Background: In recent years serologic methods have been applied to assess pneumococcal etiology of pneumonia and other respiratory tract infections. Antigen and antibody assays have shown to be insensitive, especially in young children. The aim of this study was to evaluate the usefulness of circulating immune complexes in the diagnosis of pneumococcal lower respiratory infection in children.
Material and methods: Pneumococcal immune complexes (IC) containing antibodies to species-specific C-polysaccharide, to mixtures of type-specific capsular polysaccharides or to a protein antigen, pneumolysin, were studied in the sera of 449 children with lower respiratory tract infection.
Results: Circulating ICs were found in 68 (15%) children; 46 (68%) of them were demonstrated in acute and 43 in convalescent serum. In 5 (7%) of the 68 IC-positive patients pneumococcal antigen was present in acute serum; those patients formed 18% of the 28 cases with antigenemia. An antibody response between paired sera to any of the 3 pneumococcal antigens studied was observed in 14 (21%) IC-positive children; they formed 23% of the 60 cases with an antibody response. In total ICs were positive in 51% of all the 134 pneumococcal cases diagnosed by any method.
Conclusions: We conclude that the measurement of circulating ICs is more sensitive than other serologic methods for the diagnosis of pneumococcal lower respiratory infection. In infants, however, it was as insensitive as antigen and antibody assays.