Challenges of using molecular serotyping for surveillance of pneumococcal disease

Recent advances in the molecular identification and serotyping of Streptococcus pneumoniae are useful for culture-negative samples; however, there are limitations associated with these methods. We aimed to assess the value of molecular assays for invasive pneumococcal disease (IPD) surveillance in South Africa from 2010 through 2012. Nonviable isolates and culture-negative clinical specimens were tested for the lytA gene and, if positive, were serotyped, using real-time PCRs. Multinomial regression analysis was used to determine the maximum lytA cycle threshold (CT) value useful for predicting the ability to detect a serotype for the sample. The χ(2) test was used to compare the prevalence of serotypes between viable/nonviable isolates and culture-negative clinical specimens. Of 11,224 IPD cases reported, 1,091 (10%) were culture-negative samples and 981 (90%) of these were lytA positive. Samples with a lytA CT value of ≥35 were significantly less likely to be serotyped. A serotype/group was determined for 87% (737/844) of samples with a lytA CT value of <35, of which 60% (443/737) were identified as individual serotypes. The serotype prevalence did not differ significantly between isolates and culture-negative specimens. Although molecular serotyping added 7% (737/11,224) serotyping data, the inability to resolve 40% of samples to single serotypes remains a challenge for serotype-specific data analysis.