When should you do a blood test after a tick bite? - briefly
A serologic test for tick‑borne infections should be performed 2–4 weeks after exposure, especially if the tick was attached for more than 36 hours or if any rash, fever, or joint pain appears. Testing earlier rarely yields reliable results.
When should you do a blood test after a tick bite? - in detail
A serologic evaluation for tick‑borne infections should not be performed immediately after the bite because most antibodies require time to develop. The first blood draw is typically scheduled 2–4 weeks after exposure, which allows detectable IgM and IgG responses for diseases such as Lyme borreliosis. If the initial result is negative and the patient remains asymptomatic, a second sample is often taken 6–12 weeks post‑exposure to capture any delayed seroconversion.
When clinical signs appear before the 2‑week window—fever, rash, arthralgia, or neurological symptoms—diagnostic testing should be initiated without delay. In such cases, molecular methods (PCR) for pathogens like Babesia, Anaplasma, or Ehrlichia are preferred, as they can identify the organism before antibodies are present.
Specific situations that modify the timing include:
- Prophylactic antibiotics: If a single dose of doxycycline is administered within 72 hours of the bite, baseline serology is still recommended at 2–4 weeks, with a follow-up at 6–12 weeks if symptoms develop.
- Pregnancy or immunosuppression: Earlier testing (around 1 week) may be considered, using PCR or culture, because delayed antibody production is common.
- Multiple or prolonged exposures: For individuals with repeated bites, a baseline sample is taken after the most recent exposure, and subsequent testing follows the same 2–4 week and 6–12‑week schedule.
Laboratory techniques differ by pathogen:
- Enzyme‑linked immunosorbent assay (ELISA) for initial screening of Lyme disease antibodies.
- Western blot for confirmation of positive ELISA results.
- Polymerase chain reaction (PCR) for direct detection of DNA from Babesia, Anaplasma, Ehrlichia, and early Lyme disease in skin or blood specimens.
- Culture is rarely used but may be required for certain rickettsial infections.
In practice, a clinician should document the date of the bite, assess for early symptoms, decide on prophylactic treatment, and arrange the first blood draw at the 2‑ to 4‑week mark. A repeat sample is ordered if the initial test is negative and the patient later develops signs consistent with tick‑borne illness, ensuring that late seroconversion is not missed.