Placental Transfer of IgG and IgG Subclass Antibodies Anti-Purified Escherichia coli LPS O16, O6 and O111

We evaluated 22 paired maternal and cord sera regarding the presence of IgG and IgG subclasses against purified Escherichia coli LPS O6, O16 and O111 employing ELISA for titre and avidity analysis, isoelectric focusing associated with affinity-blotting for spectrotypic analysis, and the Western-blotting technique for recognition of the various bands in lipopolysaccharide (LPS). Levels of anti-LPS IgG antibodies in cord sera were equivalent to their respective maternal sera, showing a significant correlation ( P  < 0.0001). IgG1 antibody levels were higher in cord sera than in maternal sera ( P  < 0.005 for anti-O111, P  < 0.05 for anti-O16 and P  < 0.02 for anti-O6). Cord IgG2 antibody levels were not different from the maternal levels ( P  > 0.1). The levels of IgG3 and IgG4 were undetectable. The avidity of anti-O6 and anti-O111 IgG in 10 cord sera showed an extremely significant correlation with maternal antibody avidity ( P  < 0.0001). Identical patterns of recognition were found in the paired samples analysed by Western blotting. Most of the serum samples recognized the O-repetitive chains and also the region corresponding to core and lipid A. Although the antibody spectrotypes varied among individuals, paired cord and maternal serum samples showed identical patterns. Our findings suggest the occurrence of placental transfer of IgG antibodies against LPS O6, O16 and O111, mainly involving the IgG1 or IgG2 subclasses.     

The use of cord serum and indirect viral immunofluorescence as a method of evaluating the immunological specificity of fluorescein-labelled anti-human IgA conjugate

Maternal and cord sera, obtained simultaneously, were compared by indirect immunofluorescence for the presence of IgG and IgA antibody specific for poliovirus, measles virus and herpes simplex virus. In paired maternal and cord sera virus-specific IgG is present in equal amounts; virus-specific staining by an anti-IgA conjugate was produced by maternal sera and not by cord sera. We believe that a comparison of virus-specific staining by a maternal serum with that produced by the corresponding cord serum using fluorescein-labelled anti-human IgG and IgA conjugates provides an excellent method of excluding cross-reactivity between an anti-human IgA conjugate and IgG.     

A new approach to differentiate recent vs chronic Toxoplasma infection: avidity ELISA in Toxoplasma serology

The diagnosis of toxoplamosis during pregnancy is based on maternal serology, due to the asymptomatic nature of the disease. Detection of specific IgM although is the method used all over the world to detect acute infection, persistence of IgM for long periods poses problems in distinguishing acute from chronic infection, which is of crucial importance in pregnancy. Avidity ELISA is a method recently developed to distinguish IgG antibodies developed at an early stage of infection from those that reflect past immunity. The avidity assay uses protein-denaturing agents and is a modification of an ELISA. The usefulness of this technique was tested on sera of 113 pregnant women screened for Toxoplasma specific IgG/IgM antibodies. Nine of the sixteen sera positive for IgM/IgG antibodies and three sera positive for IgG alone were subjected to avidity ELISA. Only three sera were positive for low avidity IgG indicative of recent infection. All the three sera positive for IgG alone showed high avidity.     

Prospective study of congenital toxoplasmosis screening with use of IgG avidity and multiplex nested PCR methods

Acute infection with Toxoplasma gondii during pregnancy can cause congenital toxoplasmosis. The aim of this study was to evaluate whether screening with the use of IgG avidity and multiplex nested PCR methods was effective to detect a high-risk pregnancy. In a prospective study, serum T. gondii IgG avidity was measured in consecutive 146 pregnant women testing positive for T. gondii antibody and either positive or equivocal for IgM. Multiplex nested PCR for T. gondii DNA on amniotic fluid, maternal blood, and umbilical cord blood were performed with informed consent. A total of 51 (34.9%) women presented with low IgG avidity (<30%), 15 (10.3%) presented with borderline avidity (30 to 35%), and 80 (54.8%) presented with high avidity (>35%) indices. Amniotic fluid obtained at amniocentesis or birth yielded positive PCR results in nine women with low IgG avidity indices. Of these nine women, three had congenital toxoplasmosis. None of women with high or border line IgG avidity indices had a positive PCR result in the amniotic fluid or congenital toxoplasmosis. No congenital toxoplasmosis was detected in women whose amniotic fluids yielded negative PCR results. Ingestion of raw or undercooked meat was found to be the main risk factor for acute T. gondii infection. Congenital toxoplasmosis screening with a combination of IgG avidity in the maternal blood and multiplex nested PCR in the amniotic fluid was useful for detecting a high risk pregnancy and diagnosing congenital toxoplasmosis.     

早发性牙周炎患者血清抗牙龈卟啉菌表面抗原抗体滴度和亲和力的研究(英文)

目的：检测早发性牙周炎患者血清抗牙龈卟啉菌脂多糖IgG抗体的滴度和亲和力，并探讨抗体滴度水平和亲和力之间的相关性。 方法： 受试者为15名早发性牙周炎患者、16名成人牙周炎患者和14名牙周健康者。检测了血清中抗牙龈卟啉菌脂多糖IgG抗体滴度和亲和力。血清IgG抗体滴度采用酶联免疫吸附试验（ELISA）测定；其亲和力通过二乙醇胺分离，ELISA测定。 结果： 早发性牙周炎患者和成人牙周炎患者的血清中抗牙龈卟啉菌脂多糖IgG抗体水平显著高于牙周健康者（P<0.01），而在早发性牙周炎患者和成人牙周炎患者之间无显著差异（P>0.05）。早发性牙周炎患者抗体亲和力显著高于成人牙周炎患者和牙周健康组（P<0.01），而成人牙周炎患者和牙周健康组之间无显著差异（P>0.05）。 结论： 早发性牙周炎患者和成人牙周炎患者的抗体滴度水平和亲和力之间不存在相关性。IgG抗体亲和力可以作为诊断早发性牙周炎的有效参数。     

National prevalence estimates for cytomegalovirus IgM and IgG avidity and association between high IgM antibody titer and low IgG avidity

Primary cytomegalovirus (CMV) infection of the mother during pregnancy presents risk of CMV infection of the fetus with resulting permanent disability. CMV IgM antibody is generated following primary CMV infection but also can appear during nonprimary CMV infection and is thus of limited diagnostic use by itself. In contrast, the presence of low CMV IgG avidity has been shown to be a unique and reliable serologic indicator of primary CMV infection. We measured CMV IgG and IgM antibody levels and IgG avidity in sera from a population sample of 6,067 U.S. women aged 12 to 49 years from NHANES (National Health and Nutrition Examination Survey). The CMV IgG prevalence was 58% overall and increased strongly with age. The CMV IgM prevalence was 3.0% overall and remained relatively flat across age groups. The prevalence of low IgG avidity was 2.0% overall, decreased sharply with age, and was seen mainly among IgM-positive sera. Fourteen to 18% of the CMV IgM-positive sera were low IgG avidity, presumably representing primary CMV infection. High CMV IgM antibody titer was a strong predictor of low IgG avidity. The ability to reliably identify primary CMV infection during pregnancy is important for management of the pregnancy, including possible treatment options for the fetus. Both IgM and IgG avidity measurements provide useful clinical information for evaluating primary CMV infection, although commercial tests for CMV IgG avidity are not yet widely available in the United States.     

Specific immunoglobulin M enzyme-linked immunosorbent assay for confirming the diagnosis of measles.

An enzyme-linked immunosorbent assay (ELISA) was evaluated for the detection of measles virus-specific immunoglobulin M (IgM) (MIgM). The ELISA was standardized by deriving a seronegative range of values from sera which should not contain MIgM (24 cord sera, 59 sera from immune health care workers, and 47 sera from infants before the administration of measles vaccine). These values were separable from those obtained from individuals convalescing from measles. Twenty sera containing rheumatoid factor were MIgM seronegative. Of 30 acute-phase sera from suspected measles cases, 26 contained MIgM; those that were seronegative were obtained on day 0, 0, 2, or 9. All 25 convalescent-phase samples contained MIgM. Of the 25 paired samples, 22 were IgG positive at the first sampling; 3 of the 22 did not show a rise in IgG titer. The MIgM ELISA can be used for confirming suspected measles cases, often requiring only a single serum specimen.     

Incidence and prevalence of toxoplasmosis in Indian pregnant women: a prospective study

PROBLEM: Toxoplasmosis is a major cause of congenitally acquired infections causing high degree of morbidity and mortality in the newborns. METHODS OF STUDY: IgG avidity method was used to distinguish the recent and more than 4 months old infection in a prospective cohort study for the first time in India. One hundred and eighty pregnant women presented in their first 4 months of pregnancy were included in this study. Their sera were tested for anti-Toxoplasma gondii antibodies using direct agglutination test, immunoglobulin (Ig)G and IgM-enzyme-linked immunosorbent assay, IgM-immunosorbent agglutination assay and VIDAS-IgG avidity. RESULTS: Overall IgG seroprevalence rate of toxoplasmosis was 45%. Only seven women (3.3%) were found to have IgM antibodies and only two of these showed low IgG avidity indicating recent infection of 相似文献   

Predictive value of maternal-IgG avidity for congenital human cytomegalovirus infection.

BACKGROUND: Human cytomegalovirus (HCMV) is now the most common cause of viral intrauterine infection. Fetal damage is mostly linked to maternal primary infection. It is therefore important to differentiate primary from recurrent or persistent HCMV infection in pregnant females. For this purpose, IgM tests are not reliable enough and the measurement of the IgG avidity appears to be presently the best method. OBJECTIVE: To evaluate the performance of the measurement of HCMV-IgG avidity by a 8 M urea denaturation assay in predicting congenital infection in the offspring. STUDY DESIGN: Seventy-eight women were included in this study on the basis of a HCMV IgM positive or equivocal result on a first serum during pregnancy, but without a documented seroconversion history. The IgG avidity was measured and correlated with the outcome of the pregnancy. RESULTS: In eight cases of HCMV in utero infection the maternal HCMV-IgG avidity index was below 50%. One case of HCMV in utero infection was observed despite a high avidity index during the second trimester of the pregnancy. High or intermediate HCMV-IgG avidity indexes during the first trimester of pregnancy were not associated with a congenital infection. CONCLUSIONS: Even in the presence of an IgM positive result, an HCMV IgG avidity index above 65% on a serum obtained during the first trimester of pregnancy could reasonably be considered as a good indicator of past HCMV infection. In these conditions invasive prenatal diagnosis is not necessary.     

Immunoglobulin G avidity testing in serum and cerebrospinal fluid for analysis of measles virus infection.

We studied a variety of patients with measles virus infection by using avidity testing for measles virus-specific immunoglobulin G (IgG) in serum and cerebrospinal fluid samples. For the avidity testing, an Enzygnost measles IgG enzyme-linked immunosorbent assay kit was used with an 8 M urea denaturing method. With this method, low-avidity IgG (acute primary infection, avidity of < 30% within 15 days of the onset of rash) and high-avidity IgG (subacute sclerosing panencephalitis, avidity of > 75%) could be clearly distinguished by using serum samples. One patient, who developed a typical course of measles despite a previous vaccination, showed a positive IgM response with an initial low titer of measles virus-specific IgG of low avidity, but a later sample revealed a high titer of IgG of intermediate (40%) avidity, suggesting previous immunological priming. Two patients with breakthrough infection (secondary vaccine failure), both having central nervous system involvement, showed a positive IgM response with initial high titers of serum IgG of high avidity. In addition, one of the patients had a detectable level of measles-specific IgG in cerebrospinal fluid. In this patient, the avidity of both serum and cerebrospinal fluid IgG decreased during the short follow-up period. This phenomenon has never before been reported. In subacute sclerosing panencephalitis patients, the avidity of cerebrospinal fluid IgG was consistently lower than that of serum IgG. The difference in avidity between cerebrospinal fluid and serum IgG may be used as a direct indicator of intrathecal production of IgG. In conclusion, the avidity testing is simple to perform, reliable, and highly informative in the analysis of measles virus infection.     

Follow-up of avidity and titer of anti-myeloperoxidase antibodies in sera from patients with primary ANCA-associated vasculitis

Antineutrophil cytoplasmic antibodies (ANCA) are important serologic markers for ANCA-associated vasculitis (AAV). Our previous studies in propylthiouracil (PTU)-induced AAV demonstrated that withdrawal of PTU resulted in clinical remission and significant decrease of avidity of PTU-induced myeloperoxidase (MPO)-ANCA. This study investigated the changes in avidity and titer of MPO-ANCA in sequential sera from some patients with primary AAV with different disease activities. Sequential sera samples of seven patients with MPO-ANCA-positive vasculitis at their initial onset, remission and relapse were collected. The avidity of MPO-ANCA was assessed by antigen-inhibition enzyme-linked immunosorbent assay (ELISA). The titer of MPO-ANCA was determined by a two-fold dilution of sera in MPO specific ELISA. The titer of MPO-ANCA was not significantly different between initial onset and remission. The avidity constant (aK) of MPO-ANCA in active phase is not significantly different from that in remission (724.9 ± 828.4 l/mol vs. 353.4 ± 551.7 l/mol, p = 0.303). No significant correlation could be found between aK and the level of Birmingham Vasculitis Activity Score, times of relapse, the number of organ involvement, serum creatinine, or CRP. Avidity and titer of MPO-ANCA did not decreased significantly during remission in AAV, indicating the chronic repeated antigen stimulation was not removed, which might be the reason for recurrent relapses.     

Improved diagnosis of primary Toxoplasma gondii infection in early pregnancy by determination of antitoxoplasma immunoglobulin G avidity.

The ability to discriminate between primary Toxoplasma gondii infection acquired in early pregnancy and infection that occurred prior to pregnancy was assessed by an enzyme immunoassay (EIA) to determine the avidity of toxoplasma-specific immunoglobulin G (IgG). The results were compared to those of the Platelia Toxo-IgM EIA and the dye test. The mean IgG avidity of 73 serum samples collected within 20 weeks after the estimated time of infection was 5.9%. Among 26 serum samples showing latent infection (toxoplasma-specific IgG positive and IgM negative) and 56 IgM-positive serum samples with a low dye test titer (<300 IU/ml), the mean avidities were 51.3 and 57.5%, respectively. A total of 72.8% of 92 IgM-positive serum samples with a high dye test titer (>300 IU/ml), suggesting a recent toxoplasma infection, had an IgG avidity of >20%, indicating that the infection started more than 20 weeks earlier. By introducing high IgG avidity as a criterion in the first half of pregnancy to exclude the possibility that toxoplasma infection was acquired during gestation, many women will avoid unnecessary examinations, treatment, and anxiety.     

Improvement of cytomegalovirus avidity testing by adjusting the concentration of CMV-specific IgG in test samples.

BACKGROUND: Human cytomegalovirus (CMV) is the most common cause of viral intrauterine infection. Primary CMV infection in early pregnancy bears a high risk of fetal damage. Accurate measurement of CMV-specific IgG avidity may help to improve the serodiagnosis of CMV-infected women by determining the time of infection and fetal outcome. OBJECTIVES: To study the performance of the CMV avidity assay with the fully automated Vidas analyzer (bioMérieux) as a function of the concentration of CMV-specific IgG present in the serum sample. STUDY DESIGN: Eighty-two serum samples were investigated from 3 clinical scenarios: 18 individuals with sera negative for CMV-specific IgG and IgM (control group), 20 pregnant women (44 samples) containing CMV-specific IgG- and IgM-antibodies suggesting acute or recent primary infection and 20 patients with evidence of past infection (CMV-IgG positive and CMV-IgM negative). RESULTS: In the group with presumed acute or recent primary infection 12 of 44 sera had CMV-specific IgG values above 100 arbitrary units (AU, bioMérieux)/ml and in these cases an increase in AI was measurable upon dilution of the serum sample. In two cases, AI's were shifted towards or above the cut-off value of AI>or=0.8, indicative of past infection. Dilution of sera which were CMV-specific IgM positive and had specific IgG concentrations of 相似文献   

Immunoglobulin Class of Natural Human Antibodies Reactive with Treponema pallidum

To aid the development of serological tests for incubating syphilis, a characterization was made of the background of natural anti-Treponema pallidum antibodies against which the initial immune response to syphilis takes place. Sera from presumed normal persons were studied with monospecific antisera in an indirect fluorescent-antibody procedure. Of 36 sera tested at a 1:5 dilution, all showed IgG reactivity with T. pallidum (Nichols strain), 58% showed IgM reactivity, and 20% showed IgA reactivity. The titer of IgG reactivity was considerably higher than that of the other two immunoglobulins. Heating the sera for 1 hr at 65 C abolished IgA and IgM anti-T. pallidum reactivity, but one-third of the sera retained IgG reactivity. Human Cohn fractions II and III₁ from three commercial sources contained mostly IgG antibodies reactive with T. pallidum, but IgM and IgA antibodies were also present. IgG reactivity was found in a pool of presumably normal maternal sera from 20 mothers and in a corresponding pool of umbilical cord serum from their infants. IgM and IgA reactivities were present only in the maternal serum pool.     

Effect of maternal immunotherapy on immediate skin test reactivity, specific rye I IgG and IgE antibody, and total IgE of the children

The effect of specific immunotherapy during pregnancy was studied in 14 children, 3 to 12 years after delivery. Fourteen additional children from the same allergic mothers, in whom immunotherapy was not given during the pregnancy, served as controls. The immediate skin test response to grass allergens of the children of mothers given immunotherapy. Levels of rye I IgG and total IgE were lower in the sera of children born to mothers who received immunotherapy (not statistically significant) than their control cohorts. Paired cord blood and maternal blood samples drawn at delivery showed similar levels of rye I IgG, indicating that blocking antibody freely crosses the placenta. This evidence indicates that immunotherapy during pregnancy may have an inhibitory effect on immediate skin reactivity to grass allergens in some of the offspring. Whether tolerance to other allergens can be induced in children by maternal immunotherapy remains to be determined.     

VIDAS test for avidity of Toxoplasma-specific immunoglobulin G for confirmatory testing of pregnant women

Because congenital toxoplasmosis is almost solely the result of maternal infection acquired during gestation, it is critical to determine whether infection during pregnancy has occurred. In the United States, definitive diagnosis of the acute infection and the time of its occurrence have been compromised by a lack of systematic screening and the fact that only a single serum sample is submitted for testing. In studies in Europe, and depending on the method used, the demonstration of high-avidity immunoglobulin G (IgG) toxoplasma antibodies has been shown to exclude infection having occurred in the first 3 to 5 months of pregnancy. We investigated the usefulness of determining the avidity of IgG toxoplasma antibodies with a VIDAS kit (herein referred to as the VIDAS Toxo-IgG avidity kit, the VIDAS kit essentially rules out acute infection having occurred within the 4 prior months) in the setting of a reference serology laboratory in the United States. Sera (132 samples) from 132 women in the first 16 weeks of pregnancy were chosen because at least one test in the toxoplasma serological profile (TSP) suggested or was equivocal for a recently acquired infection. High-avidity antibodies were demonstrated in 75% of 99 sera positive with the IgM enzyme-linked immunosorbent assay (ELISA) and 31.3% of 16 sera with acute TSP results. A significant percentage of sera with equivocal results wtih the IgM ELISA or TSP also had high-avidity test results. Of 39 women for whom treatment with spiramycin had been suggested to attempt to prevent congenital transmission, 19 (48.7%) had high-avidity antibodies. These findings highlight the value of the VIDAS IgG avidity kit when used in combination with the TSP to exclude recent infection, especially when only a single serum sample is available.     

Detection of immunoglobulin M (IgM), IgA, and IgG Norwalk virus-specific antibodies by indirect enzyme-linked immunosorbent assay with baculovirus-expressed Norwalk virus capsid antigen in adult volunteers challenged with Norwalk virus.

Pre- and postexposure sera collected from 17 adult volunteers challenged with Norwalk virus as described previously (D. Y. Graham, X. Jiang, T. Tanaka, A. Opekun, P. Madore, and M. K. Estes, J. Infect. Dis. 170:34-43, 1994) were examined for Norwalk virus-specific immunoglobulin M (IgM), IgA, and IgG by indirect enzyme-linked immunosorbent assays with recombinant Norwalk virus antigen bound to the solid phase. Sixteen of the 17 volunteers had evidence of past infection, all presenting with preexisting IgG antibody of high avidity; only one volunteer had no evidence of previous infection. Virus infection was detected in 14 of the 16 volunteers with evidence of past infection, and 9 of the infected volunteers had symptomatic illness. A significant rise in both virus-specific IgA and IgG titers was detected after challenge in all of the volunteers who became ill. Five of the asymptomatic volunteers who were infected had rising titers of virus-specific IgG, but only two of the five had a concomitant rise in their virus-specific IgA antibody titers. Antibody rises were detectable in eight of nine ill volunteers 8 to 11 days after challenge but in the asymptomatic volunteers only after more than 15 days had elapsed. Virus-specific IgM was detected after challenge in all 14 infected volunteers. Between symptomatic and asymptomatic volunteers there were no significant differences in titers of virus-specific IgG and IgA in serum before challenge; however, there were significantly higher titers in symptomatic volunteers between 8 and > 90 days after challenge for virus-specific IgG and 8 and 24 days after challenge for virus-specific IgA.     

Primary humoral antibody response to Coxiella burnetii, the causative agent of Q fever.

Of 147 patients with acute Q fever diagnosed during a major outbreak in Birmingham, England, in early summer 1989, 41 provided sets of sera which allowed us to make a detailed analysis of the primary humoral immune response. Antibody titers specific for Coxiella burnetii were measured by the complement fixation test and by an immunoglobulin M (IgM)- and IgG-specific indirect immunofluorescence test. The relative avidity of specific IgGs was determined by the indirect immunofluorescence test with and without treatment of antigen-antibody complexes with 8 M urea. The IgG subclass responses after primary infection and their avidities were also determined for a limited number of paired serum specimens. Specific IgM titers persisted for more than 6 months in the majority of cases and were therefore not a sufficient criterion for the diagnosis of recent infection. However, for serial samples the antibody titer ratios (IgG/IgM) and the ratios (IgG titer with treatment/IgG titer without treatment) that indicated relative avidity changed significantly, depending on the time postinfection. Within the IgG class, the C. burnetii-specific antibody response over time was almost exclusively represented by subclass 1 molecules, which thus showed affinity maturation.     

Comparison of immunoglobulin G subclass profiles induced by measles virus in vaccinated and naturally infected individuals

A total of 258 human sera positive for measles antibodies were divided into four different groups: group 1 contained 54 sera from children after natural measles infection (immunoglobulin M [IgM] positive, early infection phase), group 2 contained 28 sera from children after measles vaccination (IgM positive, early infection phase), group 3 contained 100 sera from healthy adults (natural long-lasting immunity), and group 4 contained 76 sera from healthy children (postvaccinal long-lasting immunity). In the early phase of infection, the percent distributions of measles virus-specific IgG isotypes were similar between natural and postvaccinal immune responses. IgG1 and IgG4 were the dominant isotypes, with mean levels of detection of 100% (natural infection) and 100% (postvaccinal) for IgG1 and 96% (natural infection) and 92% (postvaccinal) for IgG4. In comparison, the IgG4 geometric mean titer (GMT) in the early phase of natural infection was significantly higher than the IgG4 GMT detected in the postvaccinal immune response (80 versus 13; 95% confidence interval). In the memory phase, IgG2 and IgG3 responses decreased significantly in both natural infection and postvaccinal groups, while IgG1 levels were maintained. In contrast, the IgG4 postvaccinal immune response decreased strongly in the memory phase, whereas IgG4 natural long-lasting immunity remained unchanged (9 versus 86%; P < 0.05). The results obtained suggest that IgG4 isotype could be used in the early phase of infection as a quantitative marker and in long-lasting immunity as a qualitative marker to differentiate between natural and postvaccinal immune responses.     

Simplified assay for measuring Toxoplasma gondii immunoglobulin G avidity

A Toxoplasma gondii immunoglobulin G (IgG) avidity enzyme-linked immunosorbent assay (ELISA) was developed that combines the accuracy of assays based on end point titers and the relative ease of assays based on optical density values. Like published procedures, the new assay's avidity index (AI) was based on differential T. gondii-specific IgG reactivity in serum-treated wells washed with urea buffer versus that in wells washed with control buffer; unlike previous assays, however, the IgG reactivity was measured quantitatively using a standard curve. The assay was evaluated using 24 IgG-positive and IgM-positive sera collected within 5 months of the onset of symptoms (recent-infection group) and 25 IgG-positive and IgM-negative sera (past-infection group). All sera in the recent-infection group exhibited AI values of <0.18, whereas all sera in the past-infection group exhibited AI values of >0.27. The AI values of the recent-infection group showed significant correlation with the number of days after the onset of symptoms. A subset of 16 sera (8 recent and 8 past) was tested using a commercially available T. gondii IgG avidity ELISA based on end point titration; the results of the two assays showed highly significant correlation (R(2) = 0.9125). In addition, we confirmed and extended the findings of other investigators, showing that AI values calculated using optical density values, but not AI values calculated using quantitative IgG values, varied significantly depending on the serum dilution used. This new assay should facilitate the accurate measurement of T. gondii IgG avidity in a reference laboratory setting.