A unit of quantity for vitamin K based on biological activity, mid-20th century, determined by the following assay. Vitamin K is essential to blood clotting, and the assay is based on the clotting time in chicks. One Ansbacher unit = the least amount of vitamin K that, when injected into vitamin-K-deficient chicks weighing 70 to 100 grams, will make the prothrombin time (a standardized test¹ of the time it takes the chick's blood to clot) less than 6 minutes, within 6 hours of the injection.
One Ansbacher unit = 0.0008 milligrams menadione, and is about 20 Dam units, or about the amount of vitamin K in 67 milligrams of dried alfalfa.
It is named for the biochemist Stefan Ansbacher (1905 – 1995).
1. The Quick test:
American Journal of Physiology, volume 118, page 260 (1937).
New observation on the Vitamin K deficiency of the chick.
Science, volume page 221 (2 September 1938).
After describing his experiments, Ansbacher states “Obviously the above observations can be employed as the basis for a quantitative biological assay of vitamin K. Such a method has been used successfully with several hundred chicks and will be reported in full in the near future.” See Dam unit for a reference that compares various assays for vitamin K.
Journal of Nutrition, volume 17, page 303 (1939).
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Last revised: 15 December 2005.