Appreciating the differences between immunoassays used to diagnose maternal parvovirus B19 infection: understanding the antigen before interpreting the results

Human parvovirus B19 (B19) capsid contains two structural proteins, VP1 and VP2. During infection, immune-competent individuals produce antibodies against both linear and conformational epitopes of VP1 and VP2. However, the emergence and duration of these antibodies differ; those recognizing linear epitopes appear and decline before those made against conformational epitopes. The platforms most commonly used for detecting B19-specific immunoglobin M or immunoglobin G include enzyme immunoassays, immunofluorescent assays, and Western blots. Because suitable culture systems do not exist to produce sufficient native virus, manufacturers must rely on recombinant proteins to detect B19-specific antibodies. These proteins are most commonly expressed in Escherichia coli or baculovirus-based vectors, as linear or conformational antigens, respectively. Differences in timing of the antibodies being produced against linear and conformational epitopes can lead to differences in immunoassay performance. Using an immunoassay with a high degree of sensitivity, specificity, and positive and negative predictive values can provide an accurate picture of a patient’s immune status. This is critical for the physician faced with making decisions on the extent to which follow-up care, including fetal ultrasound, is necessary. Not only are these additional tests and procedures inconvenient to the patient, but they also add unnecessary expense to an already overburdened healthcare system. Ultimately, a serologic assay that produces far fewer inaccurate results will be more cost effective.