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

We evaluated the Precision Xtra™ ketometer during a larger study
categorizing free-ranging Steller sea lion (Eumetopias jubatus; SSL) pup
fasting status, necessitating identification of plasma β-hydroxybutyrate
concentrations ([β-HBA]) around a < and ≥0.3 mmol/L threshold.
Whole blood samples mixed with sodium heparin (NaHep) or
ethylenediaminetetraacetic acid liquid anticoagulants were tested
<10 minutes after collection (n=14; triplicate technical
replicates). Plasma (stored at -80°C, NaHep, Thaw1) measured via
our laboratory’s Reference Assay (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
(Thaw1) measurements. The ketometer produced results with linearity to
the Reference Assay for both whole blood and plasma (Thaw1). We
identified a non-linear relationship between plasma at Thaw1 and Thaw2
(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 Reference Assay β-HBA <0.2 and
>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 Reference Assay, 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.