About two units of discarded blood are required to create a unit of one of the polymerized human hemoglobin solution. One-third of the blood transfused is used for acute resuscitation and therefore, four million units could provide additional substrate for blood substitutes. Animal hemoglobin solutions are inexpensive and widely available. If bovine hemoglobin was used as the substrate, a herd of 150,000 cows could generate enough hemoglobin to make the equivalent of six million units, or about 40 units per cow. Occult pathogens such as bovine spongiform encephalitis would require eradication prior to human use.
Recombinant hemoglobin has great long-term potential.
The primary limitations in the use of recombinant hemoglobin as a substrate for blood substitutes are the cost of the facilities needed to produce gigantic volumes of bacterial culture, and the stringent purification methods required. It is estimated that only 0.1 grams of hemoglobin can be generated from a liter of Escherichia coli culture. If one assumes that 15 gm of hemoglobin equals one unit of packed red cells, then 750 liters of culture are needed to create a unit of blood. Six million units would necessitate 2.25 billion liters of culture. Currently, a unit of PRBCs has an average cost of 175 dollars in the US market.