Blood in the Stool -- The Color Tells A Lot!
Bedside Logic in Diagnostic Gastroenterology
James Christensen, M.D.
Peer Review Status: Internally Peer
Reviewed
SPECIFIC FEATURES OF BLOOD IN THE STOOL
Character of Blood in the Stool
Color of stool
When a patient says he sees blood in the stool he is rarely wrong. The only thing that he can confuse with fresh blood (hematochezia) is a red dietary pigment that might escape digestion. The pigment in beets, if eaten in large quantities, can produce a dark red stool; this is more common in children than in adults. I suppose that other red dietary pigments could cause confusion too. If you are in doubt you can ask if the blood diffuses into the water of the toilet bowl. Fresh blood diffuses rather rapidly and turns the water a familiar light yellowish pink-red color, while beet pigment goes into solution without a change in color.
Many people know that black stools (melena) can mean blood in the stool, so when a patient says he sees blood you should ask if he means black blood, red blood, or something in between ("dark blood"). Blood turns black in the gut lumen because reduced hemoglobin is dark and because bacteria alter the porphyrin pigments. Most of this conversion occurs in the right colon where the colonic bacteria are most active. Thus if blood in the stool looks entirely fresh, its source is the distal one-third of the colon; if it is completely black, its source is likely to be above the cecum; if it is mixed black and red, the source is somewhere in between. This rule holds for low to moderate rates of bleeding. Large-volume bleeding above the cecum creates a loose watery stool because of the osmotic effect of the blood constituents. The flushing of the colon does not give the bacteria time to convert the hemoglobin completely from red to black. Still, the blood rarely looks entirely fresh even in large-volume bleeding from above the cecum.
Black stools can also result from the ingestion of iron, bismuth (usually in Pepto-Bismol), and the excessive intake of spinach and other greens that are cooked and eaten in large quantities by vegetarians and enthusiasts for "ethnic” diets. You must ask about such medicines and foods when you see a patient who complains of black stools.
Stool Volume and Consistency
When a patient says he sees blood in the stools (red, dark red, or black) be sure to ask him about the stool consistency. The osmotic effect of the plasma constituents that escape into the lumen increase the luminal water content and so increase the volume of the luminal contents. The effect depends upon the amount of mucosa exposed to the osmotic effect of the blood and upon the duration of exposure. Thus bleeding into the stomach, small intestine, or right colon regularly increases stool volume and fluidity. Bleeding into the left colon affects volume and fluidity less, depending on how far up in the colon the source is located. Bleeding from the rectum and anal canal has no effect on stool consistency. Remember that the fecal mass is normally well formed by the time it reaches the sigmoid colon. If a grossly bloody stool is also soft and bulky, the bleeding source is likely to be above the sigmoid colon.
Fresh Blood in the Stool: The Relationship of the Blood to the Stool
The fecal mass forms and firms up slowly as it moves along the colon. A bleeding site at a level of the colon where the fecal mass is formed but soft produces a stool in which the blood is mixed into the stool to some degree; blood that comes from a level where the feces is already firm appears mainly to coat the stool surface. You must ask if the blood appears to be mixed into the stool or mainly coats the stool surface.
You must also ask about the temporal relationship of blood and stool. Most patients who complain of blood in the stool are bleeding from a source in the anal canal like hemorrhoids or an anal fissure. With these sources the bleeding occurs when the fragile mucosa in the anal canal is damaged by the passage of a stool, so that the blood mainly comes after the stool has passed. The blood may appear only on the toilet paper or it may be heard or felt to drip into the water in the toilet after the stool has passed. If the blood comes before the stool or if it comes without stool, the source is likely to be above the anal canal.
Chronology of Blood in the Stool
Onset
Blood in the stool is so alarming that most patients know the onset to the day and seek help immediately. The symptom is always abrupt in onset. Unfortunately, some patients seem to think that occasional hematochezia is normal, probably because hemorrhoidal bleeding is so common. Thus they may not mention previous episodes. You must ask about previous hematochezia directly if you want to be sure when the bleeding actually began.
Periodicity
If the bleeding has gone on for more than a short time, it has probably exhibited periodicity. This is characteristic of virtually all causes of rectal bleeding. The lack of periodicity, however, strongly suggests a malignant source, in the left colon if the blood is bright, higher in the colon if the blood is dark and mixed with the stool.
Aggravating and Relieving Factors in Blood in the Stool
Patients rarely can tell you anything they do that aggravates rectal bleeding or that relieves it. Anal-canal bleeding is usually worse with constipation and such bleeding often goes away with treatment of constipation, but not always, since severe hemorrhoidal disease with cryptitis and papillitis is not caused by constipation but it often aggravates constipation.
Associated Symptoms in Blood in the Stool
When rectal bleeding is associated with other major gastrointestinal symptoms the other symptoms usually dominate, but occasionally not, so you should ask specifically about them.
The bright or dark-red bleeding of colonic origin frequently occurs without associated symptoms, but you should ask about diarrhea, constipation, and cramps. The diarrhea, if present, is always rather low in volume. It may amount to no more than a consistent increase in stool softness and volume. This suggests an origin above the rectum. Mild constipation coupled with dark blood suggests a neoplasm of the right colon; with bright bleeding it suggests diverticulitis or bleeding of anal-canal origin. Cramps in relation to bright red rectal bleeding are suggestive of a partial colonic obstruction and especially suggest cancer.
The various sources of the black fluid stools produced by bleeding of upper gastrointestinal origin usually lie in the esophagus, stomach, or duodenum, for more distal intestinal sources of bleeding are rare. Ask about upper-abdominal pain, nausea, and cramps in relation to melena. The gastric lesions that bleed can be either inflammatory or neoplastic, both of which usually cause pain, but not always. Remember that one-fifth of duodenal ulcers are painless and I suspect that one-fifth of gastric cancers and gastric ulcers are too. Nausea and cramps signify that the volume of bleeding is relatively great. Some such symptoms may be present but ignored because they are usually mild. The absence of any upper-abdominal pain, nausea, and cramps suggests that the melena is of cecal or right colonic origin.
THE COMMON CAUSES OF BLOOD IN THE STOOL AS THE MAJOR SYMPTOM
This classification is based on two features of rectal bleeding: the color of the blood and the character of the stool. The rules on which this classification is based are not infallible.
A DECISION WHEEL FOR GROSS RECTAL BLEEDING
Narrowing the list of causes for gross blood in the stool requires relatively few clinical features. It depends far more on endoscopic and radiographic investigations. The important clinical features, the nature of the blood and the nature of the stool, are diagrammed in Figure 7-1.
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