Evaluation of a portable fluorometer for the quantification of vitamin E in blood at key physiological stages of dairy cattle.

Vitamin E is essential in mitigating the impact of oxidative stress on periparturient dairy cows and neonatal calves. Therefore, it is essential to measure circulating vitamin E concentrations accurately. Currently, the only reliable method is an expensive and time-consuming procedure using liquid chromatography-mass spectrometry (LC-MS). However, a cheaper and faster method has been developed, which allows the quantification of circulating vitamin E through the use of a handheld fluorometric analyzer (HFA) called the vitamin E iCheck (BioAnalyt GmbH). Our objective was to compare the accuracy of the HFA to the reference LC-MS method for measuring vitamin E in bovine samples. A total of 177 samples collected for other studies were used: 98 newborn calf serum samples from a vitamin E supplementation study (including treated and control animals) and 79 whole-blood samples from cows 1 to 7 d postcalving. Vitamin E concentrations were measured on thawed calf serum and fresh cow EDTA blood using the HFA, following the manufacturer's instructions. Whole blood from cows was then centrifuged to obtain plasma. Vitamin E was also quantified in calf serum and cow plasma at the Michigan State University Veterinary Diagnostic Laboratory using LC-MS. Calf and cow results were analyzed separately because they represent different biological matrices and physiological times. In each dataset, results between the HFA and LC-MS determinations were compared using Passing-Bablok regressions and Bland-Altman plots. The HFA showed a poor linear relationship with LC-MS for calf serum and cow plasma (intercept = 0.33 and 0.67 μg/mL, respectively). The HFA unreliably estimated vitamin E, with a mean bias of -3.2 and 0.6 μg/mL for calves (vitamin E concentration range: 0.28 to 30.75 μg/mL) and cows (0.8 to 5.88 μg/mL), respectively. Moreover, 40.4% of the calf samples read below the linear range of acceptable results for the HFA, making it unsuitable for this age group. Hence, under the conditions of our study, the HFA yielded unreliable results and cannot be recommended for field use.