In dairy cattle operations, the sustainability of reproductive efficiency depends on shortening the interval between parturition and subsequent conception, which is of great importance for farm profitability. Therefore, pregnancy diagnosis should involve not only the identification of pregnant animals but also the early and accurate detection of non-pregnant individuals. In this context, rapid test kits based on pregnancy-associated glycoproteins (PAG) provide a practical alternative diagnostic method. The aim of this study was to evaluate the diagnostic reliability of a PAG-based rapid test kit for early pregnancy detection in cattle under field conditions. The study included 10 heifers confirmed pregnant by ultrasonography at 30–40 days of gestation and 10 cows confirmed as non-pregnant at ≥110 days postpartum. Blood serum samples were analyzed using a commercial PAG rapid test kit. Test performance was assessed based on accuracy, sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV), and agreement with ultrasonographic findings was evaluated using the Kappa coefficient. In heifers tested at 30–40 days of pregnancy, accuracy, sensitivity, and PPV were all 100%. In non-pregnant postpartum cows, false-positive results were observed in 30% of cases, resulting in a specificity of 70%. Overall test performance was 85% accuracy, 100% sensitivity, 70% specificity, 77% PPV, and 100% NPV. The agreement between PAG test results and ultrasonography was good (Kappa = 0.70). Pearson correlation analysis demonstrated a moderate-to-strong positive correlation between PAG and ultrasonography results. In conclusion, PAG-based rapid test kits offer high sensitivity and practical advantages for early pregnancy diagnosis in cattle; however, their use is recommended after at least 100 days postpartum to reduce false-positive results.