The usual Hemoglobin colour test, are based on the pseudo-peroxidase activity of hemoglobulin (so-called guaiac-assay: oxidation of the substrate guajak results in a colour change). Several food components can lead to false positive or false negative results (like animal hemoglobin and myoglobin, peroxidases from animal or plant origin or antioxidants, like vitamin C).
The Hemoglobin ELISA is based on antibodies, specific for human Hemoglobin whereby above mentioned contaminants do not affect the immunoassay. Therefore, no special diet is necessary and certain drugs don't need to be discontinued before analysis.
The Hemoglobin ELISA is 100-times more  sensitive than the guaiac-assay. This avoids false negative results and - based on the antibody selection - false positive are almost excluded.

Free hemoglobin is immediately complexed by the serum protein Haptoglobin. The Haemoglobin-Haptoglobin complex is more stable during the long intestinal passage than haemoglobin alone. Even bleeding adenomas/carcinomas of the upper gastrointestinal tract can be detected. The combined use of both ELISA tests (Hemoglobin and Hemoglobin/ Haptoglobin complex) enables the early detection of colorectal carcinomas with the highest sensitivity.