PROS1 and rheumatoid arthritis: After a booster dose, levels of antibodies raised in both HC and IA, but significantly more in HC (median 1693 RU/ml) compared to all IA patients (median 1227 RU/ml, p=0.004), with the lowest levels of anti-S protein IgG antibodies in RA patients (median 1040 RU/ml, p=0.016).