What is the role of an indirect immunofluorescent antibody (IFA) assay in the workup of babesiosis?

Updated: Apr 01, 2021
  • Author: Rachel E Strength, MD; Chief Editor: Michael Stuart Bronze, MD  more...
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Indirect immunofluorescent antibody (IFA) assays of immunoglobulin M (IgM) or immunoglobulin G (IgG) B. microti titers can aid in diagnosis of babesiosis.  However, current IDSA guidelines recommend confirmatory testing for babesiosis with a blood smear or PCR. [3] . An IgM titer of 1:64 or greater is usually considered positive, while a titer of 1:32 or less could indicate prior infection. IgG Babesia titers of 1:1024 or greater typically suggest active or recent infection. [7]   A four-fold increase in Babesia IgG titer from the the time of symptom onset to the time of symptom resolution or improvement can aid in confirming the diagnosis of babesiosis. Higher titers do not necessarily indicate more severe infection.

Note that serologic studies that test for B. microti do not detect infections due to other species of Babesia (eg, B. divergens, B. bovis, B. duncani, and B. gibsoni) due to antigenic differences. [3]  If testing for B. microti is negative but suspicion for babesiosis remains high, consider testing for other strains that are endemic to areas where the patient has traveled. 

IFA for B. microti detects antibodies in 88-96% of patients with B. microti infection. These antibodies can persist for over a year regardless of whether a patient has had treatment. This can make diagnosis of acute babesiosis more difficult. If a patient has a positive Babesia IFA with negative PCR and/or blood smear, treatment is not recommended since active infection is unlikely. [3]  

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