How long after a tick bite should blood be drawn for Lyme disease testing? - briefly
Blood should be drawn roughly 2–4 weeks after the bite to permit reliable antibody formation; testing sooner than one week often yields false‑negative results.
How long after a tick bite should blood be drawn for Lyme disease testing? - in detail
The interval between a tick exposure and the collection of a blood specimen for Lyme disease diagnosis is determined by the kinetics of the immune response and the sensitivity of available assays.
During the first days after an attachment, Borrelia burgdorferi DNA may be detectable in blood by polymerase chain reaction (PCR). PCR sensitivity is highest within 1–2 weeks post‑exposure but declines rapidly as spirochetes migrate to tissues. Consequently, a PCR test performed before day 7 can identify early infection, but a negative result does not exclude disease because bacteremia is transient.
Serologic testing relies on the production of specific IgM and IgG antibodies. IgM antibodies typically become measurable 2–3 weeks after the bite, reaching peak levels around weeks 3–4. IgG antibodies appear later, usually after week 4, and persist for months. Therefore:
- Days 0–7: PCR may be useful if early dissemination is suspected; serology is usually negative.
- Days 8–14: PCR sensitivity begins to wane; IgM may start to rise but often remains below detection thresholds.
- Days 15–21: IgM antibodies become reliably detectable; a two‑tier serologic algorithm (ELISA followed by Western blot) is recommended.
- Day 28 and beyond: Both IgM and IgG are likely present; a two‑tier test provides the highest diagnostic accuracy.
Guidelines from the Centers for Disease Control and Prevention advise collecting a specimen for serology no earlier than 2 weeks after the bite when a rash or systemic symptoms are present, and at least 4 weeks after exposure for routine screening. If early treatment is initiated based on clinical judgment, repeat testing after 4 weeks can confirm seroconversion.
In summary, the optimal timing for blood sampling is:
- PCR: ≤ 7 days post‑bite, only in cases of suspected early disseminated infection.
- Serology (ELISA + Western blot): ≥ 14 days for IgM detection; ≥ 28 days for combined IgM/IgG confirmation.
Adjustments may be needed for immunocompromised patients, delayed symptom onset, or atypical presentations. Re‑testing after an initial negative result is advisable when clinical suspicion persists.