An independent blood filter device depends on flow geometry to deliver blood serum or plasma free of detrimental levels of hemoglobin. It depends critically on an upstream flow rate or pressure differential limiting control element or device that limits the rate of change of pressure differential across the filter element. Pre-evacuated versions can be used to simultaneously draw blood from a living being and provide pressure differential across the filter element between an evacuated collector and a supply end open to atmosphere. A unit pressurized by hand motion employs the external shape of a partially filled blood collection tube as a piston to produce pressure in advance of the control element or device to create the pressure differential across the filter element to a collector vented to atmosphere. The control element or device is disclosed in numerous forms, including specially sized flow constrictions and compliant arrangements.
