Why do we give "orphan" approval to biologic therapies but not to extracorporeal blood purification therapies for hyperinflammatory syndromes?

Why do we give "orphan" approval to biologic therapies but not to extracorporeal blood purification therapies for hyperinflammatory syndromes?
Minerva Anestesiol. 2019 May;85(5):465-467
Authors: Horvat CM, Carcillo JA
Abstract

PMID: 31017379 [PubMed - in process]

Source: Minerva Anestesiologica - April 26, 2019 Category: Anesthesiology Tags: Minerva Anestesiol Source Type: research