Von Dungern's indigo test for syphilis is practically unavailable for the serum diagnosis of syphilis, because we can not find any difference as to this test between syphilitic and nonsyphilitic sera.
Among the substances that the reagent contains neither indigo nor salts play any part in the inhibition of the coagulation, as von Dungern formerly thought.
The coagulation of sera in this reaction is prevented only by the alkali, as Flood proved.
The degree of alkalinity that can prevent the coagulation of serum, varies according to the individual, not according to the presence or absence of syphilis.
On the other hand, the alkalinity that prevents the coagulation of sera varies according to the concentration of the sera; that is, the more diluted the serum, the more easily may its heat coagulation be prevented by alkali.