1447
Interrelationships between methods of blood mineral measurement in early postpartum dairy cows
The objective of this study was to determine the relationship between blood minerals measured by different methods and to determine the relationship between blood total (tCa) and ionized calcium (iCa) measured on samples taken from early postpartum dairy cows, as well as the agreement of tCa and iCa for diagnosis of subclinical hypocalcemia (SCH). Seventeen multiparous Holstein dairy cows were sampled 2× in the 24 h period postpartum and 1×/d for the following 4 d. Whole blood was analyzed immediately after collection for iCa by an iSTAT Portable Clinical Analyzer (PCA), and serum was analyzed for tCa, Mg and P using both the IDEXX VetTest (VT) and colorimetric methods at a veterinary diagnostic laboratory (DL). Serum total minerals measured by VT vs. DL were highly correlated (tCa r=0.95,P<0.0001;Mg r=0.91,P<0.0001;P r=0.97,P<0.0001). A VT tCa cutpoint with the highest combined sensitivity (96%) and specificity (85%) for diagnosing SCH (defined as DL tCa ≤8.0 mg/dL) was found to be 8.9 mg/dL as determined by receiver operator characteristic (ROC) analysis. The correlation between tCa measured by DL and iCa measured by PCA was high (r=0.89,P<0.0001). Generally when iCa is used to diagnose SCH, a cutpoint of 4.0 mg/dL iCa is used based on the assumption that iCa constitutes 50% of tCa. Using this assumption, agreement (as determined by McNemar’s Test) for diagnosis of SCH (≤8.0 mg/dL tCa, ≤4.0 mg/dL iCa) was poor (Exact P=0.06;Kappa=0.45, P<0.05). Based on the two samples taken postpartum [7(±4) h and 20(±4) h postpartum], this data showed that iCa constituted 58% of tCa in the 24 h postpartum. The iCa cutpoint with the greatest combined sensitivity (91%) and specificity (87%) for diagnosing SCH was determined by ROC analysis and found to be 4.68 mg/dL. Overall, serum minerals (tCa, Mg and P) measured by standard laboratory techniques are highly correlated with minerals measured by the VT and different cutpoints can be used to accurately diagnose SCH with tCa measured by the VT. The relationship between iCa and tCa in the period immediately postpartum must be better characterized before iCa can be used for diagnosis of SCH.
Keywords: ionized calcium, subclinical hypocalcemia, serum minerals