Recent Developments for Diagnosis of Toxoplasmosis

There are four groups of individuals in whom the diagnosis of toxoplasmosis is most critical: pregnant women who acquire their infection during gestation, fetuses and newborns who are congenitally infected, immunocompromised patients, and those with chorioretinitis (14, 20, 25). Although the diagnosis of patients in each of these four groups has been hampered by a number of problems, the most frequent challenge encountered by physicians the world over is how to determine if a pregnant woman acquired the acute infection during gestation. Women who acquired their infection prior to pregnancy are essentially not at risk for delivering an infected infant (unless the woman is immunosuppressed). In the United States there is no systematic screening of pregnant women to detect seroconversion during gestation. Much of the literature is based on studies from France, where such screening is performed monthly to detect recently acquired infection. Thus, in the United States, a single serum sample from each woman is submitted for evaluation, and from this sample the physician hopes to learn if the patient has recently been infected, thereby placing the fetus at risk. The high prevalence and lifelong persistence of toxoplasma immunoglobulin G (IgG) antibodies among healthy individuals in many geographical areas preclude the use of any titer in any serologic test as reflecting recent infection. Another problem is the frequent lack of reliability when IgM, IgA, or IgE toxoplasma antibody test results are used in an attempt to discriminate between recent and more distant infection. In addition there are the vagaries associated with lack of quality control, specificity, and/or sensitivity of many commercial serologic test kits on the market. In this setting, physicians and other health care workers responsible for the care of pregnant women are confused when faced with conflicting results and disagreements about interpretations of results. This often leads to incorrect information being provided by the laboratories to the physicians as well as by the physicians to their patients. In recent years, a major effort has been made toward improving our ability to diagnose recently acquired infection in the pregnant woman and congenital infection in the fetus and newborn. We now have a number of new methods that are proving to be of great value towards this end. Among these are the serum IgG avidity test, PCR with body fluids and tissues, and Western blots of serum from mother-baby pairs. At present, the first two methods are being widely used in Europe. Hopefully they will become more readily available in the United States as well. Although our focus here is on pregnant women and newborns, these methods are finding wide use in the other groups of patients alluded to above. Our purpose here is to review each of these methods, what is known about their proper interpretation, and their strengths and shortcomings.