Proteinuria is a key to suspect autoimmune nodopathies

Abstract Background and purpose Reports of patients who have autoimmune nodopathies concurrent with nephrotic syndrome are increasing. We investigated whether proteinuria could be a biomarker of autoimmune nodopathies. Methods Qualitative urinalysis results were retrospectively obtained from 69 patients who were diagnosed with chronic inflammatory demyelinating polyneuropathy (CIDP) at a hospital in Japan. Proteinuria was graded as mild to severe (i.e., mild, 30–99; moderate, 100–299; severe, 300 mg/dL or more) according to the results of the urine dipstick test. Autoantibodies against the paranodal proteins contactin 1 (CNTN1), neurofascin 155 (NF155), and contactin‐associated protein 1 (Caspr1) and the nodal protein neurofascin 186 (NF186) were measured, and the predominant IgG subclass was determined by enzyme‐linked immunosorbent assay in sera from the 69 patients. Results Four patients (6%), five patients (7%), and one (1%) patient were positive for anti‐CNTN1, anti‐NF155, and anti‐Caspr1 IgG4 antibodies, respectively. No patients had IgG4 antibodies against NF186. Proteinuria of mild or greater levels was found in three patients with anti‐CNTN1 IgG4 and two patients with anti‐NF155 IgG4 antibodies. The autoantibody‐positive patients more frequently had proteinuria of mild or greater levels than the seronegative patients ( p  = 0.01). Conclusions Proteinuria is a possible biomarker of autoimmune nodopathies associated with autoantibodies targeting CNTN1 or NF155. Urinalysis results should be carefully checked for quick differentiation of autoimmune nodopathies from CIDP. Patients who present with nephrotic syndrome should be tested for anti‐CNTN1 IgG4 antibodies, and patients who exhibit mild proteinuria should be tested for anti‐NF155 IgG4 antibodies.