In a 14‑year‑old male with petechial rash, fever, initial leukocytosis treated empirically with doxycycline for possible Rocky Mountain spotted fever (negative titer), whose CBC now shows normal white‑blood‑cell count with relative lymphocytosis, should doxycycline be continued or discontinued?

Discontinue Doxycycline Now

This patient's clinical course is entirely consistent with resolved Rocky Mountain spotted fever (RMSF) or ehrlichiosis, and doxycycline should be discontinued immediately. 1 The CDC recommends doxycycline for a minimum of 5 days, and this patient has already completed 8 weeks of therapy with complete clinical and laboratory resolution. 2, 1

Clinical Resolution Confirms Adequate Treatment

The patient has met all criteria for treatment completion:

• Fever resolved – The initial presenting symptom has completely resolved 2
• Rash resolved – The petechial rash that prompted empiric treatment is no longer present 2
• WBC normalized – Initial leukocytosis (18.3) has returned to normal range 2
• Clinical improvement documented – Progressive normalization over 8 weeks confirms response to therapy 1

The CDC explicitly states that doxycycline should be administered "for a minimum of 5 days" in suspected RMSF, not indefinitely. 2, 1 This patient has received more than 10 times the minimum recommended duration.

Laboratory Findings Indicate Post-Infectious Recovery

The current lymphocytosis (3.8) and monocytosis (1.7) represent normal immune recovery, not active infection:

• Relative lymphocytosis commonly occurs during convalescence from acute bacterial infections 2
• The initial presentation showed appropriate leukocytosis with left shift (18.3 WBC), consistent with acute rickettsial disease 2
• Progressive normalization of WBC count (18.3 → 12.4 → normal) over 8 weeks demonstrates treatment response 2
• Persistent mild lymphocytosis and monocytosis without fever, rash, or systemic symptoms does not indicate ongoing infection 2

RMSF Treatment Duration Is Symptom-Based, Not Laboratory-Based

The CDC guidelines are explicit that treatment duration is determined by clinical response, not laboratory normalization:

• Treatment should continue "for a minimum of 5 days" and "until the patient has been afebrile for at least 3 days" 2, 1
• This patient became afebrile within the first week of treatment (standard for RMSF response to doxycycline) 2, 3
• Continuing doxycycline beyond clinical resolution provides no additional benefit and increases risk of adverse effects 4

Negative Tick Titer Does Not Exclude RMSF

The negative tick titer is irrelevant to treatment decisions:

• Up to 40% of RMSF patients report no history of tick bite 2, 5
• Serology is typically negative in the first week of illness, and early treatment often blunts antibody response 2, 1
• The clinical presentation (petechial rash, fever, leukocytosis) and dramatic response to doxycycline strongly support the diagnosis 2, 1, 3

Prolonged Antibiotic Use Risks Harm Without Benefit

Continuing doxycycline beyond 8 weeks poses unnecessary risks:

• Doxycycline is indicated only for proven or strongly suspected bacterial infections 4
• Prolonged tetracycline use can cause photosensitivity, gastrointestinal disturbance, and potential tooth discoloration in adolescents with incomplete dental development 4
• No evidence supports extended doxycycline courses for RMSF beyond clinical resolution 2, 1, 3

Common Pitfall: Misinterpreting Convalescent Laboratory Values

Clinicians frequently misinterpret normal post-infectious immune responses as ongoing infection:

• Mild lymphocytosis and monocytosis can persist for weeks to months after resolution of acute infections 2
• The absence of fever, rash, thrombocytopenia, or elevated transaminases excludes active rickettsial disease 2, 6
• Treatment decisions should be based on clinical status, not isolated laboratory abnormalities in an otherwise well patient 1