Beta-hydroxybutyrate ketone concentrations via ketometer and colorimetric assay in Steller sea lion whole blood and plasma

<p>We evaluated the Precision Xtra™ ketometer during a larger study categorizing free-ranging Steller sea lion (<em>Eumetopias jubatus</em>; SSL) pup fasting status, necessitating identification of plasma β-hydroxybutyrate concentrations ([β-HBA]) around a &lt; and ≥0.3 mmol/L threshold.  Whole blood samples mixed with sodium heparin (NaHep) or ethylenediaminetetraacetic acid liquid anticoagulants were tested &lt;10 minutes after collection (n=14; triplicate technical replicates).  Plasma (stored at -80°C, NaHep, <em>Thaw1</em>) measured via our laboratory's <em>Reference Assay </em>(Sigma Aldrich, St. Louis, MO, Kit #MAK041) served as the standard [β-HBA] for ketometer comparisons.  Our observed β-HBA range (0.0–1.6 mmol/L), consistent with published [β-HBA] of free-ranging Otariid pups, represented the lower 20% of the ketometer's range (0.0–8.0 mmol/L).  The maximal coefficient of variation (%CV) of ketometer technical replicates was 9.1% (NaHep, whole blood).   The majority of ketometer technical replicate sets (84%, including all matrices, anticoagulants, and thawings) were identical (CV=0%). We found linear relationships and agreement of ketometer [β-HBA] between whole blood preserved with different anticoagulants and between whole blood and plasma (<em>Thaw1</em>) measurements.  The ketometer produced results with linearity to the <em>Reference Assay</em> for both whole blood and plasma (<em>Thaw1</em>).  We identified a non-linear relationship between plasma at <em>Thaw1</em> and <em>Thaw2</em> (tested four months apart, NaHep), as only samples with higher SSL [B-HBA] decreased in concentration, and  all others remained the same.  With respect to categorizing SSL pup fasting, the ketometer % Accuracy, %Sensitivity, and %Specificity for samples with <em>Reference Assay</em> β-HBA &lt;0.2 and &gt;0.4 mmol/L were 100%.  We adopted a modified procedure:  plasma samples with mean ketometer concentrations ±0.1 mmol/L of 0.3 mmol/L β-HBA were re-evaluated using the <em>Reference Assay</em>, improving measurement precision from tenths (ketometer) to thousandths (assay) mmol/L.  The Precision Xtra™ ketometer was valuable to our application, over the range of [β-HBA] observed in SSL pup plasma and whole blood samples.</p>