Point-of-need visual test for the detection of γ-glutamyltransferase in calf serum

Adequate colostrum intake is critical for the passive transfer of immunity in calves, and serum γ-glutamyltransferase (GGT; EC 2.3.2.2) is a potential indicator. However, conventional assays for its detection require laboratory facilities, limiting their use in the field. Herein, a simple visual test has been developed for the semi-quantitative determination of serum GGT using immobilized reagents on paper supports. The enzyme substrate, drop volume, and membrane type were optimized, with γ-L-glutamyl-3-carboxy-4-nitroanilide, 10 µL drop volume, and a binder-free microfiber glass membrane providing the best performance. The visual test showed a limit of semi-quantification (LOSQ) of 39 U/L, while digital analysis slightly improved the sensitivity. When applied to calf serum samples, the test reliably reflected GGT evolution: negligible at birth, peak activity at 2 days (corresponding to colostrum intake), declining by day 14, and further decreasing at day 18. The visual test showed strong agreement with the conventional enzyme assay (κ = 0.94), correctly estimating 26 out of 31 samples (84 %), and digital analysis of the membranes correlated even more closely (R = 0.9924). This point-of-need (PON) device offers a rapid, low-cost, and practical tool for on-farm monitoring of colostrum intake, supporting early detection of failure of transfer of passive immunity (FTPI) and improved calf health management.