Synovial fluid was spiked with C. trachomatis persistently infected peripheral blood monocytes (C. trachomatis PBMO) per ml synovial fluid (SF) ranging from 10,000 C. trachomatis PBMO/ml SF to 0.1 C. trachomatis PBMO/ml SF in 10-fold decreasing numbers. Five independent repeats of each serial dilution were performed (n = 5) for each DNA extraction method. Amplification was performed using system number 1 (C. trachomatis-omp1 directed PCR). The median (M) of serial dilutions is given below. Sensitivity was defined as reproducibly detected lowest number of detected C. trachomatis PBMO/ml SF. Statistical significant results are indicated by *, the method compared with is indicated by number (P < 0.05). omp-1 = major outer membrane protein.