Transfusion Calculator

The expected hemoglobin rise from a red cell transfusion is fundamentally a dilution calculation. A unit of packed red cells contains roughly 200 to 250 mL of red cells at a hematocrit of 55 to 80 percent, suspended in additive solution to a total volume near 300 to 350 mL, and carries on the order of 50 to 60 g of hemoglobin. Distributing that mass through the recipient's circulating volume gives the increment, so the rise equals transfused hemoglobin mass divided by blood volume, where blood volume is estimated at about 70 mL/kg in adults, 80 mL/kg in children, and 85 to 90 mL/kg in infants. This is why one unit raises hemoglobin by roughly 1 g/dL and hematocrit by about 3 points in a 70 kg adult, but by nearly 3 g/dL in a 25 kg child. Contemporary practice is restrictive. For most hemodynamically stable hospitalized adults the transfusion threshold is a hemoglobin of 7 g/dL, raised to 8 g/dL in patients with established cardiovascular disease or undergoing cardiac and orthopedic surgery. Pediatric dosing is weight based at 10 to 15 mL/kg rather than by whole units. The predicted increment assumes no ongoing bleeding, hemolysis, or splenic sequestration, and an observed rise below about half of the prediction should prompt a search for alloantibodies or continued blood loss with a direct antiglobulin test and repeat crossmatch. A hemoglobin drawn 1 to 2 hours after the transfusion finishes is reliable in a non-bleeding patient. Transfusion is a treatment for symptoms and oxygen delivery, not for a laboratory number.

Transfusion only became reproducibly survivable after Karl Landsteiner identified the ABO blood groups in 1901, work that earned him the 1930 Nobel Prize and explained why earlier attempts had so often produced fatal hemolytic reactions. The second obstacle was clotting. In 1914 and 1915 several investigators working independently showed that sodium citrate could anticoagulate blood without poisoning the recipient, and Richard Lewisohn at Mount Sinai Hospital in New York established a practical citrate concentration, which made stored rather than direct donor-to-recipient transfusion possible. Refrigeration and citrate-glucose preservative solutions extended shelf life through the First World War and after, and in 1937 Bernard Fantus at Cook County Hospital in Chicago established and named the first hospital blood bank. Component therapy grew out of Edwin Cohn's plasma fractionation work at Harvard during the Second World War, which allowed whole blood to be separated into red cells, plasma, and derivatives so that each patient received only the component needed. The final shift was in thresholds. For most of the twentieth century a hemoglobin of 10 g/dL served as an informal transfusion trigger. The TRICC trial reported by Paul Hebert and colleagues in 1999 found that a restrictive threshold of 7 g/dL was at least as safe as 10 g/dL in critically ill patients, and a series of subsequent randomized trials extended that conclusion across surgical, cardiac, and gastrointestinal bleeding populations, producing the restrictive recommendations now issued by AABB and NICE.