4.0 Assay Pre -Analytical Procedure − The Laboratory  should (usually) receive refrigerated (not frozeni) “A” and “B” blood Samples , which have been collected in blood tubes containing an inert polymeric serum separator  gel and a clotting activation factor (for example: BD Vacutainer® SSTTM-II Plus  tubes,  EU ref 367955; BD Vacutainer® SSTTM-II Plus Advance tubes, EU ref 367954; BD Vacutainer® SSTTM tubes, US ref 367986 ) in accordance with the International Standard  for Testing and Investigation (ISTI) 16;   [Comment : Previous studies have demonstrated that IGF -I and P -III-NP concentrations remain stable if the Sample is maintained at a refrigerated temperature for up to 5 days 17.]