How long after a tick bite should blood be tested? - briefly
Blood is generally drawn 2–4 weeks after the bite to allow antibodies to develop; testing sooner may produce false‑negative results.
How long after a tick bite should blood be tested? - in detail
Blood sampling after a tick attachment must consider the pathogen’s incubation period and the diagnostic method’s sensitivity.
For Lyme disease, antibodies typically appear 2–4 weeks post‑exposure. An initial enzyme‑linked immunosorbent assay (ELISA) performed earlier than 14 days often yields a negative result; a repeat test at 4–6 weeks is recommended if early symptoms are present. Polymerase chain reaction (PCR) on blood is rarely positive in early Lyme disease but may detect Borrelia DNA in later disseminated infection; sampling after the third week improves yield.
Anaplasma phagocytophilum and Ehrlichia chaffeensis produce detectable IgM antibodies within 7–10 days. A single acute‑phase specimen taken within the first week should be paired with a convalescent sample collected 2–3 weeks later to demonstrate seroconversion. PCR on whole blood is most sensitive during the first 10 days of illness; testing after that window reduces detection probability.
Babesia microti generates parasitemia detectable by thick‑smear microscopy or PCR as early as 5 days after the bite, persisting for weeks. A single test performed at symptom onset is usually sufficient; if negative and clinical suspicion remains, repeat testing after 7–10 days is advisable.
Rocky Mountain spotted fever, caused by Rickettsia rickettsii, rarely produces a positive serology before day 7. An acute specimen should be drawn at presentation, with a convalescent draw 2–4 weeks later to confirm a four‑fold rise in IgG titer. PCR from blood or skin biopsies is most reliable during the first week of illness.
Practical schedule for laboratory evaluation after a tick bite:
- Day 0–3: Collect blood if severe systemic symptoms appear; PCR for Anaplasma/Ehrlichia and Babesia may be positive.
- Day 7–10: Perform PCR for Borrelia, Anaplasma, Ehrlichia; consider early IgM serology for Anaplasma/Ehrlichia.
- Day 14–21: First reliable Lyme ELISA; repeat if initial test negative and exposure risk persists.
- Day 21–28: Convalescent serology for Lyme, Anaplasma, Ehrlichia; repeat Babesia PCR if initial test negative and symptoms continue.
- Day 28–42: Second Lyme serology to confirm seroconversion; final convalescent IgG for Rocky Mountain spotted fever.
Testing earlier than the specified windows may miss serologic conversion or reduce molecular assay sensitivity, leading to false‑negative results. Repeating assays at the recommended intervals ensures accurate detection of tick‑borne infections.