Differential Diagnosis for Night Time Fevers in a Patient on Antibiotics with Possible Rheumatologic Cause

Single Most Likely Diagnosis

• Drug Fever: This is a common cause of fever in patients on antibiotics, where the antibiotic itself causes an allergic reaction, leading to fever. The fact that the patient is already on antibiotics makes this a highly plausible explanation for the nighttime fevers.

Other Likely Diagnoses

• Incomplete Treatment of Infection: If the antibiotics are not fully effective against the causative pathogen, or if the course of treatment is not completed, the infection may persist, causing ongoing fevers.
• Superinfection: The use of antibiotics can lead to the overgrowth of resistant bacteria or fungi, causing a new infection that presents with fever.
• Systemic Lupus Erythematosus (SLE): A rheumatologic condition that can cause periodic fevers, among other symptoms like joint pain and skin rashes. Nighttime fevers could be part of the disease's fluctuating course.
• Rheumatoid Arthritis (RA): Although less commonly associated with fever as a primary symptom, severe cases of RA can present with systemic symptoms including fever, particularly at night.

Do Not Miss Diagnoses

• Endocarditis: An infection of the heart valves that can cause persistent fever, despite antibiotic treatment. It's crucial to consider this diagnosis due to its high mortality rate if left untreated.
• Abscess Formation: An undrained abscess can continue to cause fever despite appropriate antibiotic coverage. Identifying and draining the abscess is essential for resolution.
• Lymphoma: Certain types of lymphoma can present with recurring fevers (among other symptoms like night sweats and weight loss), known as B symptoms. Although less common, missing this diagnosis could have severe consequences.

Rare Diagnoses

• Adult-Onset Still's Disease (AOSD): A rare inflammatory disorder that can cause high fevers, typically once a day, along with a salmon-colored rash and joint pain.
• Relapsing Polychondritis: An autoimmune disorder that can cause inflammation in various cartilaginous structures of the body, sometimes presenting with fever among other symptoms.
• Familial Mediterranean Fever: A genetic disorder that causes recurring episodes of painful inflammation, often accompanied by fever. It's more common in certain ethnic groups but should be considered in patients with a suggestive family history.

Each of these diagnoses has a different set of implications for treatment and patient outcomes, emphasizing the importance of a thorough diagnostic workup to determine the underlying cause of nighttime fevers in a patient on antibiotics.