Gastrointestinal Bleeding

and Ian A. D. Bouchier²

(1)

Bishop Auckland, UK

(2)

Edinburgh, Midlothian, UK

Bleeding from the alimentary tract is an important manifestation of gastrointestinal disease. It may present as an unexplained anaemia, as the passage of black stool, as the passage of red blood per rectum, or by vomiting of fresh or altered blood.

The investigation of gastrointestinal bleeding involves answering two major questions: (1) is the patient still bleeding? and (2) where is the bleeding site? Frank bleeding from the upper gastrointestinal tract, that is a site proximal to the duodenojejunal junction or ligament of Treitz, is usually obvious, presenting as a haematemesis or melaena stools. Fresh blood in the stools usually indicates rectal or colonic disease. However, it is possible for a bleeding lesion in the upper gastrointestinal tract to present with the passage of red blood per rectum; similarly a bleeding lesion in the caecum or ascending colon may cause melaena stools. The factors determining the degree of alteration of the blood in the gut include the site of the bleeding, the amount of blood lost and the motility of the bowel.

9.1 Occult Blood Tests

Between 100 and 200 ml of blood in the gut is necessary to produce a tarry stool. With smaller volumes the stools appear normal in colour, and special tests are necessary to detect the presence of blood. Tests for occult blood are used either to detect the cause of an iron-deficiency anaemia or to help in the diagnosis of those lesions of the gut which are frequently associated with bleeding such as peptic ulcer, carcinoma and polyps. An average of 0.7 ml/day of blood is normally lost in the gastrointestinal tract.

Occult bleeding can be detected by the use of chemical tests or radio-labelled erythrocytes, the microscopic examination of the stool for erythrocytes, the microscopic demonstration of crystals of haemoglobin or its derivatives, and fluorimetric and spectroscopic tests for haemoglobin or its derivative porphyrins. Of these the most widely used are the chemical tests, although the radioactive method is the most accurate and reliable.

9.1.1 Chemical Tests

Chemical tests are used universally because of their simplicity, but it is extremely difficult to devise a standard test which is neither too sensitive nor too insensitive. Faecal samples for testing may be obtained from a stool sample. This is best taken from within a lump of faeces, from the material adhering to the sigmoidoscope or proctoscope, or from the faeces on the glove after rectal examination. It is possible by vigorous digital examination to cause sufficient trauma to the rectal mucosa to give a positive test for blood in the stool, and gentle examination is essential when procuring a sample of faeces for chemical examination.

A variety of commercial tests is available. They are based on peroxidase-like activity in haemoglobin which causes the reagent used to develop a blue colour reaction. The most convenient sensitive test available is the guaiac test Hemachek, which is supplied as a kit so that the patient may send their own samples by post for examination. A specimen of stool is smeared on filter paper in a card. The reagent is added in the laboratory and development of a blue colour indicates the presence of blood in the stool. This method reliably detects amounts of bleeding of 10 ml or more daily, and usually gives positive results if there is a loss of more than 2.5 ml daily. Haemoccult is a very much less sensitive guaiac test.

The Fecatwin system depends on two levels of sensitivity for guaiac testing, with additional confirmation of the presence of human haemoglobin by an immunological technique. It is rather too elaborate for routine use. Orthotolidine tests have become less popular following the development of the guaiac-haemoperoxidase tests, but laboratories can offer this test cheaply if immediate results are not essential.

9.1.1.1 False-Positive Reactions

The main objection to sensitive tests is the occurrence of false-positive reactions. These reactions are almost exclusively dietary in origin originating from the ingestion of red meat, uncooked vegetables, unboiled milk and fruit such as bananas. Opinions differ whether or not oral iron preparations can produce positive results, but it is probable that a weakly positive result may follow the ingestion of ferrous compounds.

It must be stressed that negative tests for occult blood do not exclude ulcerative or neoplastic lesions of the gastrointestinal tract. Occult blood testing in screening ostensibly healthy subjects is used to detect early large bowel cancer, to identify those requiring colonoscopy.

In patients with a history consistent with gastrointestinal bleeding, or with iron deficiency anaemia or suspected gastrointestinal disease, a different approach is used.

1.

Haemorrhoids, gingivitis and epistaxis are excluded.
2.

Drugs, such as aspirin, which cause gastrointestinal bleeding are stopped.
3.

Three separate stool samples are tested by routine methods.
4.

If not all are positive, a radio-labelled erythrocyte study will yield further information; if this is not available, repeating the test after 3 days on a meat-free diet of only cooked vegetables (and taking bulk purgatives) may be helpful to exclude false positives.

In practice barium radiology and endoscopy are necessary in the diagnosis of difficult cases.

There is usually little doubt as to whether there is fresh blood in vomit. When altered blood or coloured material is present there may be uncertainty and the tests for occult blood mentioned above may be applied to gastric aspirate or vomit. ‘Coffee ground’ vomit is not proven haematemesis until shown to contain blood in this way.

9.1.2 Test for Dietary Iron

Although the faecal occult blood tests should not give false-positive reactions there is sometimes difficulty in evaluating very dark stools. To exclude the presence of ingested iron as a complicating factor, a simple test may be helpful. This can also be used in testing for compliance with prescribed iron preparations and in detecting surreptitious self-medication.

9.1.2.1 Method

A small button of faecal material is emulsified in 2 mol/l hydrochloric acid and a drop of the emulsion is placed in the centre of a filter paper. After 1–2 min a thin clear halo of fluid soaks into the paper around the drop. A drop of 2.5% potassium ferricyanide in aqueous solution is placed on the paper so that the haloes around the two drops meet at their periphery.

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