The amount of a phosphatidylinositol-3-phosphate phosphatase MTMR7 when measured in blood serum. The trait is level of myotubularin-related protein 7 in blood serum (Ontology of Biological Attributes identifier OBA_2042454). Also known as: blood serum phosphatidylinositol-3-phosphate phosphatase MTMR7 amount.