Systematic Review and Meta‐Analysis of Diagnostic Accuracy of Serum Refractometry and Brix Refractometry for the Diagnosis of Inadequate Transfer of Passive Immunity in Calves

Abstract

BackgroundTransfer of passive immunity in calves can be assessed by direct measurement of immunoglobulin G (IgG) by methods such as radial immunodiffusion (RID) or turbidimetric immunoassay (TIA). IgG can also be measured indirectly by methods such as serum refractometry (REF) or Brix refractometry (BRIX).ObjectivesTo determine the accuracy ofREFandBRIXfor assessment of inadequate transfer of passive immunity (ITPI) in calves.DesignSystematic review and meta‐analysis of diagnostic accuracy studies.MethodsDatabases (PubMed andCABAbstract, Searchable Proceedings of Animal Science) and Google Scholar were searched for relevant studies. Studies were eligible if the accuracy (sensitivity and specificity) ofREForBRIXwas determined using direct measurement of IgG byRIDor turbidimetry as the reference standard. The study population included calves <14 days old that were fed with natural colostrum (colostrum replacement products were excluded). Quality assessment was performed by theQUADAS‐2 tool. Hierarchical models were used for meta‐analysis.ResultsFrom 1,291 references identified, 13 studies of 3,788 calves were included. Of these, 11 studies evaluatedREFand 5 studies evaluatedBRIX. The median (range) prevalence ofITPI(defined as calves with IgG <10 g/L byRIDorTIA) was 21% (1.3–56%). Risk of bias and applicability concerns were generally low or unclear. ForREF, summary estimates were obtained for 2 different cutoffs: 5.2 g/dL(6 studies) and 5.5 g/dL(5 studies). For the 5.2 g/dLcutoff, the summary sensitivity (95%CI) and specificity (95%CI) were 76.1% (63.8–85.2%) and 89.3% (82.3–93.7%), and 88.2% (80.2–93.3%) and 77.9% (74.5–81.0%) for the 5.5 g/dLcutoff. Due to the low number of studies using the same cutoffs, summary estimates could not be obtained for BRIX.Conclusions and Clinical ImportanceDespite their widespread use on dairy farms, evidence about the optimal strategy for using refractometry, including the optimal cutoff, are sparse (especially forBRIX). When usingREFto rule outITPIin herds, the 5.5 g/dLcutoff may be used whereas for ruling inITPI, the 5.2 g/dLcutoff may be used.