In 1913, van den Bergh and Snapper1 described a test for bilirubin in the blood, which was really an application of the diazo reaction. Not much was done with respect to this test, probably because of the war.
In 1918, van den Bergh2 published a monograph giving the test in detail and also a modification of it. In this treatise he states his belief that there are different types of bilirubin and that these can be distinguished by this reaction. These two reactions are mentioned by him as the direct and the indirect reactions. He believes the direct reaction is due to the type of bile which is present as the result of mechanical obstruction. The indirect reaction is found in cases in which no such mechanical obstruction exists.
He states that the difference between the types of bilirubin is based not alone on the difference in their
GRULEE CG, MEBANE A. THE VAN DEN BERGH TEST FOR ICTERUS IN THE BLOOD OF INFANTS. Am J Dis Child. 1925;30(2):219-223. doi:10.1001/archpedi.1925.01920140079007