New therapeutic proteins that trap circulating members of the transforming 
growth factor (TGF) beta superfamily (activins and growth differentiation 
factors) show promising effects on erythropoiesis and muscular growth. They are 
dimeric recombinant fusion proteins composed of the extracellular domain of a 
human activin receptor (ActRIIA or IIB) linked to the Fc part of human IgG1. 
Sotatercept (ActRIIA-Fc) and Luspatercept (a modified ActRIIB-Fc) in particular 
are now in phase 2/3 of clinical trials against anemia and included in the 
prohibited list established by the World Anti-Doping Agency. To prevent a 
potential misuse by athletes in the near future, a robust and sensitive method 
of detection is needed. We validated an approach adapted from an electrophoretic 
method used for the detection of recombinant erythropoietins that allowed 
detection of various ActRIIA-Fc and ActRIIB-Fc proteins, including variants 
produced in different cell types, after a single immuno-extraction step. After 
separation by sodium dodecyl sulfate polyacrylamide gel electrophoresis 
(SDS-PAGE), an initial testing procedure performed by single-blotting can 
indicate the presence of an ActRII-Fc (indifferently type IIA or IIB). A 
confirmation performed by double-blotting using different antibodies for 
detection allows a more precise identification of the type of ActRII-Fc (IIA, 
IIB). Starting from a few hundred microliters of serum or plasma, this method is 
specific, sensitive, and easy to perform. It could easily be adopted by 
anti-doping laboratories.