For endogenous Threshold Substances , Markers of the “steroid profile”, or any other Prohibited Substance  that may be produced endogenously at low levels, Adverse Analytical Finding  decisions for the “A” Sample  may also be based on the applic ation of any Fit-for-Purpose  Confirmation Procedure  that establishes the exogenous origin of the Prohibited Substance  or its Metabolite (s) or Marker (s) (e.g.