Differential Diagnosis

The patient's presentation is complex, with chronic and acute components. The differential diagnosis can be organized into the following categories:

• Single Most Likely Diagnosis
  • Rheumatoid Arthritis (RA) with Felty's Syndrome: This diagnosis explains the polyarthritis, high ESR, reversal of A/G ratio (indicative of chronic inflammation and possible autoimmune process), and microcytic anemia. Felty's syndrome, characterized by the triad of RA, splenomegaly, and neutropenia, could also account for the systemic inflammation and potentially the sepsis-like presentation due to neutropenia.
• Other Likely Diagnoses
  • Systemic Lupus Erythematosus (SLE): Given the systemic inflammation, high ESR, hypergammaglobulinemia, and polyarthritis, SLE is a strong consideration. The presence of bilateral upper lobe fibrosis could be consistent with chronic inflammation seen in SLE.
  • Sjögren's Syndrome: This autoimmune disorder can cause systemic inflammation, polyarthritis, and interstitial lung disease (ILD), which could explain the bilateral upper lobe fibrosis.
  • Mixed Connective Tissue Disease (MCTD): Overlapping features of different autoimmune diseases, including polyarthritis, ILD, and high inflammatory markers, make MCTD a possible diagnosis.
• Do Not Miss Diagnoses
  • Sepsis: Despite the CRP being only mildly elevated, the clinical presentation of hypotension requiring noradrenaline is highly suggestive of sepsis or septic shock. The source of sepsis could be occult and needs to be thoroughly investigated.
  • Lymphoma: Given the chronic inflammation, hypergammaglobulinemia, and potential for anemia of chronic disease, lymphoma should be considered, especially if there are other signs such as lymphadenopathy or splenomegaly.
• Rare Diagnoses
  • Adult-Onset Still's Disease (AOSD): Characterized by high fevers, polyarthritis, and elevated inflammatory markers, AOSD could be considered, though it is less likely given the specific pattern of lung involvement and the absence of characteristic fever patterns.
  • Relapsing Polychondritis: This rare autoimmune disorder can cause inflammation of cartilaginous structures, including the lungs, and could potentially explain some of the patient's symptoms, though it is less likely given the overall clinical picture.

Each of these diagnoses requires careful consideration of the patient's clinical presentation, laboratory findings, and potentially additional diagnostic testing to confirm or rule out each condition.