The hemoglobin alone should not be the sole criterion for transfusion, since oxygen delivery is the critical issue. As a general rule of thumb, one unit of RBCs raises the hemoglobin of average-size adult who is anemic approximately 1 g/dL.
Default characteristics of packed red cell unit:
(1) hematocrit 70-80%
(2) volume about 250 mL after centrifugation
(2a) addition of preservative solutions (adenine-saline) may reduce the hematocrit to 60% and increase the volume to 330 mL
(2b) The volume of pediatric quad (4 transfer) or quint (5 transfer) pack varies from 50 to 125 mL or more, depending on the nature of the transfer packs and needs of the patient.
where:
• Hemoglobin = hematocrit / 2.941
Default target hemoglobin based on rate of blood loss:
(1) acute: target 10 g/dL hemoglobin
(2) chronic: target 8 g/dL hemoglobin
Risk of volume overload:
(1) Look for a history of congestive heart failure or current cardiac disease.
(2) Administer slow and monitor.
(3) Split units.
Massive transfusion:
(1) Look for administration of volume of blood equal to or greater than blood volume.
(2) Monitor for DIC
(3) Administer FFP, cryoprecipitate and platelets based on fibrinogen, PT and platelet count
Adult Quick and Dirty Rules
number of units packed RBCs to transfuse in the normal adult =
= (round to nearest whole number ((target hemoglobin) - (current hemoglobin)))
Calculated
From the equation:
target hemoglobin in g/dL =
= (grams hemoglobin after transfusion) / (blood volume in deciliters after transfusion) =
= (((hemoglobin before transfusion) * (blood volume in mL before transfusion) / 100) + (((product hematocrit in percent) / 2.94) * (volume of product in mL) / 100 * (number of products given))) / (((blood volume in mL before transfusion) / 100) + ((volume of product in mL) / 100 * (number of products given)))
where:
• A volume in mL is divided by 100 to determine deciliters
• Hematocrit divided by 2.94 gives hemoglobin.
• The hematocrit for packed red cells is usually between 70 and 80%. It should not exceed 80% since this results in insufficient preservative fluid for prolonged storage.
• If there has been an acute blood loss, then the patient's blood volume before transfusion is the normal blood volume minus the volume of the blood loss.
• The volume before transfusion plus volume of blood given is post-transfusion volume.
The following equation can be derived:
number of packed red cells to give =
= ((blood volume before transfusion in mL) * ((target hemoglobin) - (hemoglobin before transfusion))) / ((volume of product in mL) * (((product hematocrit) / 2.94) - (target hemoglobin)))
Limitations:
• Initial hemoconcentration or hemodilution can affect final hemoglobin.
Comments:
• Blood replacement in surgery should not be based on estimates of blood loss, as this typically results in unnecessary transfusion (Smetannikov 1996). Monitoring of hemoglobin and/or hematocrit provides a more reliable guide to replacement needs.
• Preoperative expansion of circulating blood volume with clear fluids can help decrease intraoperative hemoglobin loss.
