Making standards for quantitative real-time pneumococcal PCR

Quantitative lytA PCR is often performed using in-house standards. We hypothesised equivalence when measuring a standard suspension of Streptococcus pneumoniae by colony-forming-units (CFU) or genome-copies. Median (IQR) ratio of CFU/genome-copies was 0.19 (0.1–1.2). Genome-copies were less variable than CFU, but the discrepancy between the methods highlights challenges with absolute quantification.