Differential Diagnosis for a 50-year-old Female with Lymphocytosis, Normal WBC, and a Rash

Single Most Likely Diagnosis

• Lyme Disease: Given the rash resembling that of Lyme disease, which is often characterized by a distinctive "bull's-eye" appearance, and the presence of lymphocytosis, Lyme disease is a strong consideration. The patient's lymphocyte count is elevated (lymphocytes 4), which can occur in response to infections like Lyme disease.

Other Likely Diagnoses

• Tinea Corporis: As mentioned, the rash could resemble tinea corporis, a fungal infection of the skin. While the laboratory findings might not directly point to tinea corporis, the clinical presentation of the rash could suggest this diagnosis.
• Insect Bite Reaction: Some insect bites can cause significant skin reactions that might resemble Lyme disease or tinea corporis. The presence of lymphocytosis could be a response to the bite.
• Viral Exanthem: Various viral infections can cause rashes and lymphocytosis. The normal WBC count with a relative lymphocytosis might suggest a viral etiology.

Do Not Miss Diagnoses

• Southern Tick-Associated Rash Illness (STARI): Although less common than Lyme disease, STARI can present with a similar rash and should be considered, especially if the patient has been in areas where the Lone Star tick is prevalent.
• Rocky Mountain Spotted Fever (RMSF): This tick-borne illness can present with a rash and fever. While the laboratory findings might not specifically indicate RMSF, missing this diagnosis could be fatal due to its potential for severe complications.
• Ehrlichiosis/Anaplasmosis: These are tick-borne diseases that can cause fever, rash, and abnormalities in blood cell counts, including lymphocytosis. They are critical to diagnose due to their potential severity.

Rare Diagnoses

• Babesiosis: A parasitic disease transmitted by ticks, babesiosis can cause hematological abnormalities, although it more commonly presents with anemia and thrombocytopenia rather than isolated lymphocytosis.
• Mycoplasma pneumoniae Infection: While primarily known for causing respiratory infections, Mycoplasma pneumoniae can occasionally cause skin manifestations, including rashes, and might lead to lymphocytosis in some cases.
• Leptospirosis: This bacterial infection can cause a wide range of symptoms, including rash and lymphocytosis, especially in its early stages. However, it is less common and typically associated with specific exposures (e.g., contact with contaminated water).