Automated Time-resolved Immunofluorometric Assay for Toxoplasma gondii-specific IgM and IgA Antibodies: Study of More Than 130 000 Filter-Paper Blood-Spot Samples from Newborns

Background: To screen for congenital toxoplasmosis, we developed a time-resolved immunofluorometric assay for the simultaneous detection of Toxoplasma gondii-specific IgM and IgA in filter-paper samples collected from newborns 4–7 days after birth. Methods: The assay was performed on the AutoDELFIATM, and results were calculated based on the ToxoM WHO Third International Reference Serum. Comparison with an in-house μ-capture immunoassay was carried out retrospectively on filter-paper samples from children with confirmed congenital toxoplasmosis. Prospectively the assay was compared with a μ-capture immunoassay on 68 394 samples and a commercially available assay on another 69 467 samples. Before serum was requested from the newborn, positive samples were tested for IgA and IgM separately and in an IgM-immunosorbent agglutination assay developed for filter-paper samples. Results: Intra- and interassay variations (CVs) were 8% and 16%, respectively. The cutoff of 5 units/mL produced a 0.5% retest rate. The assay detected 13 of 18 (72%) samples from newborns diagnosed with congenital toxoplasmosis in the retrospective study. Prospectively, the assay identified 24 newborns who were later diagnosed with congenital toxoplasmosis. Results for all 24 cases were positive by the respective comparison method. No cases were detected solely by the IgA antibodies in the sample. Conclusion: Neonatal screening for congenital toxoplasmosis can be automated by use of purified europium-labeled antigen for detection of T. gondii-specific IgM and IgA eluted from filter-paper samples.

Development of a Filter Paper Blood Spot Radioimmunoassay for Thyroid Stimulating Hormone Suitable for a Regional Neonatal Screening Unit

The radioimmunoassay described measures TSH in dried whole blood spots collected from neonates onto filter paper Guthrie cards. Microgranular cellulose is added to the precipitating reagent at the critical separation stage of the assay to overcome imprecision caused by the presence of the filter paper sample disc in the tube. The method was developed for a regional neonatal screening unit and has been found to be very reliable during ten months' routine use. It was required to be as precise, sensitive, accurate, rapid, simple, and inexpensive as possible and suitable for use with automatic diluting equipment in order to process large numbers of samples. Other methods were examined for their suitability and found not to fulfil one or more of the above criteria.