The amount of a phosphatidylinositol-3-phosphate phosphatase MTMR1 when measured in blood serum. The trait is level of myotubularin-related protein 1 in blood serum (OBA_2042452, an Ontology of Biological Attributes id). Also known as: blood serum phosphatidylinositol-3-phosphate phosphatase MTMR1 amount.