What test should be taken after a tick bite? - briefly
After a tick bite, the standard diagnostic is a two‑tier serologic test for Borrelia burgdorferi—initial ELISA screening followed by confirmatory Western blot—performed if the bite occurred at least two weeks earlier or symptoms develop. Additional PCR or blood‑smear investigations may be added when other tick‑borne infections are suspected.
What test should be taken after a tick bite? - in detail
A bite from an engorged tick obliges a clinician to initiate a focused diagnostic work‑up. The first step is to identify the tick species, the estimated duration of attachment, and any immediate local reaction. This information guides the selection of laboratory investigations.
For the most common pathogen transmitted by Ixodes ticks, serologic screening is the cornerstone. The recommended sequence includes:
- Enzyme‑linked immunosorbent assay (ELISA) for IgM and IgG antibodies against Borrelia burgdorferi.
- Reflex confirmatory Western blot if ELISA is positive or equivocal.
- Polymerase chain reaction (PCR) on skin biopsy or blood when early disseminated disease is suspected, especially before seroconversion.
Additional pathogens frequently encountered in tick‑bite exposures require separate assays:
- Anaplasma phagocytophilum: quantitative PCR on whole blood; alternatively, indirect immunofluorescence assay (IFA) for IgG/IgM.
- Ehrlichia chaffeensis: PCR on blood; IFA serology for acute and convalescent samples.
- Babesia microti: thick and thin blood smears examined for intra‑erythrocytic parasites; PCR for confirmation.
- Rickettsia species (e.g., Rocky Mountain spotted fever): PCR on whole blood or skin biopsy; IFA serology with paired acute and convalescent titres.
Timing of specimen collection influences test sensitivity:
- Early localized infection (≤7 days): PCR and skin biopsy provide the highest yield; serology often remains negative.
- Early disseminated phase (7–30 days): ELISA may become positive; repeat testing after 2–4 weeks confirms seroconversion.
- Late disease (>30 days): ELISA and Western blot are reliable; PCR sensitivity declines.
Interpretation must consider clinical context. A positive ELISA/WB confirms Lyme disease, whereas a negative result early in the course does not exclude infection; repeat testing is warranted. Positive PCR for Anaplasma, Ehrlichia, or Babesia establishes active infection and directs antimicrobial therapy. In the absence of symptoms, a single serologic test is insufficient; clinicians should schedule a convalescent sample to detect rising titres.
Overall, the diagnostic algorithm after a tick exposure combines species‑specific PCR, targeted serology, and, when indicated, microscopic examination, with timing adjusted to the likely stage of infection. This approach maximizes detection of tick‑borne pathogens and informs appropriate treatment decisions.