Whole-blood metabolic biomarkers improve detection of subclinical ketosis in postpartum grazing dairy cows

Subclinical ketosis ( SCK ) is a common metabolic disorder during early lactation and is commonly identified using blood β-hydroxybutyrate ( BHB ) concentration. Portable analytical systems allow metabolic biomarkers to be assessed under field conditions, but the diagnostic value of whole-blood glucose and lactate for SCK remains uncertain. This observational study with repeated measurements evaluated portable whole-blood biomarkers for identifying SCK in postpartum grazing dairy cows using BHB as the reference standard. A total of 259 observations from 96 clinically healthy postpartum cows managed under high-altitude grazing conditions were analyzed. SCK was defined as BHB ≥ 1.0 mmol/L. Serum glucose ( SGLU ), point-of-care blood analyzer glucose ( EGLU ), glucometer glucose ( GGLU ), and point-of-care blood analyzer lactate ( ELAC ) were evaluated. Within-cow dependence was accounted for inferential analyses. ELAC had the numerically highest AUC (0.693; 95% CI: 0.633–0.754), followed by EGLU (0.627; 95% CI: 0.557–0.697), GGLU (0.595; 95% CI: 0.516–0.675), and SGLU (0.482; 95% CI: 0.397–0.567). After Holm adjustment for pairwise ROC comparisons, only the difference between ELAC and SGLU remained significant. In cow-clustered logistic regression, higher ELAC was associated with greater odds of SCK (OR = 1.74; 95% CI: 1.30–2.35), whereas lower EGLU was also associated with SCK (OR = 1.04 per 1-mg/dL decrease; 95% CI: 1.01–1.07). Results were attenuated when SCK was defined using the more restrictive BHB threshold of ≥ 1.2 mmol/L. Lactate reflects a metabolic pathway distinct from ketone body accumulation and may provide complementary metabolic information; however, it should not replace direct BHB measurement for SCK identification. Further studies are needed to determine whether lactate adds diagnostic or prognostic value beyond BHB alone.