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Clinicians often rely on laboratory testing to find evidence of infections that may explain a patient’s symptoms and help guide treatment. Direct evidence may be a specific segment of DNA or a protein that confirms the presence of the target pathogen. Indirect evidence could be antibodies and other immune biomarkers that suggest recent or past exposure to a pathogen. Galaxy offers an immunofluorescence assay (IFA) for Bartonella that provides indirect evidence of infection, resulting in what is called an “antibody titer”.
The immune system defends us from harmful microbes, or pathogens, that cause damage when their presence goes unchecked. Specialized white blood cells respond to the pathogen by recognizing a specific protein, or antigen, on its surface and using that information to mount a complex immune response. Inflammatory cells, antibodies, and signaling molecules are all important components of this response.
The immunofluorescence assay (IFA) that Galaxy offers is one method for detecting and measuring this host immune response to a pathogen. More precisely, it determines the concentration in a patient’s blood serum at a single point in time of IgG antibodies that are specific to Bartonella henselae and Bartonella quintana antigens. The result is called an antibody titer.
Antibody titers are calculated through a series of dilutions (also called “titrations”) of the test sample. Each dilution is assessed for antibody reactivity by observing whether there is fluorescence under a specialized microscope. If there are detectable antibodies present in the dilution, fluorescent green spots will appear on a slide prepared with antigens from a target pathogen. The highest dilution where reactivity, or fluorescence, can still be observed is the resulting antibody titer for the patient. This antibody titer may be considered positive or negative for the pathogen depending on the reference value. The reference value is the titer that you would expect in a population that has not been exposed to the target pathogen.
What does a positive antibody titer mean?
Interpreting the results of an antibody titer can be complex. (Find out more at our blog post The Basics of Serology.)
A positive result only confirms that there are detectable antibodies to a pathogen present in the sample. This could mean the infection was recent and the patient is currently fighting it, or the pathogen is no longer present but antibodies are still lingering from past exposure. If the antibody titer is high enough, the clinician may assume the infection is active. For example, guidelines stipulate that acute cat scratch disease can be considered active if the patient has a titer equal to or higher than 1:256. It is always important that a clinical diagnosis is made in the context of the patient’s medical history, exposure risk, other laboratory results, and current symptoms.
References
Klotz, S. A. et al. (2011). Cat-scratch disease. American Family Physician, 83(2), 152-155. https://www.aafp.org/afp/2011/0115/p152.html