ISO ANTIGENS AND THE TECHNIC OF SERUM REACTIONS 
and opalescent. The final result is the average of the readings of the 
three tubes, as indicated in the following table : 
Outline of Kahn Test and Interpretation of Results. 
Serum: Antigen suspension 
Antigen suspension, cc. . 
Serum (heated at 56° C. for 
thirty minutes) cc. 
^action 
No. 
1 . . 
+ + + + 
2 
+ + + 
3 ' '. 
+ + 
4 . . 
+ 
5 . . 
— 
6 . . 
— 
7 . . 
- 
8 . . 
— 
9 . . 
— 
10 . . 
— 
11 . . 
— 
12 . . 
— 
Tube No. 
1. 2. 
3. 
Completion of test. 
3:1 6:1 
12: 1 Tests are shaken three 
0.05 0.025 
0.0125 
minutes, 1 cc. salt sol- 
ution 
is added to first 
0.15 0.150 
0.1500 
tube 
and 0.5 cc. to 
other two tubes and 
result 
s are read. 
Interpretation of Results. 
Final result 
(Average of 
reactions of 
3 tubes). 
+ + + + 
+ + + +' 
+ + + + 
+ + + + 
+ + + + 
+ + + + 
+ + + + 
+ + + 
+ + + + 
+ + + 
+ + + 
+ + + + 
+ + 
+ + + + 
+ + 
± 
+ + + + 
— 
+ + + 
+ 
± 
+ + 
=fc 
=fc 
+ 
— 
+ 
— 
Recording Results.— Make a permanent record of findings in all tubes 
of each test at time of reading. Preferably, the tests should be read 
independently by two separate w^orkers. When two workers are not 
available, the original reading should be checked by the same worker 
after a short interval. 
Procedure With Less Than Three Tubes.— If there is insufficient serum 
for the three-tube test, examine and report as follows: (a) If enough 
serum for two tubes, employ the two lesser amounts of antigen suspen- 
sion. Report as a two-tube test. (6) If enough for one tube, employ 
the least amount of antigen suspension. Report as a one-tube test. 
(e) If less than 0.15 cc. serum is available, a one-tube test (micro test) 
may be made by employing 10 parts of serum to 1 part of antigen 
1 Weakly potent sera show most marked precipitation in the third tube because a 
small amount of reagin reacts best with a small amount of antigen suspension, the 
relatively larger amounts of suspension in the first two tubes being inhibitory to precip- 
itation. 
Strongly potent sera show 4 + precipitation in each of the three tubes, but due 
to the different amounts of antigen suspension employed, the precipitates are of unequal 
bulk, being greatest in the first tube and least in the last tube. 
In rare instances an atypical reaction is obtained in which precipitation is marked in 
the first tube and weak or negative in the second and third tubes. In such a case, a 
quantitative test should be made and if the result is 20 units or more, the qualitative 
reaction may be considered 4 + ; if less than 20 units the results of the qualitative reac- 
tion should be averaged. 
