The measurement of the avidity of cytomegalovirus (CMV) immunoglobulin G (IgG) antibodies has been shown by several investigators to be useful in identifying and excluding primary CMV infections in pregnant women. In this work, we examined the diagnostic utility of reflex testing of CMV IgM-positive specimens from pregnant women by using a CMV IgG avidity assay. The utility of this approach was directly dependent on the sensitivity of the CMV IgM assay employed during the initial screen. The higher initial reactivity rate of the AxSYM CMV IgM assay was necessary in order to detect CMV IgM in specimens containing low-avidity CMV IgG antibodies, indicative of a primary CMV infection, which other CMV IgM assays (Behring, Vidas, Captia, and Eurogenetics) fail to detect in some cases. The use of the AxSYM CMV IgM assay, followed by an avidity test, should result in more accurate diagnosis of CMV infection in pregnant women.Human cytomegalovirus (CMV) is a herpesvirus which is ubiquitously distributed in the human population. CMV is the most common cause of congenital infection, occurring in approximately 1% of all live births (1,3,5,9,21). Since CMV infections in immunocompetent individuals and pregnant women are asymptomatic or accompanied by symptoms not specific for CMV, laboratory methods are needed to diagnose CMV infection. In the absence of seroconversion, CMV-specific immunoglobulin M (IgM) is a sensitive and specific indicator of active or recent CMV infection (2,4,17,19,20). However, the presence of CMV IgM is not a specific indicator of primary CMV infection as it is often produced during nonprimary infections (2, 10, 18). Recently, the measurement of the CMV IgG avidity index has been shown to be useful in identifying and excluding primary CMV infections in pregnant women with no pregestational CMV serology (6,8,13,14,15). Detection of low-avidity CMV IgG in specimens from pregnant women indicates that primary CMV infection has occurred within the past 18 to 20 weeks, whereas detection of high-avidity CMV IgG excludes primary infection (13). In this work, we evaluated the performance of the AxSYM CMV IgM assay in conjunction with other CMV IgM assays and examined the diagnostic utility of reflex testing of CMV IgM positive specimens from pregnant women with a CMV IgG avidity assay.