TSPO and major depressive disorder: However, studies using positron emission tomography (PET) of translocator protein (TSPO), a widely used in vivo measure of microglial activation, have found no correlation between serum CRP levels and PET binding of TSPO, in both subjects with MDD and controls [51, 52], although TSPO expression may reflect other non-inflammatory conditions and its diagnostic value has been questioned [53].