Immunoglobulin M indirect-fluorescent antibody test for the diagnosis of acute toxoplasmosis during pregnancy in the avidity era: A 14-year experience at the Tuscany Reference Center for Infectious Diseases in Pregnancy, Florence, Italy Michele Trotta1 ,

Toxoplasmosis is a zoonosis, caused by Toxoplasma gondii. T. gondii is a protozoal parasite ubiquitous in nature and distributed worldwide. In recent years, the decrease in toxoplasmosis seroprevalence among populations of many industrialized countries has caused increased susceptibility among women of child-bearing age.1–5 Intrauterine transmission can occur if the infection is acquired during pregnancy.6 The risk and Received: October 15 2015. Accepted: July 17 2016. Correspondence: Dr Michele Trotta, SOD Malattie Infettive e Tropicali, Azienda Ospedaliero Universitaria Careggi, Largo Brambilla 3, 50134 Florence, Italy. Email: © 2016 Japan Society of Obstetrics and Gynecology 1 doi:10.1111/jog.13144 J. Obstet. Gynaecol. Res. 2016 severity of congenital infection depend on the time at which the mother has acquired the infection.6 Women infected before conception usually do not transmit toxoplasmosis to the fetus.6 On the other hand, the risk of transplacental infection is very low if the infection is acquired in the first gestational weeks (GW), but it increases during the course of pregnancy, reaching more than 60% in the third trimester.7 However, the earlier the transmission, the more serious the consequences tend to be.8 As the primary infection in pregnant women is not clinically recognized in the majority of cases, prenatal serological screening is adopted in many countries.9 Routine serological tests allow for the diagnosis of the infection and identify the gestational age of infection. Prenatal therapy is considered effective for reducing the vertical transmission and the incidence of clinical manifestations in infected infants.10,11 The diagnosis of T. gondii infection is commonly made by serological methods.6 The main marker for diagnosis of acute infection is the detection of specific immunoglobulin M (IgM) antibodies.6 However, IgM antibodies, measured in routine screening tests, such as immune-enzymatic tests, can persist for many months or even for many years.12 Therefore, the IgM screening test alone cannot always be considered a reliable marker of acute infection. Moreover, the detection of natural IgM is another cause of misdiagnosis of acute infection.13 An indirect-fluorescent antibody test (IFA) has been used in the past to detect IgM-specific antibodies considered markers of a toxoplasmosis infection acquired within the last 2–4 months.14–16 The IgG-avidity test, more recently introduced, assesses the strength of interaction between antibody and antigen and represents a very useful additional tool to determine more precisely the time of acquisition of the toxoplasmosis infection. Early infections are characterized by IgG antibodies with low avidity; on the contrary, an infection acquired by at least 16 GW is characterized by IgG antibodies with high avidity.17,18 The purpose of the present study was to investigate whether the IgM IFAT test is still useful for the diagnosis of T. gondii infection during pregnancy. Methods The study was carried out at the Reference Center of Tuscany for Infectious Diseases in Pregnancy (TRCIDP). TRCIDP, a referral outpatient public service, is located in a tertiary hospital in Florence, Tuscany. Pregnant women are sent to the TRCIDP from all over Tuscany, mainly from public and private gynecological outpatient services, for several infectious diseases. Concerning toxoplasmosis infection, the majority of women are referred to the TRCIDP following a positive screening test done elsewhere within the free antenatal screening program during the first trimester of pregnancy or at the first control (Ministerial Decree, 10 September 1998). The majority of these women attend the TRCIDP shortly after the screening test result has become available and before they are prescribed spiramycin. In this study, we retrospectively enrolled all of the women with T. gondii infection who were referred to our clinic because of a positive immune-enzymatic test against toxoplasmosis (IgM-positive and IgG-negative or IgM- and IgG-positive) between April 1997 and August 2012. At TRCIDP, all pregnant women were subjected to medical examination and underwent the specific serological test for T. gondii as follows: • Enzyme-linked fluorescent assay (ELFA) for IgG and IgM antibodies det