Semi-quantitative analysis of Streptococcus pneumoniae urinary antigen: Kinetics of antigen titers and severity of diseases

Detection of urinary antigen by a rapid immunochromatographic membrane test ( Binax NOW) was widely accepted as a powerful tool for diagnosis of Streptococcus pneumoniae pneumonia. This is a qualitative kit, so the value of quantitative analysis of urinary antigen, especially correlation of antigen titers and severity of diseases, remained to be determined. We examined semi-quantitative antigen titer in urines collected from urinary antigen-proven S. pneumoniae pneumonia on admission, and analyzed the kinetics of antigen titer and its relation to severity of diseases. After serial 2-fold dilution of urine, the highest dilution for positive results was determined, and this was designated as maximum dilution factor (MDF). MDFs varied from 1 to 4096 in 29 patients examined ( mean MDF, 317.8). Importantly, severe cases of S. pneumoniae pneumonia were higher values of MDFs ( mean MDF: 760.5) than those of non-severe cases ( mean MDF: 5.4). The patients with high MDFs (>= 64) demonstrated higher values of LDH, CRP and lower values of WBC and PaO2 compared to those of low MDFs group (<= 32). There was no clear correlation between CRP values and antigen titers, and conversely the majority of severe cases showed relatively weak CRP responses, despite high levels of bacterial antigen. Kinetic analysis of urinary and serum antigen titers in 4 cases of S. pneumoniae pneumonia exhibited consistently higher values of antigen titers in urine than those in serum. The half lives of urinary and serum antigen titers were calculated to be 1.0 - 3.4 and 1.1 - 2.3 weeks, respectively. These data suggest that quantitative analysis of urinary antigen may be a useful indicator for severity of disease and course of S. pneumoniae pneumonia. Our results demonstrate an application for S. pneumoniae antigen titer determination in urine and serum, which may be crucial not only for diagnostic measures, but also may provide a better understanding of the pathogenesis of S. pneumoniae infection.