Assessment of immunoglobulin-M immunosorbent agglutination assay (ISAGA) for detecting toxoplasma specific IgM.

Abstract

An immunoglobulin-M immunosorbent agglutination assay (ISAGA) was introduced to detect toxoplasma specific IgM. This assay incorporates mu chain capture and use of entire toxoplasma trophozoites as an antigen source. The performance of the ISAGA was compared with that of a double sandwich enzyme linked immunosorbent assay (DS-ELISA) currently used in the Public Health Laboratory Service Toxoplasma Reference Laboratories. The ISAGA was found to be more sensitive than DS-ELISA but there was no demonstrable difference in the specificity or reproducibility between the two assays. The ISAGA is suitable for the diagnosis of acute toxoplasmosis in immunocompetent patients and as a screening test for recent infection in pregnant women. The persistence of ISAGA reactivity, however, is such that additional serological assessment is required to define the risk of congenital infection.

Selective antenatal screening for toxoplasmosis and the latex agglutination test

“…Reference centres attempt to overcome these problems by the use of several IgM tests with comparison of findings. In our experience the ISAGA is significantly more sensitive than DS-ELISA for the detection of toxoplasma specific IgM but the specificity of the two assays are comparable [8]. Our findings of 85 ISAGA 411 reactive sera, negative on examination by DS-ELISA reflects the comparative sensitivity of the assays.…”

“…However, the results of the present study could have been biased by initial screening of sera in primary laboratories so that specimens producing low titre LAT results were not sent for IgM assessment. Previously we were unable to demonstrate ISAGA or DS-ELISA reactivity amongst 50 sera producing LAT titres < 16 [8]. Nor have we detected toxoplasma-specific IgM in LAT negative samples taken from AIDS sufferers or transplant recipients.…”

“…The non-specific signs and symptoms of toxoplasma infection make diagnosis based on clinical examination unreliable [11]. Limited availability of serum samples showing recent seroconversion and the failure of Western blots or parasite isolation to define recent infection lead to considerable difficulties in the assessment of IgM assays [8]. The absence of a definitive reference has resulted in commercial toxoplasma immunoglobulin-M assays being made available without quantitative assessment of the sensitivity and specificity of the technique.…”

“…The LAT uses disrupted toxoplasma trophozoites as an antigen source. In consequence, the assay detects antibodies with affinity for cytoplasmic and membraneous antigens of T. gondii [8]. Western blot studies have shown wide variation in the individuals response to toxoplasma infection, both in the number of antigens recognized and the relative amount of 413 antibody directed to different antigens [13].…”

“…The latex agglutination test (LAT) findings for the initial sample of serum only were recorded when serial samples were received. Toxoplasmaspecific IgM was measured using a It-chain capture double sandwich enzyme linked immunosorbent assay (DS-ELISA) and an immunosorbent agglutination assay (ISAGA) as previously described [8]. The LAT was performed according to the manufacturer's instructions [9].…”