To the Editor: In the paper, "Four Methods for Detecting Fibrinogen Degradation Products" (New Eng J Med 283:663, 1970), Thomas et al., using a log,-log2 graph, suggest that a significant correlation exists between the levels of fibrinogen degradation products (FDP) obtained by the serum "Fi" test and by the tanned red-cell hemagglutination immunoinhibition assay (TRCHII) in patients with consumption coagulopathy. A word of caution seems indicated to these authors before they assign such accuracy to the "Fi" test.The serum "Fi" test has been used most often only as a semiquantitative screening test for the presence of FDP, usually performed . . .