206 OPSONIC INDEX 



No attempt is made to count the phagocyted bacteria. The relative 

 amount of phagocytosis with the immune serum in various dilutions is 

 compared with the normal controls, and the result is expressed as the 

 bacteriotropic titer. 



Simon's Method. This method of counting the number of empty 

 leukocytes with a given dilution of serum is followed; a similar count is 

 made of the normal serum control in the same dilution; thus, if in the 

 control film 25 per cent, of leukocytes were empty, and in the patient's 

 film, 50 per cent., the index would be -f|- = 0.5. A study of the results 

 obtained by this method, and by careful counting after Wright's method, 

 shows that they are fairly comparable, and the method may be used 

 where it is only necessary to determine whether the index is high or 

 low. 



Precautions. 1. If phagocytosis is entirely absent, one should not 

 conclude that bacteriotropins are not present. The leukocytes may 

 have been injured, especially if heterologous leukocytes have been 

 present; control examinations with homologous leukocytes (i. e., from 

 the same animal) as the serum should result in phagocytosis. 



2. The time during which the tubes were in the incubator may have 

 been too short or too long. Most microorganisms require from one- 

 half to two hours meningococci require about one-half hour; piieumo- 

 cocci usually need at least two hours; typhoid and cholera bacilli about 

 fifteen minutes to thirty minutes, as they undergo extracellular or even 

 intracellular lysis quite readily. 



3. If the control of bacteria and leukocytes alone shows well-marked 

 phagocytosis the test should be repeated with another strain. 



PRACTICAL VALUE OF THE OPSONIC INDEX 



1. In competent hands, the opsonic index of normal persons to most 

 pathogenic organisms has been found to vary from 0.8 to 1.2. As 

 previously mentioned, it is difficult to find a perfectly normal serum for 

 such microorganisms as the Bacillus coli, the staphylococci, Bacillus 

 tuberculosis, etc., as it is unlikely that any individual can altogether 

 escape active infection at some period of his life. As menstruation ap- 

 proaches, even wider fluctuations occur, the normal index being re- 

 established by the second or third day. During the first year of life the 

 opsonic index varies to such a degree that it has little or no practical value. 



2. Although a large amount of work has been done with the op- 

 sonins in disease, it is the consensus of opinion that the determination of 



