Evaluating the feasibility of using digital Brix refractometry to determine the passive immune status of lambs

Determining serum immunoglobulin G (IgG) concentrations is critical for the assessment of passive immune status in lambs. This study aimed to assess the accuracy of the Brix refractometer for the determination of passive immune status in lambs.

Blood samples were collected from 417 randomly selected healthy 1‒7-day-old lambs from nine farms in western Türkiye. Digital Brix refractometry was used as the index test to determine the lambs' serum IgG concentrations, and the radial immunodiffusion (RID) technique was used as the reference test.

A strong correlation was detected between the serum RID‒IgG and Brix percentages (r = 0.86). The prevalence of lambs with serum IgG concentrations less than 6 g/L, 6 to less than 10 g/L, 10 to less than 15 g/L, 15 to less than 20 g/L, 20 to less than 25 g/L, 25 to less than 30 g/L, 30 to less than 35 g/L, 35 to less than 40 g/L and 40 g/L or more were 13.4%, 10.3%, 8.6%, 16.8%, 15.1%, 14.2%, 8.4%, 4.1% and 9.1%, respectively. The optimum thresholds determined for the digital Brix refractometer to detect lambs with serum IgG concentrations less than 6, less than 10, less than 15, less than 20, less than 25, less than 30, less than 35 and less than 40 g/L were 8.2 or less, 8.4 or less, 8.4 or less, 8.8 or less, 9.1 or less, 9.3 or less, 9.7 or less and 9.9 or less, respectively. The sensitivity and specificity at each IgG threshold were 80% or above.

Production type was not taken into consideration, which may influence serum Brix thresholds to determine passive immunity. Data are clustered by farm and analysis has not taken this into account; thus, prevalence estimates may not be reliable with regards to the true prevalence of inadequate transfer of passive immunity.

This study demonstrated that the digital Brix refractometer could be used as an accurate tool for the determination of passive immune status in lambs.