Haem in the gut. Part II. Faecal excretion of haem and haem‐derived porphyrins and their detection

Only a fraction of haem (ferroprotoporphyrin) finding its way into the gut lumen is absorbed; the major portion enters the colon. There, unabsorbed haem, together with any haem of haemoproteins shed directly into the colonic lumen as haemoglobin or other haemoproteins, are converted by bacteria to a range of haem-derived porphyrins (HDP) lacking iron. This conversion is a slow and incomplete process and the amount converted in this way depends on colonic transit rate, site of bleeding and amount of luminal haem. As a consequence, faeces contain variable proportions of haem and HDP. The guaiac and tetramethylbenzidine tests give a qualitative index of faecal blood; they depend on the pseudoperoxidase activity of intact haem and cannot detect HDP. These tests perform better for large bowel bleeding than for more proximal bleeding. The fluorimetric HemoQuant assay quantitates both haem and HDP; it performs well for both proximal and distal bleeding. Neither type of test can allow for intestinal absorption of haem or HDP. Quantitation of gastrointestinal bleeding derived from measurement of faecally excreted haem and HDP is, therefore, likely to underestimate haem delivered into the gut lumen. In a given clinical situation, the choice of a haem-dependent occult blood test must take into account the possibility of colonic conversion of haem to HDP and the possible value of quantitation as opposed to qualitative detection.