Use of Tritiated Water for Measurement of Total Body Water in Animals

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Yousif Hussein Abdalla
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The problem of overestimation of body associated with the use of tritium was investigated by exploring problems connected with the use of tritium in measuring total body water in laboratory and domestic livestock. Several in vitro and in vivo experiments were carried out to measure and study TBW in rabbits, sheep and goats. Methods used included extrapolation and equilibration methods. Samples were assayed using whole plasma and protein free plasma. Quench correction was made by the internal standard and the channel's ratio methods. The results showed that the assay of protein free plasma coupled with quench correction measured body water more accurately than whole plasma. In a preliminary in vitro experiment conducted prior to the in vivo experiments to measure constant volume of plasma water, it was found that the error with protein free plasma was almost 0.21% whereas with whole plasma an overestimation of 4.5% was noticed. There was no significant difference between the results when applying either channels ratio or internal standard methods for measuring quenching. Also, the use of different types of scintillation fluids has no effects on accuracy of measurement. It was also found that the most suitable volume of scintillation fluid to be used was 12 ml. In rabbits experiment, TBW was measured by direct (freeze drying) and indirect methods (tritiated water dilution technique in the same animal). There was a close agreement between values estimated by the two methods. It was also found that extrapolation method for determination of tritiated water space had the same accuracy as equilibration technique.
Use of Tritiated Water for Measurement of Total Body Water in Animals