Noninferiority of cycle threshold values from 0.9% sterile saline compared with PBS as a collection medium for Tritrichomonas foetus RT-rtPCR testing

Abstract

Tritrichomonas foetus infection can cause reproductive losses and subsequent economic losses in cow-calf herds. Our objective was to determine if 0.9% sterile saline (saline) was noninferior to PBS as a transport medium for T. foetus reverse-transcription real-time PCR (RT-rtPCR) testing. Transport tubes were prepared with either 1.6 mL of PBS with preputial washing (n = 30) or 1.6 mL of saline with preputial washing (n = 30) and inoculated at 3 different concentrations of T. foetus organisms (high = 100 org/100 µL; moderate = 10 org/100 µL; low = 1 org/100 µL) for a total volume of 1.7 mL/tube. Samples were submitted to a diagnostic laboratory for RT-rtPCR analysis. The effect of concentration and medium on mean Ct values was tested using linear regression. Normalized means were used to conduct a noninferiority t-test, with a noninferiority margin of 1 Ct. For all analyses, α = 0.05. Mean Ct values (high = 18.5; moderate = 22.7; low = 30.4) differed by concentration (p <0.0001), whereas medium (p = 0.67) and the interaction of medium and concentration (p = 0.87) were not significant. The normalized mean Ct for PBS was 22.6 (min = 16.2; max = 27.0), and for saline was 22.8 (min = 20.9; max = 26.4). The normalized mean Ct of saline was noninferior to PBS (p = 0.037). The use of saline as a transport medium for T. foetus RT-rtPCR should not affect Ct results.