Comparison of Three Different Analyzers to Measure Canine Serum Progesterone

The canine estrous cycle is unique compared to other domestic animal species. During estrus, serum progesterone concentrations ([P4]) rise two days prior to ovulation. In fact, the luteinization of pre-ovulatory follicles resulting in this initial increase in [P4] (1.5-2.5 ng/ml) cannot be temporally dissociated from the onset of the surge in luteinizing hormone. For this reason, measuring progesterone from daily blood samples is commonly used to determine the optimal breeding day in female dogs. In addition, the fall in [P4] (<2 ng/ml) prior to parturition can be used for the purposes of determining the timing of an elective C-section in dogs. There are several methods veterinarians can use to measure [P4] but directly comparing results between assays without formulaic adjustments often yields unreliable results. The objective of this research was to compare [P4] measured on three different veterinary analyzers (enzyme linked fluorescent assay (ELFA), colorimetric immunoassay (CIA), and chemiluminescent immunoassay (CLIA)). It was hypothesized that irrespective of analyzer used, the [P4] measurement would be reliable for determining timing for breeding or C-section. Venous blood samples (n=116) were collected from privately-owned female dogs (n=44) at the Waipahu Waikele Pet Hospital in Honolulu, Hawaii. Dogs were fasted 6 to 8 hours prior to each blood collection and blood samples were collected into tubes containing a clot activator but not serum separating gel. Blood samples were allowed to clot for 30 to 60 minutes and then were centrifuged for five minutes at 3,500 rpm to allow for serum removal. In accordance with the manufacturer’s instructions, [P4] was determined from three analyzers. Data were managed in Google Sheets and analyzed using R. Using simple linear regression, the coefficients of determination (R2) between CIA and ELFA, CIA and CLIA, and CLIA and ELFA was 0.88, 0.914, and 0.957, respectively. The simple linear regression was also used to determine the regression equation, which was CLIA = 0.536*CIA+0.688, CLIA = 0.417*ELFA+0.321, and CIA = -0.45*ELFA+0.73. Using a Passing-Bablok regression, the Pearson correlation coefficients were 0.938, 0.956, and 0.978, respectively. However, analysis of the residuals showed an increase in the spread as the [P4] increased. The results from this study show that comparison of [P4] between the analyzers is accurate at lower values (<5 ng/mL) but the variability in [P4] increases as the value increases