Differential Diagnosis for Fever with Lymphocytosis

Single Most Likely Diagnosis

• Viral Infections (e.g., influenza, mononucleosis): These are common causes of fever and lymphocytosis, especially in the context of an upper respiratory infection or exposure to a viral illness.

Other Likely Diagnoses

• Tuberculosis: Can cause fever and lymphocytosis, particularly if the infection is disseminated or in its early stages.
• Lymphoma: Certain types of lymphoma, such as Hodgkin lymphoma, can present with fever and lymphocytosis, although lymphopenia or a normal lymphocyte count is more common.
• Infectious Mononucleosis (caused by Epstein-Barr Virus): Characterized by fever, sore throat, and lymphocytosis, often with atypical lymphocytes.

Do Not Miss Diagnoses

• HIV Infection: Acute HIV infection can present with fever and lymphocytosis, and it is crucial to diagnose early due to the implications for treatment and prevention of transmission.
• Severe Bacterial Infections (e.g., sepsis): Although more commonly associated with neutrophilia, certain bacterial infections can cause a relative lymphocytosis, especially in the early stages or in specific contexts like typhoid fever.
• Toxoplasmosis: Particularly in immunocompromised patients, toxoplasmosis can cause fever and lymphocytosis, and missing this diagnosis can have severe consequences.

Rare Diagnoses

• Brucellosis: A zoonotic infection that can cause fever, lymphocytosis, and other systemic symptoms, more common in individuals with occupational exposure to animals.
• Cytomegalovirus (CMV) Infection: Can cause fever and lymphocytosis, especially in immunocompromised individuals or congenitally infected newborns.
• Toxoplasmic Encephalitis: In immunocompromised patients, particularly those with HIV/AIDS, this condition can present with fever and lymphocytosis among other neurological symptoms.

Each of these diagnoses has a unique set of additional symptoms, risk factors, and diagnostic tests that can help in narrowing down the differential diagnosis. The approach to diagnosis should include a thorough history, physical examination, and appropriate laboratory tests based on the suspected cause.