Dextran is the most satisfactory plasma substitute. It is a polysaccharide produced when the bacterium Leuconostoc mesenteroides is grown in a sucrose-containing medium. The organism secretes an enzyme that converts to dextran according to the following equation:

Different strains produce dextrans of two main groups:

1. Long, practically unbranched chains of glucose units joined by 1:6 glucosidic linkages
2. Highly branched polymers consisting of short chains of 1:6 units joined by 1:4 and 1:3 linkages to branches

Branched chains are more likely to give rise to allergic reactions when injected, and when used for plasma substitutes in dextran, the linkages should be almost entirely of the 1:6 type. This is achieved by choosing a suitable specially developed strain of the organism that produces dextran in which about 95% of the linkages are 1:6.

Dextran 40 injection: A number of conditions, including severe burns, crush injuries and acute peritonitis are accompanied by a severe degree of sludging in the blood. This can be reduced by the administration of Dextran 40 injection, which because of the presence of polymers of low molecular weight, lowers plasma viscosity and improves capillary flow. Both these changes reduce cell aggregation, which in turn further improves the flow.

A crude dextran of low molecular weight is manufactured by including very small template molecules in the fermentation medium. Then fractionation is used to produce the clinical material, which has an average molecular weight of 40,000.