Pre-Analytic Sources of Error with the Serum PAG-Pregnancy Test for Cows

The pregnancy associated glycoprotein (PAG) test for pregnancy detection in cows necessitates transportation of blood samples to the laboratory. This investigation addresses preanalytic sources of error that might compromise its reliability. During shipping blood samples undergo substantial temperature fluctuations (Experiment 1). Temperatures of whole blood beyond 0°C had no effect, whereas freezing reduced measurements by 22% at -10 °C and by 25% at -20 °C (Experiment 2). Freezing of blood with low PAG content (Experiment 3) caused an increase from 2.4 to 3.7 ng/ml (P < 0.01). Cryopreservation of serum with various PAG concentrations (Experiment 4) brought about increases to varying degrees. The presence of heparin and EDTA in collecting tubes had no effect on PAG measurements, whereas citrate caused an initial reduction, but remained stable thereafter (Experiment 5). In blood stored six months at chilling temperature no change in PAG values occurred as long as samples contained heparin or EDTA (Experiment 6). In Experiment 7 vortexing of whole blood showed no effect, whereas freezing and dilution with water seriously compromised results. In summary, to obtain reliable PAG measurements, contamination with water must be avoided; freezing of whole blood or serum and the use of collecting tubes containing citrate will result in inaccuracies without altogether distorting results. High ambient temperature, physical agitation and long term storage at chilling temperature in the presence of heparin or EDTA will have no impact. PAG determination in blood may thus be considered a reliable pregnancy test for cows in most situations.