Background: The Precision Xtra® electrochemical meter is widely used to measure blood β-hydroxybutyrate concentration ([BHB]b) in dairy cattle. The meter uses an algorithm optimized for human blood that assumes the hematocrit in cattle is the same as in humans,
and equivalent intra-erythrocyte β-hydroxybutyrate concentration ([BHB]e) and plasma β-hydroxybutyrate concentration ([BHB]p).
Objectives: The primary objective was to characterize the analytical performance of the meter for measuring [BHB]b and [BHB]p in dairy cattle. Secondary objectives were to
characterize the influence of hematocrit and sample temperature on [BHB]p measured by the meter.
Methods: Blood and plasma samples were obtained from 106 periparturient Holstein cattle and 15 lactating Holstein cows with experimentally-induced electrolyte and acid-base
imbalances. Meter performance was evaluated using Deming regression and Bland-Altman plots. Multivariable linear regression was used to determine the effect of hematocrit and sample temperature on the [BHB]b and [BHB]p, respectively.
Results: The meter was linear up to [BHB] = 3.0 mmol/L as measured by the reference method, equivalent to meter values for [BHB]b > 4.5 mmol/L and [BHB]p > 5.2 mmol/L. An
increase in hematocrit increased the measured value for [BHB]b; this result was partially explained by [BHB]e being much lower than [BHB]p. Changes in sample temperature caused a linear change in the measured value for [BHB]p whenever [BHB]p > 3.0 mmol/L.
Conclusions: Meter accuracy was markedly dependent on the [BHB]e to [BHB]p ratio and therefore the hematocrit; consequently, the algorithm used by the meter should be revised when applied to bovine blood in order to improve accuracy.
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