Use of a Point-of-Care Progesterone Assay to Predict Onset of Parturition in the Bitch

Abstract

An assay of circulating progesterone (P4) is commonly used to estimate progress through late gestation in the bitch. Point-of-care assays provide rapid results, a major advantage over laboratory-based assays. This study aims to compare P4 levels determined by the Catalyst® Progesterone point-of-care assay with those determined by chemiluminescent immunoassay (CLIA) and to identify the expected distribution of Catalyst P4 levels at time intervals 3 days prior to the onset of parturition in pregnant bitches. Twenty-eight pregnant bitches carrying two or more fetuses were admitted to a specialist veterinary reproduction hospital 53 days after the onset of cytological diestrus or, when that date was not known, 57 days after the last mating. Vaginal speculum examinations were performed every 6 h until the onset of cervical dilatation (TCD). Serum samples were collected twice daily (08h00 and 18h00) until TCD. For most samples, fresh serum was assayed for P4 immediately using the Catalyst assay (CatP4), then frozen until assayed by CLIA (IMMULITE 2000; ImmP4). However, for some samples, CatP4 was not analyzed prior to freezing. For these data points (n = 33), CatP4 for fresh serum was estimated from CatP4 assayed on frozen-thawed serum, based on a comparison between CatP4 on fresh vs. frozen-thawed sera. In comparison to ImmP4, CatP4 levels up to and including 7 nmol/L appear to have a constant bias of -1.69 nmol/L (limits of agreement -4.91 to 1.52), while levels >7 nmol/L appear to have a proportional bias of -17.9% (limits of agreement -68.6% to 32.7%). Bootstrapped percentiles of CatP4 results spanned 0.4-9 nmol/L within 12 h of TCD, 0.9-11 nmol/L 12-24 h from TCD, and 2.2-13.5 nmol/L 24-36 h from TCD. A CatP4 >9 nmol/L indicates a bitch that is unlikely to reach TCD within 12 h. Bitches with CatP4s below 3.5 nmol/L are likely to reach TCD within 36 h and bitches with a CatP4 below 2.2 nmol/L are likely to reach TCD within 24 h. In conclusion, the Catalyst Progesterone assay provides rapid assessment of circulating P4 in the bitch, with clinical application in the monitoring of late term pregnant bitches.

Keywords: Catalyst; IDEXX; P4; chemiluminescent immunoassay; dog; in-house analyzer; vaginoscopy; whelp.

Figure 1

Serum progesterone levels measured with the Catalyst and IMMULITE assays in 28 bitches before and at the first detected time of cervical dilatation (TCD, shown as time zero). Time 1–5 represent the regularly scheduled bleeding times at 0800h and 1800h that occurred before TCD. Time 1 could occur 0–14 h before time zero.

Figure 2

Passing–Bablok regression of 134 progesterone levels measured with the Catalyst and IMMULITE assays.

Figure 3

Bland-Altman plot showing agreement between 134 progesterone levels measured with the point-of-care Catalyst assay and those measured with chemiluminescent immunoassay (IMMULITE).

Figure 4

Progesterone levels were determined using the Catalyst point-of-care assay (CatP4) paired with fresh and frozen-thawed sera. Duplicate CatP4 was determined for both the fresh and frozen-thawed aliquots. Linear regression was performed after the exclusion of the outlier.

Figure 5

Serum progesterone levels determined with the point-of-care Catalyst assay (CatP4) at regular times every day (08h00 and 18h00) and at the time of cervical dilatation (TCD; Time 0) in 28 bitches. The data are aligned to TCD level. Shaded boxes indicate estimated distribution, determined using 100,000 bootstrapped samples and reference interval statistics to infer levels for the central 95% of a larger population similar to the study population.