Agreement of a Handheld Glucose Meter with Laboratory Serum Glucose Measurements in Dairy Cows

Blood glucose concentration is commonly used to assess energy metabolism in dairy cows. Handheld glucose meters allow rapid measurement under farm conditions, but their performance in cattle may differ from that obtained with laboratory methods. This study evaluated the agreement between capillary whole-blood glucose measured with the Accu-Chek® Instant handheld glucose meter and venous serum glucose measured with a laboratory biochemical analyzer. A total of 93 Holstein–Friesian animals provided 117 paired glucose measurements; 24 animals were sampled on two occasions. Capillary blood was collected from the vulvar mucosa and measured immediately, while venous blood was collected from the coccygeal vein and processed to serum for laboratory analysis. The mean glucose concentration was 3.27 ± 0.51 mmol/L in capillary whole blood and 3.44 ± 0.42 mmol/L in venous serum. The mean paired difference was 0.17 mmol/L (95% CI, 0.06–0.27 mmol/L; p = 0.0018). The correlation between methods was weak (r = 0.267), and Lin’s concordance correlation coefficient was 0.246. Bland–Altman analysis showed limits of agreement from −0.94 to 1.27 mmol/L. A significant proportional bias was also identified (β = −0.302; 95% CI, −0.562 to −0.042; p = 0.023). Categorical agreement was limited (Cohen’s κ = 0.241), and 47 of 117 paired measurements were classified into different glucose categories. These findings show that the two measurement approaches are not interchangeable for individual-cow assessment. The observed differences may reflect not only device performance but also differences in sampling site, sample matrix, and sample handling.