Brief Report: Spuriously Low Serum IgG4 Concentrations Caused by the Prozone Phenomenon in Patients With IgG4‐Related Disease

To determine the frequency of the prozone effect in patients with IgG4-related disease (IgG4-RD).After identifying the prozone effect in an index patient with IgG4-RD, we examined additional samples to determine the frequency of this phenomenon. Thirty-eight serum samples obtained from patients with IgG4-RD whose results had been reported previously were retested. The serum IgG4 concentrations determined by this repeat analysis were compared...