Differential Diagnosis for Low C8 and C3 with Positive MCTD and Sjögren's
- Single Most Likely Diagnosis
- Mixed Connective Tissue Disease (MCTD): This is the most likely diagnosis given the positive MCTD and the presence of low C8 and C3, which can indicate immune complex deposition and consumption of complement components, a common feature in MCTD.
- Other Likely Diagnoses
- Sjögren's Syndrome: Although mentioned as part of the initial presentation, Sjögren's can coexist with or mimic other autoimmune diseases. The low C8 and C3 levels could be seen in active Sjögren's, especially if there's systemic involvement.
- Systemic Lupus Erythematosus (SLE): SLE can present with low complement levels (including C3 and C8) due to immune complex formation and consumption. The presence of positive MCTD and Sjögren's antibodies might also be seen in SLE due to its overlapping nature with other autoimmune diseases.
- Do Not Miss Diagnoses
- Infectious Diseases (e.g., bacterial endocarditis, hepatitis B or C): These conditions can cause low complement levels and mimic autoimmune diseases. Missing these diagnoses could lead to inappropriate treatment and severe consequences.
- Post-Streptococcal Glomerulonephritis: Although less common in adults, this condition can cause low C3 levels and should be considered, especially if there's a history of recent streptococcal infection.
- Rare Diagnoses
- C8 Deficiency: A rare primary immunodeficiency that could lead to recurrent infections, particularly with Neisseria species. This would be an unusual cause of isolated low C8 but should be considered if other explanations are ruled out.
- Atypical Hemolytic Uremic Syndrome (aHUS): This condition can cause low C3 levels due to alternative pathway dysregulation. It's a rare but critical diagnosis to consider due to its severe prognosis and specific treatment options.