The term avidity used to express the strength of the bond between a serum specimen and a multi- valent antigen. It is known that the avidity increases with time after antibody challenge and measurement of the avidity has been used diagnostically. Recently an assay measuring the IgG avidity of various virus infections and of toxoplasmosis was used to distinguish between acute and chronic infection. Our study was focused on the method to distinguish acute and chronic Toxocara infection, zoonosis caused by the larvae of dog and cat ascarids Toxocara canis and Toxocara cati known all over the world for the possibility of provoking the infestation of man, accompanied by visceral or ocular clinical manifestations. The infection is generally diagnosed by demonstration of specific immunoglobulins to Toxocara excretory-secretory antigens (TES) in sera of infected patients. Highly sensitive assays with specific antigens are necessary for detection of antibodies. The test proved clinically useful is the ELISA reaction with TES antigens. This method detects the antibodies for months or even years after infection and this is the reason why the discrimination between chronic and recent infection is very difficult. For disrupting the hydrogen bond urea has been used. The index of avidity was calculated as the ratio of IgG values in sera treated with urea and the value of IgG in non-treated sera, multiplied by 100. An index up to 40 is considered as low avidity, that means freshly acquired infection (36 to 40 borderline) and more than 40 is high avidity. In the group of 1 376 patients only 5.09% low avidities were found. It means that predominantly patients in the chronic stage of infection attend examination.

        Key words: larval toxocariasis – serology – IgG avidity test.