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Background <p>Systemic juvenile idiopathic arthritis (sJIA), a severe autoinflammatory subtype of JIA, presents diagnostic challenges due to non-specific clinical features and the absence of definitive biomarkers. Current ILAR classification criteria emphasize exclusion of mimicking conditions, yet misdiagnosis rates persist (20%-35%), particularly in early disease stages.</p> Case presentation <p>A 15-year-old female presented with persistent high-grade fever, anemia (Hb: 84→64→73→91&#xa0;g/L), thrombocytosis (PLT 592→1,044 × 10⁹/L), and iron overload. Initial evaluations excluded infections (CMV-positive, treated) but revealed hypocellular marrow (30%) and a polyclonal hypergammaglobulinemia. Naproxen-responsive fever, generalized lymphadenopathy (PET/CT) and evanescent rash suggested autoinflammatory etiology. Hyperinflammation (CRP 265→311&#xa0;mg/L, ferritin 1,083→1,271&#xa0;µg/L) and elevated IL-6 (272 ng/L) emerged, with bone marrow ruling out malignancy (initially suspected MGUS based on the presence of M protein bands in serum protein electrophoresis and immunofixation electrophoresis). Final diagnosis aligned with systemic juvenile idiopathic arthritis (sJIA) per ILAR criteria, supported by rash, arthralgia, and IL-6 elevation. Tocilizumab rapidly resolved fever and inflammation. Ongoing multidisciplinary monitoring continues.</p> Conclusions <p>This case report highlights a pediatric sJIA patient exhibiting atypical monoclonal (M) protein expression—a finding not typically associated with sJIA—that complicated diagnostic evaluation. It illustrates how hematological abnormalities may complicate immunophenotypic interpretation in JIA, potentially contributing to diagnostic delays.</p>

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Diagnostic challenges of systemic juvenile idiopathic arthritis with aberrant M-protein expression: a case report

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年4月2日

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  • Shanshan Liang,
  • Wenliu Yu,
  • Peiying Zhong,
  • Chuanmin Tao,
  • Chengyao Jia,
  • Li Zhang

摘要

Background

Systemic juvenile idiopathic arthritis (sJIA), a severe autoinflammatory subtype of JIA, presents diagnostic challenges due to non-specific clinical features and the absence of definitive biomarkers. Current ILAR classification criteria emphasize exclusion of mimicking conditions, yet misdiagnosis rates persist (20%-35%), particularly in early disease stages.

Case presentation

A 15-year-old female presented with persistent high-grade fever, anemia (Hb: 84→64→73→91 g/L), thrombocytosis (PLT 592→1,044 × 10⁹/L), and iron overload. Initial evaluations excluded infections (CMV-positive, treated) but revealed hypocellular marrow (30%) and a polyclonal hypergammaglobulinemia. Naproxen-responsive fever, generalized lymphadenopathy (PET/CT) and evanescent rash suggested autoinflammatory etiology. Hyperinflammation (CRP 265→311 mg/L, ferritin 1,083→1,271 µg/L) and elevated IL-6 (272 ng/L) emerged, with bone marrow ruling out malignancy (initially suspected MGUS based on the presence of M protein bands in serum protein electrophoresis and immunofixation electrophoresis). Final diagnosis aligned with systemic juvenile idiopathic arthritis (sJIA) per ILAR criteria, supported by rash, arthralgia, and IL-6 elevation. Tocilizumab rapidly resolved fever and inflammation. Ongoing multidisciplinary monitoring continues.

Conclusions

This case report highlights a pediatric sJIA patient exhibiting atypical monoclonal (M) protein expression—a finding not typically associated with sJIA—that complicated diagnostic evaluation. It illustrates how hematological abnormalities may complicate immunophenotypic interpretation in JIA, potentially contributing to diagnostic delays.