Congenital syphilis represents an important public health challenge in the United States, and its prevalence has been increasing for the past 10 years because of many factors. The diagnosis can be difficult given its various and nonspecific clinical manifestations in newborns, and the possibility of false negative results during prenatal care. The prozone phenomenon, caused by an excess of antibody, which interferes with the regular screening tests, is a cause of false negative tests. This could delay the diagnosis and increase morbidity and mortality in the newborn. We present a case of congenital syphilis in a 3-month-old infant whose mother had prenatal care and negative tests for syphilis, which contributed to the late diagnosis. In the face of clinical findings suggestive of congenital syphilis and negative maternal syphilis tests healthcare providers should consider the possibility of maternal false negative test caused by the prozone phenomenon.
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