Antibody Detection
Diagnosis of Babesia infection should be made by detection of parasites in patients' blood smears.  However, antibody detection tests are useful for detecting infected individuals with very low levels of parasitemia (such as asymptomatic blood donors in transfusion-associated cases), for diagnosis after infection is cleared by therapy, and for discrimination between Plasmodium falciparum and Babesia infection in patients whose blood smear examinations are inconclusive and whose travel histories cannot exclude either parasite.

The indirect fluorescent antibody test (IFA) using B. microti parasites as antigen detects antibodies in 88-96% of patients with B. microti infection.  IFA antigen slides are prepared using washed, parasitized erythrocytes produced in hamsters.  Patients' titers generally rise to >1:1024 during the first weeks of illness and decline gradually over 6 months to titers of 1:16 to 1:256 but may remain detectable at low levels for a year or more.  Specificity is 100% in patients with other tick-borne diseases or persons not exposed to the parasite.  Cross-reactions may occur in serum specimens from patients with malaria infections, but generally titers are highest with the homologous antigen.

The extent of cross-reactivity between Babesia species is variable.  A negative result with B. microti antigen for a patient exposed on the West Coast may be a false-negative reaction for Babesia infection.  Individuals whose exposure could have occurred on the West Coast should be tested also for antibodies to the Babesia WA1 species, because of the lack of cross-reactivity with B. microti.

A

A: Positive IFA result with B. microti antigen.

Reference:

Krause PJ, Telford S RI, Ryan R, et al. Diagnosis of babesiosis: Evaluation of a serologic test for the detection of Babesia microti antibody. J Infect Dis 1994;169:923-926.