How many days after a tick bite should a blood test be performed? - briefly
Blood testing is generally recommended no sooner than 2 weeks after the bite, with most protocols advising sampling at 3–4 weeks to allow antibodies to develop. Earlier testing may yield false‑negative results.
How many days after a tick bite should a blood test be performed? - in detail
After a tick attachment, serologic testing is most reliable once the immune response has had time to develop. Antibodies to the causative spirochete typically appear 2‑3 weeks after exposure; testing earlier yields a high false‑negative rate.
Recommended timing
- Day 0‑7: No blood work is advised. Clinical observation is preferred; symptoms such as erythema migrans may appear, allowing immediate treatment without laboratory confirmation.
- Day 8‑14: Early serology may be performed, but sensitivity remains low (≈30‑40 %). Results should be interpreted with caution and combined with clinical findings.
- Day 15‑21: Antibody levels rise sharply. ELISA screening reaches ≥70 % sensitivity, making this the earliest window for a reasonably reliable test.
- Day 22 and beyond: Sensitivity exceeds 90 % for both ELISA and confirmatory Western blot. Testing at this stage provides the most accurate serostatus.
Test selection
- ELISA (enzyme‑linked immunosorbent assay): First‑line screening; detects IgM (early) and IgG (later) antibodies.
- Western blot: Confirmatory assay; required when ELISA is positive. IgM bands are interpreted only if drawn ≤30 days after exposure; IgG bands are valid at any later point.
Special considerations
- If prophylactic antibiotics are administered within 72 hours of removal, serology may be suppressed; retesting after 4 weeks is advisable.
- In immunocompromised patients, antibody production may be delayed; extending the interval to 4‑6 weeks improves detection.
- Persistent symptoms without serologic evidence may warrant polymerase chain reaction (PCR) testing of joint fluid or skin biopsy, particularly in later disease stages.
In summary, the earliest reliable serologic assessment occurs around the third week post‑bite, with optimal accuracy achieved after three weeks and continuing improvement thereafter. Testing before this period should be reserved for cases where immediate clinical decision‑making outweighs the risk of false‑negative results.