S 2238 [ H-D-phenylalanyl-L-pipecolyl-arginine-nitroanilide,H-D-phenylalanyl-Pip-Arg-p-nitroanilide,H-Phe-Pip-Arg-p-nitroanilide,H-Phe-Pip-Arg-pNA,S 2238 dihydrochloride,S-2238,chromogenic substrate S-2238,H-D-Phe-Pip-Arg-pNA ] chromogenic substrate for thrombin; used in amidolytic assay; patterned after N-terminal portion of A alpha chain of fibrinogen; synonym S-2238 refers to di-HCl