The activity of HNMT, unlike that of DAO, cannot be measured by blood (serum) analysis.
Organs that produce DAO continuously release it into the bloodstream. DAO is stored in vesicular structures associated with the plasma membrane in epithelial cells. As a result, serum DAO activity can be measured, but not HNMT. This is because HNMT is primarily found within the cells of internal organs like the brain or liver and is not released to the bloodstream. Measuring intracellular HNMT directly is challenging. Therefore, diagnosis of HNMT activity is typically done indirectly by testing for known genetic variants.