A rough estimate is that it takes about 14 hours for blood to be broken down within the intestinal lumen; therefore if transit time is less than 14 hours the patient will have hematochezia, and if greater than 14 hours the patient will exhibit melena. : 322   One often-stated rule of thumb is that melena only occurs if the source of bleeding is above the ligament of Treitz although, as noted below, exceptions occur with enough frequency to render it unreliable. Etymology
The origin of melena is dated to the early 19th century via modern Latin, via Greek melaina (feminine of melas, black). See also
Blood in stool
Dieulafoys lesion
Hematemesis

References


== External links ==
WFH as % of median reference value is calculated this way:
  
    
      
        
          W
          F
          H
        
        =
        
          
            
              weight of a given child
            
            
              median weight for a given child of that height
            
          
        
        ×
        100
      
    
    {\displaystyle \mathrm {WFH} ={\frac {\mbox{weight of a given child}}{\mbox{median weight for a given child of that height}}}\times 100}
  Cutoff points may vary, but <80% (close to −2 Z-score) is often used.