How should a blood test be performed after a tick bite? - briefly
Obtain a venous sample 2–4 weeks post‑exposure, screen with an ELISA for Borrelia‑specific IgM/IgG, and confirm any positive result by Western blot. Use the same protocol for subsequent monitoring if symptoms develop.
How should a blood test be performed after a tick bite? - in detail
A laboratory evaluation after a tick attachment must follow a strict protocol to ensure reliable results and early detection of vector‑borne infections.
The decision to order serologic testing depends on the duration of attachment, geographic prevalence of disease, and the presence of clinical signs such as erythema migrans, fever, or arthralgia. If exposure exceeds 36 hours or the patient resides in an endemic area, a baseline blood sample should be obtained as soon as possible, ideally within 2–4 weeks of the bite.
Specimen collection
- Use a standard vacutainer system with a serum separator tube (SST) for antibody assays; an EDTA tube is required for PCR and complete blood count.
- Draw at least 5 mL of blood for serology and an additional 3 mL for molecular testing.
- Label each tube with patient identifier, date, time of draw, and test request.
- Gently invert SST tubes 5–10 times to mix clot activator; avoid vigorous shaking of anticoagulated tubes.
Timing of draw
- Initial serology: collect 2–4 weeks post‑exposure to allow IgM and IgG antibodies to become detectable.
- Follow‑up serology: repeat after 4–6 weeks if the first result is negative but clinical suspicion remains.
- PCR: can be performed as early as the day of presentation, especially for early disseminated disease.
Laboratory assays
- First‑line: enzyme‑linked immunosorbent assay (ELISA) for IgM and IgG antibodies against Borrelia burgdorferi.
- Confirmatory: Western blot performed on any ELISA result that meets or exceeds the positive threshold.
- Supplemental tests: polymerase chain reaction (PCR) on whole blood or skin biopsy for early infection; complete blood count, liver function tests, and inflammatory markers (CRP, ESR) to assess systemic involvement.
Specimen transport and storage
- Keep SST tubes at room temperature; process within 2 hours to separate serum, then refrigerate serum at 2–8 °C if testing is delayed beyond 24 hours.
- EDTA tubes for PCR should be kept at 4 °C and processed within 24 hours; freeze plasma at –20 °C for longer storage.
- Use insulated containers with ice packs for transport over distances exceeding 30 minutes.
Result interpretation
- Positive ELISA with confirmatory Western blot indicates established infection; isolated IgM positivity before 3 weeks may represent early disease.
- Negative serology in the first 2 weeks does not exclude infection; repeat testing is required.
- PCR positivity supports early diagnosis, especially when serology is indeterminate.
- Elevated liver enzymes, thrombocytopenia, or leukocytosis may suggest co‑infection with other tick‑borne pathogens and warrant additional testing.
Adhering to these steps maximizes diagnostic accuracy, facilitates timely treatment, and reduces the risk of complications associated with tick‑transmitted illnesses.