- “B” Confirmation Procedure  for Non-Threshold Substances  and exogenous Threshold Substances  For Non-Threshold Substances  (including those with Minimum Reporting Level s as specified in the TD MRPL)  and exogenous Threshold Substances , the “B” Sample  results shall only confirm the presence of the Prohibited S ubstance (s) or its Metabolite(s)  or Marker(s)  identified in the  “A” Sample  (in compliance with the TD IDCR) for the Adverse Analytical Finding  to be valid 16.