Simultaneous estimation of neonatal total body water by antipyrine and H2(18)O dilution

Total body water was estimated simultaneously by dilution of antipyrine (antipyrine space; APS) and H2(18)O (18O-space; 18OS) in 5 baboon neonates and 2 two-month-old lambs (total of 14 studies). Calculations of 18OS were made either from a single plasma delta 18O measured 4 h after injection of the marker (18OS4) or by extrapolation to time zero of several plasma delta 18O obtained at precisely timed interval (18OS0). Mean (+/- SD) 18OS4 was on the average 23% lower than mean 18OS0 (796 +/- 70 ml/kg vs. 650 +/- 67 ml/kg, p less than 0.001). Mean APS was not statistically different from mean 18OS0 (664 +/- 127 ml/kg vs. 658 +/- 57 ml/kg), but APS ranged from 41% lower to 30% higher than 18OS0. These data and information from the literature suggest that antipyrine is of doubtful reliability and should be abandoned as a marker for total body water.