184 ANTIGENS AND THE TECHNIC OF SERUM REACTIONS 
Determination of Spijial-fiuid Reacting (Kahn) I nits.— The number 
of reacting units in spinal fluid is determined by the formula S = 4 D, 
where S is the potenc}^ of spinal fluid in terms of Kahn reacting units, 
and D is the highest dilution ratio in which the concentrated globulin 
solution or its equivalent dilution of spinal fluid shows a definite posi- 
tive precipitation reaction. If only the undiluted globulin solution 
gives a positive reaction, then the spinal fluid is considered as contain- 
ing 4 reacting (Kahn) units. If a positive reaction is obtained in the 
case of the dilution ratio 5, then the spinal fluid contains 4 X 5 or 20 
units. If the original unconcentrated spinal fluid (dilution ratio 10) 
gives a positive reaction, then the spinal fluid contains 4 X 10 or 40 
units. The following table gives some typical reactions. 
Typical Reactions with the Quantitative Spinal-fluid Procedure. 
Dilution ratio of 
Highest dilu- 
Spinal fluid 
globulin solution . 
1. 
5. 
10. 
15. 
20. 
tion ratio of 
globulin solu- 
potency in 
terms of 
Diluted globulin sol- 
tion giving 
reacting units 
ution, cc. . 
0.15. 
0.15. 
0.15. 
0.15. 
0.15. 
definite pre- 
cipitation 
(equivalent 
to 4 D). 
Antigen suspension, 
reactions. 
S. 
CO 
0.01. 
0.01. 
0.01. 
0.01. 
0.01. 
D. 
Reaction No. 
1 
Pos. 
1 
4 
2 
Pos. 
Pos. 
5 
20 
3 
Pos. 
Pos. 
Pos. 
10 
40 
4 
Pos. 
Pos. 
Pos. 
Pos. 
15 
60 
Highly Potent Spinal Fluid.— li a spinal fluid gives a definite pre- 
cipitation reaction when the dilution ratio is 20, examine still higher 
dilutions until a negative reaction is obtained. Returning to the 
qualitative spinal-fluid procedure, it should be emphasized that highly 
potent spinal fluids give positive reactions directly without concen- 
trating the globulins by means of ammonium sulphate. It is recom- 
mended therefore, that in examining spinal fluids from clinically 
diagnosed cases of neurosyphilis, 0.15 cc. fluid be mixed with 0.01 ec. 
antigen suspension, the antigen being employed in the same titer as 
for tests with serum. If the spinal fluid is definitely positive ( + + + + , 
-f- + + or ++) in the unconcentrated state, the reaction may be 
interpreted as 4+ in the qualitative procedure and as at least 40 units 
in the quantitative procedure. (To complete this last procedure, 
dilutions of spinal fluid with saline are made and tested as indicated 
above.) If the reaction is very weak or negative (+, ± or — ), the 
spinal fluid is examined in the usual manner, after concentrating the 
globulins. 
Relationship Between Kahn and Wassermann Test.— It is likely that 
the basic interaction between the serum and antigen substances is the 
same in both methods. The dift'erence appears to be that in the Kahn 
test the end-result of this interaction is a visible precipitate, while in the 
Wassermann test the end-result not being visible, a special indicator 
