An in vitro method of simultaneously detecting total alpha-synuclein and S129 phosphorylated alpha-synuclein, comprising: a) contacting a sample with: (i) a donor complex comprising an antibody that specifically binds to alpha-synuclein linked to a support and a donor label, which is light excitable to activate oxygen to produce singlet oxygen; (ii) a first acceptor complex for detecting pS129 alpha-synuclein comprising an antibody that binds to pS129 alpha-synuclein, wherein the antibody has an association constant at least five times higher for pS129 alpha-synuclein than for unphosphorylated alpha-synuclein, linked to a support and a first acceptor label; wherein if pS129 alpha-synuclein is present in the sample, the antibodies of the donor complex and first acceptor complex bind to the pS129 alpha-synuclein forming a first sandwich in which the donor label is light excitable to produce singlet oxygen to activate the first acceptor label to produce a first signal; (iii) a second acceptor complex for detecting total alpha-synuclein comprising an antibody that binds to alpha-synuclein regardless of S129 phosphorylation, the antibody being linked to a support and a second acceptor label; wherein if total alpha-synuclein is present in the sample, the antibodies of the donor complex and second acceptor complex bind to total alpha-synuclein forming a second sandwich in which the donor label is light excitable to produce singlet oxygen to activate the second acceptor label to produce a second signal; (b) exciting the donor label with a light to activate oxygen to produce singlet oxygen, thereby producing the first signal if pS129 alpha-synuclein is present and the second signal if total alpha-synuclein is present; and (c) detecting the first and second signals, if any, to indicate presence of pS129 alpha-synuclein and total alpha synuclein, respectively. a) contacting a sample with: (i) a donor complex comprising an antibody that specifically binds to alpha-synuclein linked to a support and a donor label, which is light excitable to activate oxygen to produce singlet oxygen; (ii) a first acceptor complex for detecting pS129 alpha-synuclein comprising an antibody that binds to pS129 alpha-synuclein, wherein the antibody has an association constant at least five times higher for pS129 alpha-synuclein than for unphosphorylated alpha-synuclein, linked to a support and a first acceptor label; wherein if pS129 alpha-synuclein is present in the sample, the antibodies of the donor complex and first acceptor complex bind to the pS129 alpha-synuclein forming a first sandwich in which the donor label is light excitable to produce singlet oxygen to activate the first acceptor label to produce a first signal; (iii) a second acceptor complex for detecting total alpha-synuclein comprising an antibody that binds to alpha-synuclein regardless of S129 phosphorylation, the antibody being linked to a support and a second acceptor label; wherein if total alpha-synuclein is present in the sample, the antibodies of the donor complex and second acceptor complex bind to total alpha-synuclein forming a second sandwich in which the donor label is light excitable to produce singlet oxygen to activate the second acceptor label to produce a second signal; (i) a donor complex comprising an antibody that specifically binds to alpha-synuclein linked to a support and a donor label, which is light excitable to activate oxygen to produce singlet oxygen; (ii) a first acceptor complex for detecting pS129 alpha-synuclein comprising an antibody that binds to pS129 alpha-synuclein, wherein the antibody has an association constant at least five times higher for pS129 alpha-synuclein than for unphosphorylated alpha-synuclein, linked to a support and a first acceptor label; wherein if pS129 alpha-synuclein is present in the sample, the antibodies of the donor complex and first acceptor complex bind to the pS129 alpha-synuclein forming a first sandwich in which the donor label is light excitable to produce singlet oxygen to activate the first acceptor label to produce a first signal; (iii) a second acceptor complex for detecting total alpha-synuclein comprising an antibody that binds to alpha-synuclein regardless of S129 phosphorylation, the antibody being linked to a support and a second acceptor label; wherein if total alpha-synuclein is present in the sample, the antibodies of the donor complex and second acceptor complex bind to total alpha-synuclein forming a second sandwich in which the donor label is light excitable to produce singlet oxygen to activate the second acceptor label to produce a second signal; (b) exciting the donor label with a light to activate oxygen to produce singlet oxygen, thereby producing the first signal if pS129 alpha-synuclein is present and the second signal if total alpha-synuclein is present; and (c) detecting the first and second signals, if any, to indicate presence of pS129 alpha-synuclein and total alpha synuclein, respectively.