An observational study of early neonatal biochemical parameters in twins

Early neonatal biochemical values are generally considered to reflect maternal values. However, studies have been inconclusive due to the statistical methods used. We hypothesized that there would not be important differences in plasma biochemistry between newborn twins, suggesting a common mechanism of control, and that differences between unrelated infant pairs would be greater. All twins over a 5-year period who had plasma biochemistry measured within 6 hours of birth were identified retrospectively. An unrelated control infant was matched to one of each twin pair. The 95% limits of agreement for plasma urea, creatinine, and sodium were calculated for twin pairs and unrelated matched infant pairs. Fifty-three twin pairs were studied. For urea, creatinine, and sodium, 95% of differences between twins were less than or equal to 1.5 mmol/L (8.9 mg/dL), 9.2 μmol/L (0.1 mg/dL), and 5.1 mmol/L (11.7 mg/dL), respectively. In unrelated infant pairs, the corresponding values were 4.0 mmol/L (24.3 mg/dL), 33.9 μmol/L (0.4 mg/dL), and 8.2 mmol/L (18.9 mg/dL). Differences between unrelated infant pairs were significantly wider than differences between twins ( p < 0.001). This study has demonstrated a close and significant agreement in urea, sodium, and creatinine when measured soon after birth in twin pairs compared with unrelated infants, implying a common mechanism of control.