This procedure is sometimes used instead of blood transfusion. Plasma the fluid part of blood from which the cells have been separated may be dried and in powder form kept almost indefinitely; when wanted it is reconstituted by adding sterile distilled water. In powder form it can be transported easily and over long distances. Transfusion of plasma is especially useful in the treatment of shock. One advantage of plasma transfusion is that it is not necessary to carry out testing of blood groups before using it.