Assay of fibrinogen in small blood samples.

T IS DIFFICULT to perform accurate fibrinogen assays on small samples of blood. A significant fraction of the plasma is lost when it is separated from the cells in 0.1or 0.2-mi. samples of blood and the plasma is diluted to a variable, usually undetermined, extent by the addition of anticoagulant. This can be minimized by using heparin dried in capillary tubes, but heparin interferes with subsequent clotting of the fibrinogen in the assay procedure. One alternative is to dilute the blood sample with a relatively large, and hence easily measured volume of anticoagulant that is isotonic with red cells. In the procedure reported here the blood is drawn and mixed with 3 or 4 volumes of isotonic buffer, the packed cell volume is determined, and the diution of the plasma can then be calculated. This procedure was developed to determine the plasma fibrinogen level in rats 5-6 times in 24 hours. Samples larger than 0.2 ml. resulted in significant hemodilution and artificial changes in fibrinogen level in response to sampling. Attempts to use small syringes or needles fitted with glass capillaries similar to those used by Natelson (1, 2) resulted in variable dilution of the plasma or in partial or complete coagulation of the blood.