This code system http://hl7.org/fhir/v2/0372 defines the following codes:

CodeDisplay
BLDWhole blood, homogeneous
BSEPWhole blood, separated
PLASPlasma, NOS (not otherwise specified)
PPPPlatelet poor plasma
PRPPlatelet rich plasma
SEDSediment
SERSerum, NOS (not otherwise specified)
SUPSupernatant