Measurement of water turnover using a stable isotope technique: calculation of error propagation

Water turnover may be determined with stable isotope techniques from the product of tracer dilution space and tracer elimination rate. Tracer concentrations must be measured in two body fluid samples which are separated by a sampling interval. The tracer concentration of the first sample may be charged with an error which may be due to analytical errors, incomplete tracer dilution or because the necessary steady-state assumption (water influx equals water efflux) is violated at the moment of tracer loading. Error propagation shows that the influence of this error on the estimation of water turnover is partially compensated for. The goodness of compensation depends on the sampling interval and on the magnitude of the error of the first sample. The error is best compensated using sampling intervals of 1.5 tracer half life.