8S4 THE KAHN REACTION 



for a complete test, the quantity of serum chosen for each of the three proportions 

 was 0.15 cc, making the quantities of suspension 0.05, 0.025, and 0.0125 cc respec- 

 tively. 



The precipitation reaction between serum and antigen suspension lends itself to 

 the determination of the relative number of reacting substances in s\'philitic serum. 

 Since such determinations would be made in practice most frequently in cases under- 

 going anti-syphilitic treatment, a serum-suspension proportion of somewhat greater 

 sensitiveness than that employed in any of the tubes in the routine test, namely, 

 15:1, was chosen for a quantitative procedure. A series of serum dilutions with 

 normal saline — 1:1, 1:5, 1:10, 1:20, and so on up to 1:60 — are prepared, and 0.15 

 cc. of each of the dilutions are tested with o.oi cc. of antigen suspension. As will be 

 shown later, if undiluted serum alone gives a precipitation reaction, the final result 

 is four units; if the diluted sera also give precipitation reactions, then the highest 

 dilution multiplied by 4 gives the final result. 



THE SHAKING OF THE INGREDIENTS 



Strongly potent serum when mixed with standard antigen suspension in the 

 proper proportion will produce a precipitate after shaking for about ten seconds. 

 Weaker sera may require several minutes' shaking to bring forth a precipitate. The 

 optimum shaking speed was found to be about 275 oscillations per minute. This 

 optimum applies especially to weaker reactions. Most strongly reacting sera show 

 somewhat bulkier precipitates at a speed under 275. When the oscillations extend 

 beyond 300 per minute, the tendency is to break up the particles, making the reading 

 of the tests somewhat more difficult. A shaking period of three minutes is ample for 

 the routine diagnostic procedure, although increased shaking up to seven or ten 

 minutes tends to bring forth more marked precipitation, especially in some weakly 

 positive sera, without apparently producing false reactions. The standard shaking 

 period is three minutes, at 275 oscillations per minute, with a stroke-length of about 

 1.5 inches. Rapid hand shaking approximates this speed. 



The effect of incubation on the test — either at room (21° C), icebox (6° C), or 

 incubator (37° C.) temperatures — is inversely proportional to the effect of shaking. 

 With but little shaking, incubation will bring forth many precipitates. The effect of 

 incubation following shaking for three minutes is comparatively insignificant, al 

 though an occasional positive reaction will be produced following a negative reaction 

 after the shaking period. After five or six minutes' shaking, many reactions following 

 incubation are weaker, the precipitates going into solution — more correctly, into a 

 state of dispersion. To a lesser degree, weaker reactions are also obtained following 

 incubation after shaking for three minutes. 



THE READING AND INTERPRETATION OF THE RESULTS 



A definite precipitate suspended in a clear medium is read + + + + . Proportion- 

 ally weaker reactions are read -f + + , + + , +, and +, respectively. After reading 

 the results in each of the three tubes of the routine test, the final result is computed 

 by taking the average of the readings of the three tubes. It is obvious that for a final 

 reaction to be interpreted as -}- + + + , each of the tubes of the routine test must show 



