MMP9 and Pseudoxanthoma elasticum: Although correlations of MMP-9 and MMP-2 between serum and plasma samples exist in patients with gestational hypertension and periodontal disease [23] (but they are obviously unknown for pseudoxanthoma elasticum patients), comparative measurements in other patient groups [24] showed that the high unspecific "background" concentration of MMP-9 in serum obviously was not related to the true pathological process of interest, thus impairing the potential diagnostic performance of MMP-9 biochemical evaluation [24].