FIXATION OF COMPLEMENT 125 



mistake of giving a positive result by absence of 

 hemolysis in the front row tube is avoided by finding 

 a like result in the control tube in the back row. In 

 reading the results, varying degrees of hsemolysis may 

 be observed in tests of different sera, ranging from 

 complete haemolysis in negative cases, to absence of 

 haemolysis in strongly positive cases. Even slight de- 

 grees of inhibition of haemolysis are generally to be re- 

 garded as positive, the closeness of reading of border- 

 line cases being dependent on clinical facts and the 

 experience of the worker. The stronger the serum in 

 syphilitic " reagin," the more complement will be 

 fixed, less being left free for haemolysis. In a serum 

 containing smaller amounts of the Wassermann sub- 

 stance, only part of the complement will be fixed, leav- 

 ing the remainder free, and we get partial haemolysis 

 (see Plate II). 



Quantitative Estimation, — While the degree of 

 haemolysis produced gives an idea of the strength of 

 the reaction up to absorption of one unit of comple- 

 ment, yet it does not make any measure of positive 

 cases in which the serum is capable of absorbing more 

 than one unit of complement. In other words, absence 

 of haemolysis as seen by the usual method means a 

 strongly positive reaction, but indicates no difference 

 in the degree of strength of strongly positive sera; 

 so that a serum may show no signs of weakening under 



