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This research aimed to define thresholds for ewe colostrum and lamb serum Brix refractometer measurements in lowland Scottish sheep. This would facilitate the use of this convenient, sheep-side test, enabling quick and accurate identification of poor quality colostrum and prevention of failure of transfer of passive immunity (FTPI) in lambs. Secondary aims were to identify risk factors for poor colostrum quality and FTPI in lambs. Serum samples (n = 233) were collected from lambs between 24 and 48hrs after birth, from four lowland Scottish meat sheep farms. Pre-suckle colostrum samples (n = 112) were also collected from ewes on two of these farms. Farmers provided information on litter size, ewe body condition score, ewe breed and dystocia. Duplicate digital Brix refractometer measurements were compared with immunoglobulin G (IgG) radial immunodiffusion (RID) testing for all colostrum and serum samples. Receiver operating characteristic (ROC) curves were used to redefine thresholds for Brix testing in colostrum and serum. Linear regression models were constructed with colostrum and serum IgG concentration as the outcomes of interest. Colostrum and serum IgG concentrations were highly variable. The prevalence of inadequate colostrum quality (using <50 g/L IgG on RID) was 4.5% (95% CI = 1.5 - 10.1) and the prevalence of FTPI (using <15 g/L IgG in serum on RID) was 7.73% (95% CI = 4.64-11.93). A ewe's colostrum IgG concentration was significantly and positively associated with the serum IgG concentration of her lamb(s) (p = 0.02). ROC analysis defined a Brix threshold for adequate colostrum quality of > 22.10% (sensitivity 80% (95%CI=28.4-99.5), specificity 90% (95%CI=82.3-94.8)). ROC analysis defined a Brix threshold for serum of > 8.65% for adequate passive transfer of immunity in Scottish lambs (sensitivity 94% (95%CI=72.7-99.8), specificity 82% (95%CI=76.6-87.2)). To optimise passive transfer of immunity in lambs, we suggest that ewe colostrum Brix measurements be defined as 'poor' (<22%); 'fair' (22-26%) and 'good' (>26%); and lamb serum as 'poor' (<8%); 'fair' (8-9%) and 'good' (>9%). It is recommended that these tests are used as for flock screening, using samples from multiple animals., Competing Interests: Declaration of Competing Interest Funding was provided by the Ruminant Bursary from MSD Animal Health Ltd. The funders were not involved in study design, data collection, data interpretation or publication. None of the authors has any other financial or personal relationships that could inappropriately influence or bias the content of the paper., (Copyright © 2023 The Authors. Published by Elsevier B.V. All rights reserved.)