Differential Diagnosis

The patient's presentation suggests an autoimmune disease with several key laboratory findings. Here's a breakdown of potential diagnoses:

• Single most likely diagnosis

  • Systemic Lupus Erythematosus (SLE): High ENA (extractable nuclear antigen) levels, particularly ENA6, are associated with SLE. The presence of high kappa light chains free in serum, along with high normal lambda light chains and a high normal kappa/lambda ratio, can be seen in various autoimmune conditions, including SLE. Low vitamin B6 and potassium levels can also be found in SLE due to the disease's impact on multiple systems and potential renal involvement.

• Other Likely diagnoses

  • Sjögren's Syndrome: This autoimmune disorder can present with high ENA levels and abnormalities in light chain production. It often coexists with other autoimmune diseases like SLE.
  • Rheumatoid Arthritis (RA): While RA is more commonly associated with rheumatoid factor and anti-CCP antibodies, some patients may have elevated ENA levels. The abnormal light chain levels could be indicative of a monoclonal gammopathy, which can occur in RA.
  • Mixed Connective Tissue Disease (MCTD): MCTD combines features of SLE, scleroderma, and polymyositis/dermatomyositis, and patients often have high ENA levels, particularly anti-RNP antibodies.

• Do Not Miss

  • Multiple Myeloma: Although less likely given the clinical context, multiple myeloma must be considered due to the abnormal light chain levels. The high kappa free light chains and the kappa/lambda ratio could indicate a clonal plasma cell disorder. Missing this diagnosis could be fatal due to the aggressive nature of myeloma.
  • Amyloid Light-chain (AL) Amyloidosis: This condition, associated with plasma cell dyscrasias, can present with renal failure, which might explain the low potassium and potentially the low vitamin B6 levels. It's crucial not to miss this diagnosis due to its poor prognosis if left untreated.

• Rare diagnoses

  • Light Chain Amyloidosis with Sjögren's Syndrome or SLE: In rare cases, patients with autoimmune diseases like SLE or Sjögren's can develop light chain amyloidosis, which would explain the abnormal light chain levels.
  • Monoclonal Gammopathy of Undetermined Significance (MGUS): MGUS is a precursor condition to multiple myeloma and other plasma cell disorders, characterized by the presence of a monoclonal protein (M protein) in the blood. It could be considered in the differential due to the abnormal light chain levels, although it's less likely given the autoimmune disease context.