What blood test should be ordered after a tick bite? - briefly
The recommended test is a two‑tier Lyme disease serology, beginning with an ELISA screen and confirming any positive result with a Western blot.
What blood test should be ordered after a tick bite? - in detail
After a bite from an ixodid arthropod, the clinician must select laboratory assays that correspond to the pathogens most likely to be transmitted in the area and to the patient’s clinical picture. The core panel includes:
- Enzyme‑linked immunosorbent assay (ELISA) for Borrelia burgdorferi antibodies, performed 2–4 weeks after exposure. A positive result requires confirmation with a Western blot (IgM and IgG bands) to distinguish early infection from past exposure.
- Polymerase chain reaction (PCR) for Babesia microti DNA in whole blood, indicated when hemolytic anemia or fever is present. PCR is preferred to microscopy because it detects low‑level parasitemia.
- PCR for Anaplasma phagocytophilum and Ehrlichia chaffeensis in peripheral blood, ordered when leukopenia, thrombocytopenia, or elevated transaminases are observed. Results become reliable within 3–5 days of symptom onset.
- Indirect immunofluorescence assay (IFA) for Rocky Mountain spotted fever rickettsial antibodies, collected in acute and convalescent phases (7–14 days apart) to demonstrate seroconversion.
- IgM/IgG ELISA for tick‑borne encephalitis virus, applicable in endemic regions, with a confirmatory neutralization test if initial screening is positive.
Additional supportive tests:
- Complete blood count with differential to identify leukopenia or thrombocytopenia.
- Liver function panel (ALT, AST) and lactate dehydrogenase, which may be elevated in anaplasmosis or ehrlichiosis.
- Serum creatinine and electrolytes to monitor renal involvement in severe infections.
Timing considerations:
- Acute‑phase specimens (within 1 week of bite) are useful for PCR and for detecting early IgM responses.
- Convalescent samples (2–4 weeks later) are required for serologic confirmation of Lyme disease and rickettsial infections.
- Re‑testing after 4–6 weeks may be necessary if initial results are negative but clinical suspicion persists.
Interpretation must account for regional tick species, exposure duration, and symptom chronology. Positive PCR indicates active infection; serology reflects current or past exposure and requires paired samples to confirm recent disease.