Differential Diagnosis for Chronically Declining Kidney Function

The patient's complex clinical picture, including a history of Sjögren's syndrome, frequent pneumonia infections, long-term mildly decreased WBC, and long-term high serum protein, necessitates a comprehensive differential diagnosis. The following categories outline potential causes for the patient's declining kidney function:

• Single Most Likely Diagnosis

  • Sjögren's Syndrome-associated Interstitial Nephritis: This is a common renal manifestation of Sjögren's syndrome, where chronic inflammation leads to interstitial nephritis, explaining the decline in kidney function. The patient's existing diagnosis of Sjögren's makes this a highly plausible cause.

• Other Likely Diagnoses

  • Hypertension: Uncontrolled hypertension is a common cause of chronic kidney disease, and even if not previously diagnosed, it could explain the decline in renal function.
  • Diabetes Mellitus: Undiagnosed or poorly controlled diabetes can lead to diabetic nephropathy, causing a decline in kidney function over time.
  • Chronic Pyelonephritis: Recurrent or chronic infections of the kidney can lead to scarring and a decline in renal function, especially if the patient has a history of urinary tract infections.
  • Analgesic Nephropathy: Long-term use of certain analgesics, such as NSAIDs, can cause chronic kidney damage, particularly in patients with Sjögren's syndrome who may use these medications for pain management.

• Do Not Miss Diagnoses

  • Vasculitis (e.g., ANCA-associated vasculitis): Although less common, vasculitis can cause rapid decline in kidney function and is associated with significant morbidity and mortality if not promptly treated.
  • Lupus Nephritis: Given the patient's autoimmune background with Sjögren's, the possibility of systemic lupus erythematosus (SLE) with renal involvement should be considered, as it can cause significant kidney damage.
  • Amyloidosis: This condition, which can be primary or secondary to chronic diseases like Sjögren's, involves the deposition of amyloid proteins in the kidneys, leading to their dysfunction.

• Rare Diagnoses

  • Tubulointerstitial Nephritis and Uveitis Syndrome (TINU): A rare condition characterized by the combination of tubulointerstitial nephritis and uveitis, which could be considered in the differential given the patient's Sjögren's syndrome.
  • Light Chain Deposition Disease: A condition associated with the deposition of light chain immunoglobulins in the kidney, leading to renal dysfunction, particularly if there's suspicion of a plasma cell dyscrasia.
  • Sarcoidosis: Although rare, sarcoidosis can affect the kidneys and cause a decline in function, and might be considered if other systemic symptoms suggestive of sarcoidosis are present.
  • Multiple Myeloma: Given the patient's long-term high serum protein, multiple myeloma should be considered as it can lead to renal impairment through various mechanisms, including light chain deposition disease.
  • Immunodeficiency: The patient's history of frequent pneumonia infections and mildly decreased WBC count could suggest an underlying immunodeficiency, which might be contributing to the kidney dysfunction, possibly through recurrent infections or an underlying condition affecting both the immune system and the kidneys.

Given the complexity of the patient's condition, it would be advisable to refer her to a Nephrologist for further evaluation and management of her declining kidney function. Additionally, considering the potential for underlying immunodeficiency and the presence of Sjögren's syndrome, consultation with a Rheumatologist and an Immunologist may also be beneficial to fully address the patient's condition.