When should you have blood tests after a tick bite? - briefly
Test at 2–3 weeks after the bite to detect early infection, and repeat at 6–8 weeks if symptoms develop or the initial result is inconclusive.
When should you have blood tests after a tick bite? - in detail
A tick attachment creates a window for pathogen transmission that varies by species. The earliest laboratory evaluation should occur as soon as the bite is recognized if the tick is known to be a vector for serious infection or if the patient belongs to a high‑risk group (immunocompromised, pregnant, young children). An initial blood sample establishes a baseline for later comparison and can detect early circulating organisms through polymerase chain reaction (PCR) or antigen assays.
If the bite was from a nymph or adult Ixodes scapularis in an endemic area, the following schedule is recommended:
- Day 0–3: Baseline serology (IgM/IgG) and PCR for Borrelia burgdorferi, Anaplasma phagocytophilum, and Babesia microti. PCR is most sensitive before antibodies appear.
- Week 2–4: Repeat serology for Lyme disease; IgM may become positive. Consider a second PCR if the first was negative but symptoms have emerged.
- Week 6–12: Convalescent serology to assess seroconversion. A fourfold rise in IgG confirms Lyme infection. Additional testing for Rickettsia rickettsii (Rocky Mountain spotted fever) or Ehrlichia chaffeensis is advisable if fever, rash, or laboratory abnormalities persist.
- Month 3–6: For persistent or relapsing symptoms, repeat PCR or culture for Babesia and consider quantitative PCR to monitor parasite load. Repeat serology for any previously negative but clinically suspected infections.
Testing intervals may be adjusted based on clinical presentation:
- Acute febrile illness within days of the bite warrants immediate PCR for bacterial agents and a complete blood count with differential to detect leukopenia or thrombocytopenia.
- Rash, arthralgia, or neurologic signs appearing after one week should trigger serologic testing for Lyme disease and a lumbar puncture if meningitis is suspected; cerebrospinal fluid analysis can be supplemented with serum‑CSF antibody ratios.
- Late manifestations (e.g., migratory arthritis, cardiac conduction disturbances) require serology at 6–12 weeks, as antibodies typically peak during this period.
High‑risk exposures (multiple bites, prolonged attachment >24 h, residence in hyperendemic zones) justify a more aggressive schedule, adding weekly PCR for the first month and earlier serologic follow‑up at 10‑14 days.
Laboratory interpretation must consider the window period: PCR is reliable before antibodies develop; serology gains sensitivity after 2–3 weeks. Negative results early in the course do not exclude infection and should be repeated according to the timeline above.