ASK THE EXPERT: How reliable are Brix refractometers for measuring passive transfer?

Be aware of the limitations of total serum protein or Brix assessment for evaluation of passive transfer in individual calves! Serum total protein and % Brix can be successfully used at the herd level to monitor FPT in calves; however, interpretation of individual calf values on a daily basis can be difficult due to individual variation.

The evaluation of serum total protein (STP) and serum total solid values (%Brix) by optical and digital Brix refractometers at 24 hours of life is widely used to determine the presence of failure of passive transfer in calves (FPT, serum IgG at 24h < 10 g/L). A recent study [Deelen SM J. Dairy Sci 2014; 97(6):3838] using 400 calf serum samples demonstrated a high correlation between digital Brix refractometer (% Brix), STP, and serum IgG values, suggesting either method could be used to monitor calves.

It is important to understand that while there is sufficient correlation among the measurements to be useful on a herd basis there are limitations such that both serum protein and %Brix should be used with caution for interpretation of individual calf passive transfer status.

We evaluated 157 serum samples randomly collected from calves that received maternal colostrum and analyzed at Saskatoon Colostrum Company Ltd. (SCCL) for STP, %Brix, and IgG concentration by radial immunodiffusion (RID). As in previous reports, there was a linear correlation between STP, %Brix, and serum IgG (r2 = 0.88 and 0.82, respectively). The sensitivity and specificity of 5.5 g/dL STP to detect FPT was 94.1% and 82.1%, respectively. The sensitivity and specificity of 8.4% Brix to detect FPT was 76.1% and 88.7%, respectively. The positive and negative predictive values of < 5.5 g/L STP to diagnose FPT were 59.2% and 98% respectively. The positive and negative predictive values of < 8.4% Brix to diagnose FPT were 61.5% and 94% respectively.

Often STP values of 5.5 g/dL and Brix of 8.4% are used as “cut-offs” for predicting adequate passive transfer.  Using these criteria most calves are correctly categorized but this was not true for all calves. In the following table are presented some individual values of %Brix, STP, and IgG in serum to illustrate this. Among the data from the 10 calves shown here all have adequate passive transfer (>10 g/L serum IgG) yet all have %Brix less than 8.4% and only one calf serum protein levels of 5.5 g/dL or greater.  

% Brix  STP (g/dL) IgG (g/L)
8.3 5.2 10.2
8.1 5.2 12.8
8.1 5.1 14.7
8.3 5.1 15.7
8.0 5.0 16.6
8.3 5.5 16.7
8.2 5.4 20.6
8.3 5.2 14.5
8.0 4.7 10.1
8.3 5.3 16.7

Table 1: Sample of test results from individual calf serum samples


Manuel F. Chamorro
Manuel F. Chamorro
DVM, PhD, DACVIM - Technical Veterinary Consultant, SCCL and Assistant Professor of Livestock Medicine and Field Service, College of Veterinary Medicine, Kansas State University
United States
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