A technique for dating toxoplasmosis in pregnancy and comparison with the Vidas anti-toxoplasma IgG avidity test

Abstract

A comparative evaluation of 384 selected sera was performed using the Beckman Coulter Access and Abbott Axsym Toxo-IgG assays. The Axsym assay yields positive early results following infection, while the Access assay gives higher titres during chronic infection. The ratio between the two complementary tests, Axsym Toxo-IgG/Access Toxo-IgG (Ax/Ac), was compared with the Vidas anti-Toxoplasma IgG avidity index (AI). The Ax/Ac ratio decreased progressively as the time between infection and sampling increased. The mean Ax/Ac values (+/-SE) were 2.50 (+/-0.26), 2.14 (+/-0.13), 2.33 (+/-0.22), 1.34 (+/-0.09), 1.32 (+/-0.10), 0.92 (+/-0.08) and 0.74 (+/-0.07) for groups of sera sampled at 1, 2, 3, 4-5, 6-8, 9-12 and 13-24 months, respectively, after infection in pregnant women. These values were much smaller for cases with chronic infection (>24 months), i.e., 0.56 (+/-0.03), 0.44 (+/-0.04) and 0.53 (+/-0.04), respectively, for pregnant women and immunodepressed patients with and without reactivation. Taking a ratio of 1 as a threshold for recent infection, the patients in the groups sampled at 1, 2 and 3 months had Ax/Ac ratios >1 in 49/50 (98%), 53/55 (96.4%) and 36/36 (100%) cases, respectively. Thus, an Ax/Ac ratio of <1 in serum from a pregnant woman allows a recent infection (<3 months) to be excluded. This technique has the advantage of yielding positive results that develop much more rapidly than the AI, thereby helping to reassure large numbers of pregnant women and avoiding costly and unnecessary prophylactic treatment and follow-up.

Bicentric Evaluation of Six Anti-Toxoplasma Immunoglobulin G (IgG) Automated Immunoassays and Comparison to the Toxo II IgG Western Blot

“…In Europe, the Access II test replaced the Access I test during the year 2008. The Access I (4,8), AxSYM (4,8,11,19), and Vidas (6,8,11,17) tests use the WHO second international standard (IS), whereas the Access II, Immulite (10), and Modular (6) tests use the third WHO IS. It is to be noted that neither the second nor the third WHO IS was tested in parallel by the six automated techniques in the present study.…”

Toxoplasma Seroconversion with Negative or Transient Immunoglobulin M in Pregnant Women: Myth or Reality? A French Multicenter Retrospective Study

“…Consequently, serological tests other than those for IgG and IgM detection have been developed to facilitate the diagnosis of seroconversion. The measurement of IgG avidity was developed almost 20 years ago to exclude any infection acquired in the preceding 3 or 4 months using most commercial tests (16,17,18,19,20,21,22,23,24). Immunoblot analysis (Toxo II IgG test; LDBio, Lyon, France) may promptly confirm the seroconversion when IgG concentrations detected with routine tests are negative or equivocal (25,26,27).…”

Comparison of Four Commercially Available Avidity Tests for Toxoplasma gondii-Specific IgG Antibodies

“…Because in-house tests often lack automation and standardization, the use of commercial IgG avidity tests is highly recommended. For this purpose, most major in vitro diagnostic companies have produced kits based on various approaches, including recombinant antigen-based technology (8)(9)(10)(11)(12). Since 2006, the objective of the French National Reference Center for Toxoplasmosis (NRCT) has been to investigate the methods used for the serological diagnosis of toxoplasmosis, with the aim of reducing the cost of the French screening program (13).…”