Differential Diagnosis for Worsening Bilateral Flank Pain

Given the patient's recent diagnosis of a urinary tract infection (UTI) and the initiation of Macrobid (nitrofurantoin), with subsequent resolution of dysuria and frequency but worsening bilateral flank pain, the following differential diagnoses are considered:

• Single Most Likely Diagnosis

  • Pyelonephritis: This is a likely complication of an untreated or inadequately treated UTI, especially if the bacteria have ascended to the kidneys. The worsening flank pain, particularly if it is more severe on one side, could indicate an upper urinary tract infection involving the kidneys.

• Other Likely Diagnoses

  • Kidney Stones (Nephrolithiasis): The pain from kidney stones can be severe and is often described as flank pain that can radiate to the groin. The fact that the UTI symptoms have resolved but flank pain persists or worsens could suggest a stone, especially if it is obstructing the flow of urine.
  • Obstructive Uropathy: This condition, which can be caused by kidney stones, blood clots, or other obstructions, leads to a buildup of urine and increased pressure within the kidneys, resulting in flank pain.

• Do Not Miss Diagnoses

  • Sepsis: Although less likely if the patient is otherwise stable and the UTI symptoms have improved, sepsis is a potentially life-threatening condition that can arise from an untreated or inadequately treated infection. Severe flank pain could be a sign of a systemic infection.
  • Emphysematous Pyelonephritis: A rare but serious infection of the kidney where gas forms in the kidney tissue. It typically occurs in diabetic patients and can present with severe flank pain and systemic symptoms.

• Rare Diagnoses

  • Papillary Necrosis: A condition where part of the kidney tissue dies due to a lack of blood supply. It can be associated with analgesic abuse, diabetes, or severe pyelonephritis and presents with flank pain and possibly hematuria.
  • Xanthogranulomatous Pyelonephritis: A rare form of chronic pyelonephritis characterized by the destruction of kidney tissue and the presence of granulomatous tissue. It often presents with flank pain, recurrent infections, and a non-functioning kidney on the affected side.