408 



VITAMIN K GROUP 



more specifically on the blood prothrombin level and, in turn, the effect 

 of vitamin K supplements on the prothrombin level. The improved method, 

 while requiring more preliminary preparation, has several advantages over 

 simple blood-clotting time, namely : 



1. A larger sample of blood is used and is, therefore, more likely to be 

 an accurate sample from the chick. 



2. Measurements are made in seconds rather than minutes, and the 

 gathering of the final data is expedited. Determinations can be made in 

 duplicate. 



3. Conveniently measurable values can be obtained from every chick. 



4. The variability of the results within a test group is much lower than 

 that of the simple blood-clotting times^^- ^^ (Table II). 



TABLE II 



Means, Standard Errors, and Coefficients of Variability of Simple 

 Blood-Clotting Times, and Whole Blood-Prothrombin 



Times 



Reference 



standard of 



vitamin K per 



kilogram of diet, 



mg. 



No. of 

 chicks 



Blood-clotting time 



Mean and standard 

 error, min. 



CoeflScient 



of 

 variability 



Whole blood-prothrombin time 



Mean and Coefficient 



standard of 



error, sec. variability 



3 



6 



12 



15 

 15 

 15 



10.07 ± 2.31 

 3.42 ± 0.55 

 1.97 ± 0.37 



61.1 

 60.2 

 72.1 



51.2 ± 1.9 



37.3 ± 1.0 

 30.5 ± 0.5 



17.1 

 9.6 

 6.1 



Blood samples may be taken simply by cutting off the head of the chick. ^ 

 Samples have also been taken from a carotid artery ,2^ or from a jugular 

 vein,^^' ^* or from a wing vein.^^' -^ Prothrombin has been determined by an 

 elaborate two-stage method,^^- ^^ or by a method in which the ratio of the 

 strength of a clotting agent (thromboplastin) required to clot the plasma 

 in a specified time to the strength required to clot normal plasma-^ is de- 

 termined. The reader should consult the original papers for details of these 

 methods. 



5. Computation of Results 



It was found that over the sensitive assay range a plot of the reciprocal 

 mean prothrombin time against the logarithm of the vitamin K dosage 



26 H. J. Almquist, Physiol. Revs. 21, 194 (1941). 



26 H. Dam and J. Glavind, Biochem. J. 32, 485 (1938). 



2' F. W. Stamler, R. T. Tidrick, and E. D. Warner, J. Nutrition 26, 95 (1943). 



28 E. D. Warner, J. Exptl. Med. 68, 831 (1938). 



29 H. J. Almquist and E. L. R. Stokstad, J. Nutrition 14, 235 (1937). 



'« H. P. Smith, E. D. Warner, and K. M. Brinkhous, /. Exptl. Med. 66, 801 (1937). 



