Serological diagnosis of pneumococcal infection in children with pneumonia using protein antigens: A study of cut-offs with positive and negative controls

The etiological diagnosis of infection by Streptococcus pneumoniae in children is difficult, and the use of indirect techniques is frequently warranted. We aimed to study the use of pneumococcal proteins for the serological diagnosis of pneumococcal infection in children with pneumonia. We analyzed paired serum samples from 13 Brazilian children with invasive pneumococcal pneumonia (positive control group) and 23 Finnish children with viral pharyngitis (negative control group), all aged < 5 years-old. Children with pharyngitis were evaluated for oropharyngeal colonization, and none of them carried S. pneumoniae. We used a multiplex bead-based assay with eight proteins: Ply, CbpA, PspA1 and 2, PcpA, PhtD, StkP and PcsB. The optimal cut-off for increase in antibody level for the diagnosis of pneumococcal infection was determined for each antigen by ROC curve analysis. The positive control group had a significantly higher rate of ≥ 2-fold rise in antibody levels against all pneumococcal proteins, except Ply, compared to the negative controls. The cut-off of ≥ 2-fold increase in antibody levels was accurate for pneumococcal infection diagnosis for all investigated antigens. However, there was a substantial increase in the accuracy of the test with a cut-off of ≥ 1.52-fold rise in antibody levels for PcpA. When using the investigated protein antigens for the diagnosis of pneumococcal infection, the detection of response against at least one antigen was highly sensitive (92.31%) and specific (91.30%). The use of serology with pneumococcal proteins is a promising method for the diagnosis of pneumococcal infection in children with pneumonia. The use of a ≥ 2-fold increase cut-off is adequate for most pneumococcal proteins.