Differential Diagnosis

The patient's presentation of intermittent fever, loose stools, and unresponsiveness to antibiotics, along with laboratory findings of hyperferritinemia, hepatocellular injury, and a low likelihood of bacterial infection, suggests a range of potential diagnoses. These can be categorized as follows:

• Single Most Likely Diagnosis

  • Hemophagocytic Lymphohistiocytosis (HLH): The combination of fever, hyperferritinemia (ferritin 1089 ng/mL), hepatocellular injury (elevated ALT and LDH), and a significant inflammatory response (CRP 18 mg/L, ESR 24 mm/hr) in the absence of a clear bacterial infection (low procalcitonin) points towards HLH. This condition is characterized by an excessive and uncontrolled immune response, often triggered by infections, autoimmune disorders, or malignancies.

• Other Likely Diagnoses

  • Autoimmune Hepatitis: The presence of hepatocellular injury (ALT 162 U/L) and hyperferritinemia, along with a lack of response to antibiotics, could suggest an autoimmune process affecting the liver.
  • Lymphoma: Given the lymphocytosis (77% lymphocytes) and the presence of systemic symptoms like fever, lymphoma should be considered, especially if there's an underlying malignancy triggering HLH.
  • Systemic Juvenile Idiopathic Arthritis (sJIA): Although less common in adults, sJIA can present with fever, rash, and systemic inflammation, including hyperferritinemia, and should be considered, especially if other autoimmune markers become positive.

• Do Not Miss Diagnoses

  • Tuberculosis (TB): Despite the lack of cough or lymphadenopathy, TB can present atypically, especially in immunocompromised individuals, and can cause fever, hepatomegaly, and elevated inflammatory markers.
  • Endocarditis: Although the patient does not have the typical presentation of endocarditis (e.g., heart murmur, embolic phenomena), atypical presentations can occur, and this diagnosis would be critical not to miss due to its high mortality if untreated.
  • Sepsis of Unidentified Source: The patient's fever and elevated inflammatory markers could still represent a bacterial infection not responsive to the given antibiotics, necessitating a thorough search for the source of infection.

• Rare Diagnoses

  • Castleman Disease: A rare lymphoproliferative disorder that can present with fever, lymphadenopathy (not present in this case), and systemic inflammation, including hyperferritinemia.
  • Familial Hemophagocytic Lymphohistiocytosis: If there's a family history of similar illnesses, genetic forms of HLH should be considered, although the patient's age makes this less likely to be the primary diagnosis.

Each of these diagnoses requires careful consideration of the patient's clinical presentation, laboratory findings, and response to initial treatments to guide further diagnostic testing and management.