What blood test should be taken if bitten by a tick? - briefly
The standard evaluation is a serologic test for Borrelia burgdorferi, usually an ELISA with confirmatory Western blot if the initial result is positive. Testing is typically performed 2–4 weeks after the bite to allow antibody development.
What blood test should be taken if bitten by a tick? - in detail
A tick bite introduces the possibility of several infectious agents that may be detectable only through laboratory analysis. Immediate assessment includes a thorough history of exposure, the interval since the bite, and any emerging symptoms such as fever, rash, or joint pain.
Testing is divided into two temporal phases. During the first week after attachment, direct detection methods are preferred because antibody responses may not yet be measurable. After two to three weeks, serological assays become reliable for most tick‑borne pathogens.
The recommended panel comprises:
- Polymerase chain reaction (PCR) on whole blood for early identification of Borrelia burgdorferi, the causative agent of Lyme disease, and for Babesia microti if hemolytic anemia is suspected.
- Enzyme‑linked immunosorbent assay (ELISA) for IgM and IgG antibodies against Borrelia, followed by a confirmatory Western blot if ELISA results are positive.
- Indirect immunofluorescence assay (IFA) for antibodies to Anaplasma phagocytophilum and Ehrlichia chaffeensis, useful when leukopenia or thrombocytopenia is present.
- Serology for Rickettsia rickettsii if a rash consistent with spotted fever appears.
- Complete blood count with differential to detect cytopenias indicative of Babesia or ehrlichial infection.
- Liver function tests to monitor hepatic involvement in severe cases.
Interpretation guidelines:
- A positive PCR result confirms active infection, regardless of serology.
- Serological positivity for Borrelia requires both ELISA and Western blot criteria; isolated IgM without IgG may reflect early infection or false positivity.
- Rising antibody titers between acute (≤1 week) and convalescent (≥4 weeks) samples support recent exposure for Anaplasma, Ehrlichia, and Rickettsia.
- Negative results in the early phase do not exclude infection; repeat testing after the appropriate interval is advised.
Follow‑up testing should be scheduled at 2–4 weeks post‑exposure to capture seroconversion, and again at 6–12 weeks if clinical suspicion persists. Prompt initiation of antimicrobial therapy is guided by the specific pathogen identified, with doxycycline serving as the first‑line agent for most bacterial tick‑borne diseases.