A blood filtration device encapsulates a filter pad assembly Within a flexible housing. The filter pad assembly includes first and second filter media layers. A first heat and pressure sealed region integrally bonds the peripheries of the first and second media layers together. The flexible housing comprises a peripheral rim aligned with the first heat and pressure sealed region of the first media layer. A second heat and pressure sealed region, formed after the first heat and pressure sealed region, joins the rim of the flexible housing to the first heat and pressure sealed region, thereby forming a composite seal that encapsulates the filter pad assembly within the housing.