Peripheral blood hematocrit in critically ill surgical patients: an imprecise surrogate of true red blood cell volume.
BACKGROUND
Peripheral blood hematocrit (red blood cell volume/total blood volume) is conventionally used to determine the need for blood transfusions. In critically ill surgical patients, this variable may not accurately approximate true red blood cell volume. We compared peripheral blood hematocrit to (1) plasma volume, (2) estimated circulating blood volume, and (3) a normalized hematocrit to clarify their relationships.


METHODS
Consecutive patients admitted to the surgical intensive care unit were evaluated using the BVA-100 Blood Volume Analyzer (Daxor Corporation, New York City, NY). Plasma volume was directly measured by serial tagged albumin concentration. Red blood cell volume was calculated using plasma volume and the peripheral blood hematocrit result. All volumes were presented as percentage deviation from ideal volumes. These ideal volumes were obtained using a patented formula incorporating ideal body weight as determined by Metropolitan Life tables. The peripheral blood hematocrit was compared with a "normalized" hematocrit, defined as the hematocrit value if plasma volume was adjusted to a normal whole blood volume.


RESULTS
Eighty-six data points were recorded for 40 patients with average age 61 +/- 20 yr, APACHE II score 20 +/- 6, and a 13% mortality rate. The primary reasons for admission were severe sepsis/septic shock (n = 11), hemorrhagic shock (n = 7), respiratory failure (n = 20), and cardiac failure (n = 2). Bland-Altman analysis showed a mean difference of 3.4 +/- 7.8 hematocrit percentage points between normalized and peripheral blood hematocrit methods, with a 95% confidence interval of 1.7-5.1 and limits of agreement of +/-15.2 hematocrit percentage points. Peripheral blood hematocrit was lower than the normalized hematocrit in 48% of measurements, higher in 17%, and equivalent in 35%.


CONCLUSIONS
Peripheral blood hematocrit may not accurately estimate red blood cell volume in a cohort of critically ill surgical patients. This remains to be validated in a larger group of patients, comparing these results with the double isotope technique.
Takanishi
Danny M
DM
Department of Surgery, John A. Burns School of Medicine, University of Hawaii, Honolulu, HI 96813, USA. dtakanis@hawaii.edu
Yu
Mihae
M
Lurie
Fedor
F
Biuk-Aghai
Elisabeth
E
Yamauchi
Hideko
H
Ho
Hao Chih
HC
Chapital
Alyssa D
AD
D018806:APACHE; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001803:Blood Transfusion; D001810:Blood Volume; D001811:Blood Volume Determination / Q000379:methods*; D003422:Critical Care; D004911:Erythrocyte Volume; D005260:Female; D006400:Hematocrit; D006801:Humans; D007362:Intensive Care Units; D008297:Male; D008875:Middle Aged; D008954:Models, Biological; D018579:Patient Selection; D010953:Plasma Volume; D019106:Postoperative Hemorrhage / Q000175:diagnosis* / Q000401:mortality / Q000503:physiopathology / Q000628:therapy; D011237:Predictive Value of Tests; D011865:Radioisotope Dilution Technique; D011933:Reagent Kits, Diagnostic; D015203:Reproducibility of Results
D011933:Reagent Kits, Diagnostic