The invention relates to a method for detection of an analyte in a test sample
by a specific binding reaction among the analyte, a specific binding partner for
the analyte, and an (immuno)reactant provided with a label, characterized in that
the label is a lanthanide ion-ligand complex wherein the lanthanide ion is neodymium(III)
ion (Nd3+), ytterbium(III) ion (Yb3+), or erbium(III) ion
(Er3+) and the ligand comprises or is in contact with a sensitizing
moiety which absorbs in the 400-1000 nm region, and preferably in the 400-800 nm
region. Further, a diagnostic kit is disclosed as well as a method of detecting
an analyte in a matrix of biomedical interest through an oligonucleotide, an antigen,
or an antibody attached to a material, preferably core-shell latex or with specific
binding sites wherein the antigen or antibody is labeled with the lanthanide ion-ligand
complex and brought into contact with the analyte, after which the analyte with
the lanthanide-ion complex is immobilized on the material, and, optionally, residual
lanthanide-ion complex is removed, after which the sample obtained is irradiated
with light in the 400-1000 nm region, and the emitted light from said sample is
detected if said analyte is present in the matrix of biomedical interest.