Performance of Total Blood Volume Algorithms in Obesity and Severe Obesity

Amid the ongoing obesity epidemic, the estimation of total blood volume (TBV) in obese people remains hotly contested without common agreement among apheresis practitioners. We compare the results of estimated TBV from different formulas across a spectrum of obese BMIs from a cohort of 155 individual patients. We also plot the difference between the resulting TBVs, assigning a threshold of functional significance of 500 mL, at which the choice of formula might make an impact on patient care. We compare the mean TBV estimated in non-severe versus severe obesity for all the above algorithms and determine the impact of algorithm choice on various apheresis procedures. The choice of algorithm has a significant impact on apheresis procedures; for example, we found differences of up to ~3-4 plasma units for a therapeutic plasma exchange depending on the choice of algorithm applied. We additionally find that algorithm performance varies widely in both men and women, particularly in morbid obesity, and often produces values that fall outside an empirically chosen expected range. We do not find a clinically significant difference in mean estimated TBV between non-severe and severe obesity in any algorithm tested, suggesting that physiological changes in obesity may fail to be captured by these algorithms. We hope these results are useful to other apheresis practitioners and help them make an informed choice of algorithm to estimate TBV in their obese patients. However, our current algorithms for estimating TBV may be flawed, and the field may wish to move toward measurement of TBV using recently available commercial options.