To the Editor:— In this column there recently appeared a note by Dr. Armand J. Quick dealing with prothrombin tests as applied to the control of vitamin K therapy (June 17, p. 2552). One such test, advocated by our laboratory, consists simply of mixing thromboplastin (a simple saline extract of lung) with whole blood and observing the clotting time (Ziffren, S. E.; Owen, C. A.; Hoffman, G. R., and Smith, H. P.:Proc. Soc. Exper. Biol. & Med.40:595 [April] 1939). The clotting time is normally from twenty-five to thirty seconds but is typically prolonged when the prothrombin level is low. Dr. Quick objects to this test on the ground that clotting times cannot be measured accurately at the bedside. We have had no particular difficulty in this respect. A stop-watch is desirable, but even this is not essential, for an error of two seconds affects the result by