A modification to the isotope-dilution technique for estimating milk intake of pigs using pig serum

The aim of this study was to compare the measurement of deuterium oxide (D2O) directly in pig serum with that in sublimed whole blood. This was to assess whether excluding vacuum sublimation before analysis would cause any significant loss of accuracy in estimates of pig milk intake. Water and serum standards were made in deionized water and serum, respectively, and were assayed with samples under the same conditions on a fixed-filter, infrared spectrophotometer. The mean concentration of D2O in sublimed samples was 2,244 microg/mL of body water, and the mean concentration of D2O in serum samples was 2,184 microg/mL of body water. The mean ratio of D2O concentration in deionized water to the D2O concentration in serum was 1.0275, which was used as a correction factor to convert serum D2O concentration to D2O concentrations in body water. Using this method, the mean concentration of D2O in all serum samples was identical to that in sublimed samples (i.e., 2,244 microg/mL of body water). Mean milk intake of pigs based on sublimed samples was 1,006 g/d and that based on serum samples was 1,012 g/d. This confirms that milk intake determined from measurement of D2O directly in pig serum is sufficiently precise.