Background The Abbott Realtime m2000 system (m2000) is a qualitative real-time PCR assay for the detection of CT and NG that has the capability to provide a relative measure of target DNA. Results from the m2000 were used to determine CT and NG organism load by comparing the delta cycle (DC) value of each specimen to a set of lab developed standards containing known concentrations of each organism.
Methods Vaginal swabs and male urine specimens were evaluated. Six standards of each organism were prepared by inoculating collection tubes with lab strains in concentrations ranging from 0 to 4×105 organisms. The log10 organism load for each positive specimen was determined by comparing the DC value to the calibration curve. Self-reported symptoms were available for each patient.
Results A total of 99 vaginal and 284 urine specimens were available for analysis. There was no statistical difference in DC value of mean organism load by gender. Neither was there a difference based on the presence or absence of symptoms in people infected with CT. For NG, there was a significant difference in mean DC value and organism load by gender (p<0.001 for both DC and organism load) with men having higher loads. In NG positive men the mean DC was 15.2 [95% CI 14.7 to 15.8] and 11.4 [95% CI 9.0 to 13.7] for men with and without symptoms (p=0.003). This translated in mean log10 organism loads of 6.5 [95% CI 6.3 to 6.6] and 5.4 [95% CI 4.7 to 6.1] for men with and without symptoms (p=0.005). In NG positive women there was no difference in organism load based on presence or absence of symptoms (p=0.220).
Conclusions Advantages to using this methodology include being able to quantify organism load from specimens obtained for routine diagnostic testing, using standardised test reagents that can be purchased commercially, and using an automated platform. Even in those settings that do not have the capacity for calibration, the DC values may provide useful relative loads. This exploratory study demonstrated the feasibility of using this method to obtain relative quantitation measures. Application of this tool to epidemiologic questions using larger data sets may prove useful.