2). Although this result suggests that the prevalence of factor H autoantibodies increases with age, there was no relationship between age and autoantibody titer in the individual groups (AMD patients,
r 2 = 0.090,
P = 0.375; age-matched control subjects,
r 2 = −0.086,
P = 0.401; blood donor control subjects,
r 2 = 0.034,
P = 0.739). The threshold for determining autoantibody positivity in the AMD patients was calculated in two ways. First, we used the mean antibody titer +2SD from the blood donor control subjects. This is the method that most groups,
16,17 including our own,
33 have used, but it does not take into account the non-normal distribution. The mean antibody titer +2SD in the blood donor control subjects was 810 RU, and accounting for individual sample variance, a value over 900 RU was taken as indicative of the presence of a complement factor H autoantibody. In both the AMD patients and the age-matched control subjects, there were eight individuals with an autoantibody titer greater than this threshold. In the blood donor control group, there was only one. Second, to take into account the non-normal distribution of antibody titer in all three groups, we used the 0.975 fractile, as recommended by the International Federation of Clinical Chemistry,
34 of the blood donor control subjects and derived a threshold of 624 RU. With this threshold, there would be 21 age-matched control subjects and 10 AMD patients who were autoantibody positive. This frequency is significantly different between the two groups (χ
2 = 4.895,
df = 1,
P = 0.027). The two thresholds are shown in
Figure 1.