154 CLINICAL BACTERIOLOGY AND HEMATOLOGY 



Lastly, the films are examined with the oil-immersion lens. The 

 polynuclear leucocytes will be found to contain the bacteria, and 

 it will be necessary to count the number in each of fifty leucocytes 

 in both your preparations — i.e., in that made with the patient's 

 serum and in the control made with that from a healthy person. 

 The ratio between the two gives the opsonic index. For example, 

 in one case the number of tubercle bacilli contained in fifty poly- 

 nuclear leucocytes taken at random amounted to 78. In the 

 control specimen from a healthy person the same number of poly- 

 nuclears contained 172. The ratio i^=0'45 gives the opsonic 

 index ; it shows that the patient has less than half the normal 

 amount of opsonin, and probably, therefore, less than half the 

 normal resisting power to the tubercle bacillus. 



The diagnostic value of the examination is considerable. In 

 acute infections the index is almost always low to the organism 

 causing the disease, but normal to others. For example, in 

 a case of severe furunculosis the index to staphylococci was 

 0-65, and in a case of pustular acne 0-7. In a case of septicaemia 

 due to streptococci the index (tested with a culture of strepto- 

 cocci obtained from the patient's blood) was o'6 on several 

 occasions. 



When spontaneous cure takes place it is accompanied by, and 

 is apparently due to, an increase of the opsonic power of the blood. 

 Thus, in pneumonia the opsonic index tested with pneumococci is 

 below normal until the crisis is reached, when there is a sudden 

 rise above normal, so that the patient is for a short period more 

 resistant than the healthy person. In staphylococcic lesions the 

 rise is as a rule more gradual and irregular, and the lesions may 

 persist when the index is high, though in most cases this heralds a 

 rapid improvement. 



The behaviour of the opsonic index in tuberculosis is very in- 

 teresting. The main feature is that it is very variable, especially 

 in patients in whom the disease is progressing rapidly and in those 

 who are taking exercise. This variability is supposed to be due 

 to auto-inoculation, bacilli being detached from the lesions and 

 lodged in the tissues, where they act as small doses of vaccine. 

 As a general rule it is thought that (in lupus especially*) a high 

 index is of favourable import, and vice versa, but there is not the 

 same more or less direct relation between a high index and the 

 process of cure that there is in pneumococcic and some other 

 * Bulloch, Trans. Path. Soc, 1905. 



