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Background  Transfer 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 ).    Objectives  To determine the accuracy of  REF  and  BRIX  for assessment of inadequate transfer of passive immunity ( ITPI ) in calves.    Design  Systematic review and meta‐analysis of diagnostic accuracy studies.    Methods  Databases (PubMed and  CAB  Abstract, Searchable Proceedings of Animal Science) and Google Scholar were searched for relevant studies. Studies were eligible if the accuracy (sensitivity and specificity) of  REF  or  BRIX  was determined using direct measurement of IgG by  RID  or 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 the  QUADAS ‐2 tool. Hierarchical models were used for meta‐analysis.    Results  From 1,291 references identified, 13 studies of 3,788 calves were included. Of these, 11 studies evaluated  REF  and 5 studies evaluated  BRIX . The median (range) prevalence of  ITPI  (defined as calves with IgG <10 g/L by  RID  or  TIA ) was 21% (1.3–56%). Risk of bias and applicability concerns were generally low or unclear. For  REF , 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/ dL  cutoff, 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/ dL  cutoff. Due to the low number of studies using the same cutoffs, summary estimates could not be obtained for BRIX.    Conclusions and Clinical Importance  Despite their widespread use on dairy farms, evidence about the optimal strategy for using refractometry, including the optimal cutoff, are sparse (especially for  BRIX ). When using  REF  to rule out  ITPI  in herds, the 5.5 g/ dL  cutoff may be used whereas for ruling in  ITPI , the 5.2 g/ dL  cutoff may be used.

journal of veterinary internal medicine

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