Tehran University of Medical Sciences Iranian Journal of Parasitology 1735-7020 21 3 2026 09 01 326 334 EN Mostafa Tork 1. Toxoplasmosis Research Center, Communicable Diseases Institute, Mazandaran University of Medical Sciences, Sari, Iran 2. Student Research Committee, School of Medicine, Mazandaran University of Medical Sciences, Sari, Iran Shahabeddin Sarvi 1. Toxoplasmosis Research Center, Communicable Diseases Institute, Mazandaran University of Medical Sciences, Sari, Iran 2. Department of Parasitology and Mycology, Faculty of Medicine, Mazandaran University of Medical Sciences, Sari, Iran Hossein Asgarian-Omran 1. Department of Immunology, School of Medicine, Mazandaran University of Medical Sciences, Sari, Iran 2. Gastrointestinal Cancer Research Center, School of Medicine, Mazandaran University of Medical Sciences, Sari, Iran Mitra Sadeghi 1. Toxoplasmosis Research Center, Communicable Diseases Institute, Mazandaran University of Medical Sciences, Sari, Iran 2. Department of Parasitology and Mycology, Faculty of Medicine, Mazandaran University of Medical Sciences, Sari, Iran Bahare Basirpour 1. Toxoplasmosis Research Center, Communicable Diseases Institute, Mazandaran University of Medical Sciences, Sari, Iran 2. Student Research Committee, School of Medicine, Mazandaran University of Medical Sciences, Sari, Iran Eisa Nazar 1. Orthopedic Research Center, Mazandaran University of Medical Sciences, Sari, Iran 2 Psychiatry and Behavioral Sciences Research Center, Addiction Institute, Mazandaran University of Medical Sciences, Sari, Iran Seyed Abdollah Hosseini 1. Toxoplasmosis Research Center, Communicable Diseases Institute, Mazandaran University of Medical Sciences, Sari, Iran 2. Department of Parasitology and Mycology, Faculty of Medicine, Mazandaran University of Medical Sciences, Sari, Iran Ahmad Daryani 1. Toxoplasmosis Research Center, Communicable Diseases Institute, Mazandaran University of Medical Sciences, Sari, Iran 2. Laboratory of Molecular Parasitology, Scientific Center of Zoology and Hydroecology, NASRA, 7P. Sevak st. Yerevan 0014, Armenia Sargis A. Aghayan 1. Laboratory of Molecular Parasitology, Scientific Center of Zoology and Hydroecology, NASRA, 7P. Sevak st. Yerevan 0014, Armenia 2. Laboratory of Zoology, Research Institute of Biology, Yerevan State University, Yerevan, 0025, Republic of Armenia 2025 04 08 2026 09 01 Background: Toxoplasmosis is a significant global health concern. Definitive diagnosis of toxoplasmosis is a serious challenge due to false positive serological results. This study focuses on optimization and standardization of an in-house IgG avidity ELISA for Toxoplasma gondii using freshly prepared Toxoplasma lysate antigen (TLA). Methods: Samples were collected during routine pregnancy screening. Following tachyzoite purification, TLA concentration was determined by the Bradford assay at 20 μg/mL. The indirect ELISA assay was then performed with a urea treatment step (6 M). Using 75 serum samples classified as acute or chronic toxoplasmosis, the IgG avidity test was performed by ELISA in two rows of microplates with 6 M urea and PBS, and then the avidity index was calculated. Results: After calculating the cutoff values, the sensitivity and specificity of the in-house Toxoplasma IgG avidity kit were 92% (95% CI: 74–99%) and 100% (95% CI: 86–100%), respectively, with an area under the ROC curve (AUC) of 0.977 (95% CI: 0.951–1.000). Conclusion: The results of the IgG avidity test designed in this study along with standardizing the method and interpreting the results showed the promising performance of this assay to differentiate the acute from the chronic stages of toxoplasmosis. https://ijpa.tums.ac.ir/index.php/ijpa/article/view/4567 https://ijpa.tums.ac.ir/index.php/ijpa/article/download/4567/1508