title: Changes in hematocrit based on incremental blood sampling: mathematical models perform poorly.
abstract: OBJECTIVE
  Excessive blood sampling, with its inherent risks, is of growing concern among clinicians. We performed this study to measure the changes in hematocrit (Hct) during a laboratory investigation where multiple blood samples are collected. The performance of a simple mathematical model, used in clinical practice to predict Hct changes, is evaluated.
  
  
  METHODS
  Eight healthy male volunteers participated in this study. The equation Hct(f) = Hct(i)*(EBV-BL)/EBV is used to predict changes in Hct. Where Hct(f) and Hct(i) are, respectively, the final and initial Hct, EBV is the estimated blood volume and BL is the blood loss.
  
  
  RESULTS
  Thirty-five pharmacokinetic samples per subject were collected totalling 314 mL of BL. The Hct decreased from 44.2% +/- 2.2% to 39.9% +/- 2.5% (P = 0.001). On average, model predictions tended to have a discrete tendency to underestimate the Hct changes (-0.5% points of bias). While the predictions of the Hct were very accurate in 50% of the subjects, the discrepancy of the Hct predictions was clinically significant in the other 50% of the subjects.
  
  
  CONCLUSIONS
  Consistent with the model prediction, this study demonstrated a significant reduction in the Hct values in healthy subjects undergoing incremental phlebotomy. On average, the model successfully predicted the decrease in Hct. However, the inter- and intra-individual variabilities in the Hct changes are clinically significant. In clinical settings, which are not well controlled environments, the variability is likely to be greater and the clinical use of the model cannot replace the need to monitor the Hct.
authors.affiliation: Department of Anesthesiology, Duke University Medical Center, Durham, North Carolina, USA. licorti@med.puc.cl
authors.forename: Luis I
authors.initials: LI
authors.lastname: Cortinez
authors.forename: Jacques
authors.initials: J
authors.lastname: Somma
authors.forename: Kerri M
authors.initials: KM
authors.lastname: Robertson
authors.forename: John C
authors.initials: JC
authors.lastname: Keifer
authors.forename: David R
authors.initials: DR
authors.lastname: Wright
authors.forename: Yung-Wei
authors.initials: YW
authors.lastname: Hsu
authors.forename: David B
authors.initials: DB
authors.lastname: MacLeod
authors.forename: Eugene W
authors.initials: EW
authors.lastname: Moretti
mesh_terms: D000328:Adult; D006400:Hematocrit; D006801:Humans; D008297:Male; D008433:Mathematics; D008962:Models, Theoretical; D018962:Phlebotomy / Q000009:adverse effects*