23 
color with guaiacum, and iron and copper can be excluded, the proba- 
bilities are that it is blood. According to Klein (87) the guaiacum 
test is of great value, especially when it leads to a negative result. 
Liman (105) is also of the opinion that when a substance under exami- 
nation gives a negative result with the van Deen test, it is not blood, 
but that a positive result does not prove the presence of blood without 
further investigation. Mecke and Wimmer (113) are also of the 
same opinion as Liman regarding the significance of this test. Marx 
(111) also calls attention to the negative aspect of the van Deen test 
as b^ing the most valuable. According to Schaer (140) the guaiacum 
test is chiefly valuable as a control on the hematin test and as a con- 
trol in the recognition of hemin crystals. Schwartz (167) agrees with 
Liman that if the result of the van Deen test is negative blood is 
absent, but that a positive result does not necessarily mean that 
blood is present. According to Foulis (56) the guaiacum test alone 
is not sufficient to prove the presence of blood in urine, but is useful 
if applied with other confirmatory tests. By others (4) it is looked 
upon as confirmative of the spectroscopic and hemin tests. Aschern 
(8) is of the opinion that the guaiacum test as applied to the exami- 
nation of urine (Alme’n) is more delicate than the method of Bird (20) 
or of Struve (175); when the test is positive it should be confirmed 
with the microscope. According to Fluhnefeld (73) one can con- 
clude with certainty that if a spot or substance under examination 
does not show these color reactions it is not blood; a positive result, 
however, does not necessarily show blood, as a number of substances 
can give such a reaction. This author describes methods whereby 
such confusing substances can be gotten rid of. Schuster (165) has 
arrived at the conclusion that the chief distinguishing criterion for 
blood lies in the fact that the ox}"gen carrier present in blood is not 
destroyed by boiling. Siegel (171), from the results of his exhaustive 
studies on the detection of blood coloring matter in feces, concludes 
that if one obtains a negative guaiacum or benzidin test, it indicates 
that the substance contains no blood or at least only minimal traces; 
if the guaiacum test on feces is positive, it indicates the presence of 
blood originating in the organism, or at least very probably so, 
especially if meats and foodstuffs containing blood have been excluded 
from the diet, and provided further that the test has been carried 
out in an absolutely clean reagent glass. McNamara (110) says that 
the guaiacum test is not positive for blood, but is valuable if applied 
with other tests. According to Schumm (163) the benzidin test is 
of considerable practical significance. Lefort (100) comes to the 
conclusion that the guaiacum reaction is valuable if used with other 
corroborative tests. According to Babcock (10) the guaiacum reac- 
tion certainly indicates the absence of blood when negative, and that 
under the conditions which he enumerates it is reliable and certain. 
