Case Report: Autoimmune-associated tubulointerstitial nephritis with predominant Sjögren features in a child presenting with recurrent sterile pyuria.

Tubulointerstitial nephritis (TIN) is an important cause of renal tubular dysfunction in children but may be misdiagnosed as urinary tract infection (UTI) because of non-specific urinary manifestations. Autoimmune-associated TIN is particularly challenging when overlapping autoimmune features are present. Here, we report a pediatric case of TIN with an autoimmune-associated phenotype initially presenting as recurrent culture-negative urinary symptoms suggestive of UTI. A 5-year-old boy presented with recurrent urinary frequency, persistent pyuria, and negative urine cultures. Renal ultrasonography revealed hydronephrosis and bladder abnormalities, while renal scintigraphy suggested pyelonephritis. Despite antibiotic therapy, the urinary abnormalities persisted. Further evaluation showed polyuria, polydipsia, and hyposthenuria, indicating tubular concentrating impairment. A renal biopsy demonstrated severe tubulointerstitial inflammation with tubular injury, mild glomerular changes, negative immunofluorescence, and absence of electron-dense deposits. An immunological evaluation revealed high-titer antinuclear antibodies, anti-double-stranded DNA antibody positivity, elevated rheumatoid factor, and hypergammaglobulinemia, whereas the results of anti-Sjögren syndrome-related antigen A antibody and anti-Sjögren syndrome--related antigen B antibody tests were negative. Salivary gland abnormalities were identified by ultrasonography, while a labial biopsy showed chronic inflammation without diagnostic focal lymphocytic sialadenitis. The patient did not fulfill the 2016 ACR/EULAR classification criteria for Sjögren syndrome, and systemic lupus erythematosus remained insufficiently supported as a definitive diagnosis. Based on comprehensive evaluation, the case was considered autoimmune-associated TIN with predominant Sjögren features. Glucocorticoid and immunosuppressive therapy led to the resolution of urinary abnormalities. This case highlights the need to consider autoimmune-associated TIN in children with recurrent culture-negative urinary symptoms and persistent pyuria.