Black, Tarry Stools

What Causes Black Tarry Stools?

Black tarry stools can have various causes, ranging in severity from 'needs attention' to 'life-threatening'.  Finding the true cause means ruling out or confirming each possibility – in other words, diagnosis.

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Diagnosis is usually a complex process due to the sheer number of possible causes and related symptoms.  In order to diagnose black tarry stools, we could:

  • Research the topic
  • Find a doctor with the time
  • Use a diagnostic computer system.
The process is the same, whichever method is used.

Step 1: List all Possible Causes

We begin by identifying the disease conditions which have "black tarry stools" as a symptom.  Here are four possibilities:
  • Stomach Ulcers
  • Gastrointestinal Bleeding
  • Possible Urgent Medical Need
  • Carcinoid Cancer

Step 2: Build a Symptom Checklist

We then identify all possible symptoms and risk factors of each possible cause, and check the ones that apply:
regular aspirin use
attempting suicide
significant right iliac pain
frequent unexplained vomiting
frequent unexplained nausea
severe upper-right pain after food
acute abdominal pain
medium-term abdominal pain
low systolic blood pressure
major unexplained weight loss
burning epigastric pain after meals
pain between shoulder blades
... and more than 30 others

Step 3: Rule Out or Confirm each Possible Cause

A differential diagnosis of your symptoms and risk factors finds the likely cause of black tarry stools:
Cause Probability Status
Gastrointestinal Bleeding 99% Confirm
Possible Urgent Medical Need 12% Unlikely
Stomach Ulcers 0% Ruled out
Carcinoid Cancer 0% Ruled out
* This is a simple example to illustrate the process

Arriving at a Correct Diagnosis

The Analyst™ is our online diagnosis tool that learns all about you through a straightforward process of multi-level questioning, providing diagnosis at the end.

If you indicate irregular stools, The Analyst™ will ask further questions including this one:
Do you ever pass black, tarry stools?
Possible responses:
→ No / don't know
→ Yes, sometimes slightly black/tarry
→ Yes, often slightly black/tarry, sometimes very
→ Yes, my stools are usually/always very black/tarry
Based on your response to this question, which may indicate slightly black/tarry stools, black/tarry stools or very black/tarry stools, The Analyst™ will consider possibilities such as:
A Potentially Urgent Medical Need

Blood, as seen in the stool, can originate anywhere along the intestinal tract.  A black stool (called melena) usually means that the blood is coming from the upper part of the tract.  At least 4 Tablespoons (60ml) of blood must have been lost in order to make the stool black.  Maroon-colored stools suggest that the blood is coming from the middle portion of the intestinal tract, and bright red from the large bowel or rectum.  If stools change color suddenly, it can be evidence of internal bleeding which requires immediate medical attention.

The ingestion of black licorice, Pepto-Bismol, or blueberries can all cause black stools or false melena.  Stools should be tested for the presence of hidden blood.

Gastric/Peptic/Duodenal Ulcers

Bleeding from an ulcer may occur in the stomach or the duodenum, and sometimes is the only symptom.  Rapid bleeding causes bowel movements to become black or even bloody.

Gastrointestinal Bleeding

Black, tarry stools usually result from bleeding that occurs high up in the digestive tract – for example, in the stomach or first segment of the small intestine (duodenum); blood in the stomach turns black when exposed to stomach acid and enzymes.  A single severe bleeding episode can produce tarry stools for as long as a week, so continuing tarry stools do not necessarily indicate persistent bleeding.

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