Detection of BLyS/APRIL HTs by using a bead-based HT immunoassay. The assay has a broad detection range (~25 ng/ml to ~100 pg/ml; see standard curve, inset), with an LOQ of ~100-313 pg/ml in the presence of serum (EC50~6 ng/ml). The average serum sample size required is 25 μl, and the total assay time is 2 hours. The assay does not detect BLyS or APRIL homotrimers. APRIL, a proliferation-inducing ligand; BLyS, B-lymphocyte stimulator; EC50, 50% effective concentration; HT, heterotrimer; LOQ, limit of quantitation.