Nursing care
Why melena points to upper GI bleeding and how to read it
Written and reviewed by Dana Whitfield, RN, MSN · 4 min read · Updated October 2026
Short answer
Melena is black, tarry, foul-smelling stool caused by blood that has been digested on its way through the gut. Haemoglobin is altered by stomach acid, enzymes and bacteria, turning it black. Because this takes time and distance, melena usually points to bleeding above the ligament of Treitz, although slower bleeding from the small bowel or right colon can also cause it.
How blood turns black in the gut
Blood entering the stomach meets acid that converts haemoglobin into darker compounds, and digestive enzymes and intestinal bacteria break it down further during transit. By the time it reaches the rectum, the blood has become black, sticky and tar-like with a distinctive smell. A substantial amount of blood, roughly 100 to 200 mL in the upper tract, is needed to produce melena.
Transit time is the key variable. The longer blood stays in the gut, the darker it becomes, which is why a source high in the tract usually produces melena. Melena can continue for several days after bleeding has stopped, as blood already in the bowel is passed, so one black stool does not by itself prove bleeding is ongoing.
Reading melena against bright red blood
Bright red blood per rectum, or haematochezia, usually means a lower source with little time for digestion. The important exception is a brisk upper GI bleed: when blood moves through quickly, it can emerge red or maroon. In that situation red blood is a sign of a large, fast bleed, and it often comes with haemodynamic instability.
Coffee-ground or red vomit, a history of peptic ulcer, NSAID use, alcohol use or known varices support an upper source. Black stool from oral iron or bismuth can mimic melena, but those stools are usually not tarry or sticky and test negative for occult blood. Ask about medicines before concluding, and report true melena rather than assuming a supplement explains it.
What matters more than stool colour
Stool colour hints at location; vital signs show how much blood has been lost. Rising heart rate, falling blood pressure, postural dizziness, pallor, cool clammy skin, reduced urine output and new confusion suggest significant loss and possible shock. Orthostatic changes are a useful clue in a stable client, but they should not be checked when the client is already hypotensive or symptomatic.
Nursing actions follow the circulation: secure large-bore intravenous access as ordered, send blood for haemoglobin, crossmatch and coagulation, keep the client nil by mouth pending endoscopy and monitor closely. Haemoglobin may not fall immediately after acute loss because the blood lost is whole blood, so a near-normal first value can underestimate the bleed until fluid shifts dilute it.
What the source of melena usually is
Common upper GI causes include peptic ulcers, which are linked to Helicobacter pylori infection and NSAID use, gastritis and oesophageal or gastric varices in clients with liver disease. Less often, bleeding from the small bowel, such as from abnormal blood vessels, produces melena when the upper tract looks normal. Endoscopy identifies the source and often treats it at the same time.
History helps the nurse anticipate needs. A client with known cirrhosis and melena may have variceal bleeding and needs close watching for massive haemorrhage and encephalopathy. A client taking anticoagulants or antiplatelet medicines may bleed more heavily, and the prescriber needs to know about these promptly. Record the medicines, the timing and number of black stools and any vomiting.
Worked scenario: black stool and a fast pulse
A hypothetical client taking an NSAID for arthritis reports two black, sticky stools and feels light-headed when standing. Heart rate is faster than at the last check. Options are to reassure the client that iron causes black stool, to request an order for a stool softener, or to assess vital signs and report a suspected upper GI bleed.
Assessing and reporting is the strongest answer. Sticky black stool with NSAID use and postural symptoms fits upper GI bleeding with volume loss. Reassurance about iron assumes a medicine the client may not be taking and delays escalation, and a stool softener addresses nothing relevant. Keep the client in bed for safety, confirm IV access and prepare for blood tests.
Sources and further reading
Merck Manual Professional: Overview of gastrointestinal bleeding. Melena definition, blood volume needed, persistence after bleeding stops, iron and bismuth mimics, brisk upper bleeding causing haematochezia and orthostatic changes.
NIDDK: Symptoms and causes of GI bleeding. Black tarry stool with upper bleeding, bright red blood with lower bleeding, signs of shock and common causes.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our gastrointestinal practice questions are the closest set to what this page covers.
Common questions
Can melena come from the lower GI tract?
Sometimes. Slow bleeding from the small bowel or right colon can produce dark stool, but melena most often reflects an upper GI source.
Why can an upper GI bleed produce bright red stool?
When bleeding is brisk, blood passes through too quickly to be digested. Red stool from an upper source usually means a large, fast bleed and possible instability.
How can the nurse tell iron-related black stool from melena?
Iron or bismuth stool is usually dark but not tarry or sticky and is negative for occult blood. Ask about medicines, but report suspected melena rather than assuming.