LIMITATION OF THE METHOD 195 



Purpose of the Method. The opsonic index aims to serve as a guide: 



1. In diagnosing the presence of bacterial infection, or rather in dis- 

 covering whether the natural protective powers of the patient's blood 

 have been diminished or increased as the result of the immunizing in- 

 fluence of the infection. 



2. In connection with vaccine therapy, to guard against diminishing 

 the opsonin content of the patient's blood; to assure ourselves that 

 our efforts to increase them have been successful, and occasionally to 

 ascertain how long the store of opsonin that has been obtained for the 

 patient remains in the blood. 



Limitation of the Method. In ascertaining the opsonic index of a 

 patient's serum, we must take it for granted although it has not been 

 proved : 



1. That the bacteria act the same in the body as they do in the test- 

 tube. This is known not to be the case, for virulent organisms resist 

 phagocytosis, whereas a non-virulent strain of the same bacterium is 

 easily phagocyted. If, therefore, a laboratory culture of attenuated 

 organisms is used in making the opsonic index, the result can hardly 

 be accepted as a criterion of the power of the patient to overcome the 

 "resistant" or more virulent organism as it occurs in the body. This 

 source of error can be overcome in a manner if the microorganism is 

 isolated and used at once before attenuation occurs. 



2. That the leukocytes are a constant factor, and need not be taken 

 into account. Investigation has shown that, as a result of infection, 

 the leukocytes probably undergo qualitative changes and it is hardly 

 fair to accept phagocytosis by normal leukocytes as a criterion of pha- 

 gocytosis with the patient's own leukocytes, as it occurs in the body dur- 

 ing the infection. 



3. The method assumes that phagocytosis by the polynuclear leuko- 

 cyte plays a large part in overcoming the infection. In many cases, 

 however, this is by no means proved. For example, in tuberculosis 

 it is not this form of leukocyte, but the mononuclear form or the lympho- 

 cyte, which seems to be more important, and hence it is difficult to un- 

 derstand how the index with the polynuclear leukocyte can aid the 

 question of diagnosis or treatment. 



4. The chances for error are considerable. To be of any value, the 

 work requires experience and painstaking care. The results obtained 

 by competent workers with the same blood may show variation, but 

 it must be said that, with strict attention to technic and insistence upon 

 perfect preparations, the worker may usually obtain valuable results. 



