362 ANIMAL BIOCHEMISTRY 



occurrence of substances interfering with the measurements. Many 

 instances of serious error of this and other types might be cited, and 

 others can be reasonably expected in the future. Thus some sort of 

 standards to be met by the analytical methods employed will at least 

 aid in reducing the size and frequency of errors. 



Criteria for Useful Assays 



Whatever the method, it should satisfy the following conditions if 

 at all possible. 



1. Values obtained at various levels of the factor in the assay should 

 agree over at least a four-fold range of concentration. In other words, 

 quadrupling the amount of the factor should produce a proportional 

 increase in the property measured. 



2. Consistent values should be obtained on replication at different 

 times. 



3. The assay must agree with other acceptable methods of analysis 

 within reasonable limits of experimental error. 



4. The method should be specific for the factor under test. Test 

 systems or organisms should be supplied with sufficient materials of 

 all other kinds in order that the one in question be the only variable. 



5. Recovery values at various concentration levels should agree with 

 known amounts added. Recovery values are obtained by mixing 

 known amounts of the factor with standard quantities of the unknown 

 source. This procedure checks for interference from the unknown 

 source and shoidd give additive values. 



6. There should be no drift in results; that is, there should be no 

 trend either high or low in the analysis of standard samples. 



Satisfaction of these criteria indicates a useful method for assay of 

 vitamins or almost any other physiological factor. Since the number 

 of methods is becoming enormous, procedures cannot be discussed in 

 detail. For the present, the assays are grouped into the following 

 four categories. 



Biological Methods 



As the name implies, biological methods depend on the use of 

 experimental animals to determine the presence or absence of a specific 

 vitamin in a food mixture. Animals most commonly chosen for vita- 

 min assays are rats, mice, pigeons, and chickens. Guinea pigs are 

 employed almost exclusively for biological estimations of ascorbic acid. 

 Hamsters have been used for certain types of vitamin assays, and 

 higher animals, such as the dog, have been utilized to a limited extent. 



