MedWire News: Measuring peripheral blood hematocrit can under- or over-estimate critically ill patients' true red blood cell volume, say researchers.
"This may incorrectly lead clinicians to either over- or under-transfuse red blood cells, both of which may have negative consequences," the researchers write in the journal Anesthesia & Analgesia.
Danny Takanishi (University of Hawaii, Honolulu, USA) and colleagues note that their results add to the growing weight of evidence that peripheral blood hematocrit is an inaccurate reflection of total red blood cell volume.
Despite this, "peripheral blood hematocrit is currently viewed by many clinicians as the best surrogate available for estimation of total body hematocrit, and is the value most commonly used in contemporary clinical practice," says the team.
Takanishi et al estimated the red blood cell volume of 40 patients from their plasma volume and peripheral blood hematocrit.
The patients were aged an average of 61 years. Eleven were admitted to intensive care for severe sepsis/septic shock, seven for hemorrhagic shock, 20 for respiratory failure, and two for cardiac failure.
The team calculated normalized hematocrit values for the patients based on a validated formula incorporating gender, height, weight, and ideal body weight and adjusted them to measured plasma volume.
On average, the estimated red blood cell volume deviated from normalized values by 3.4 hematocrit percentage points. The estimated volume was lower than the normalized value in 48% of measurements, higher in 17%, and similar in 35%.
This may be partly due to the poor correlation between peripheral blood hematocrit, plasma and estimated total blood volume, the researchers note. Plasma volume explained 35.3% of the variation in peripheral blood hematocrit, while estimated total blood volume explained less than 1%.
In a related editorial, Robert Hahn (Clinical Research Center, Södertälje, Sweden) stressed that Takanishi et al looked at normalized rather than measured hematocrit, and that several factors in intensive care unit patients may affect the relationship between measured and normalized hematocrit.
These include use of anesthetics, changes of position, and multiple blood sampling.
However, Hahn noted that blood volume is infrequently assessed in intensive care patients. "Despite all controversies, the return of blood volume as a clinical parameter in the intensive care unit would be most welcome," he concluded.