Drinking Blood

During the medicine sub-i, Dr. Ashar handed us a really interesting paper from 1940s.  I will review this paper here for both the strangeness and the results derived. 

Paper: Schiff, L., Stevens, R.J., Shapiro, N., and Goodman, S.  Observations on the oral administration of citrated blood in man. March, 1941 - Presented at American Society of Clinical Investigation

Objective: In the context of an upper GI bleed, tarry stool or frank blood in stool is assumed to signify sever hemorrhage.  The paper goes about trying to quantify how much blood actually needs to be lost in an upper GI bleed before stool quality changes.

How much blood is necessary to produce tarry stool?
The authors used 18 subjects (3 controls and 15 other patients in hospital for non-GI related diseases), who drank varying amounts of venous blood mixed with 100 to 200 mL of Vichy water (mineral water) to "help disguise the taste."  The result of the study was that grossly tarry stools were detected starting from subjects who drank at least 100 mL of blood (4/7 patients who drank 100 mL of blood had tarry stools).  Everyone who drank 200 mL of blood had tarry stools.

After a bleed in upper GI tract, how long does it take for blood to show up in stool?
On five subjects (unknown identities), 1000 to 2000 mL of blood was poured down nasogastric tubes. To prevent reflex diarrhea from irritation by stored blood products, subjects were pre-treated with codeine and atropine.  After blood was introduced through NG tube, the first bloody stool appeared starting from 4 hours to 12 hours post procedure.  Bloody or tarry stools lasted for three to five days.

After an upper GI bleed, how long does occult blood last in stool?
When two subjects drank anywhere from 75 to 250 mL of blood, the average number of days with guaiac positive stool ranged from three (for 75 mL of blood) to ten days (for 250 mL of blood).  When more than 1000 mL of blood was introduced via the NG tube, guaiac positive stool lasted for five (for 1000 mL of blood) to 12 days (for 2000 mL of blood).

Conclusion: Tarry stools can appear after as little as 100 mL of blood from upper GI bleed - not a massive hemorrhage.  It takes roughly about four to twelve hours for the blood to show up in stool after a big bleed of one to two liters (although patients were pre-treated with anti-motility agents).  Subsequent to a bleed, the occult blood can last for one to two weeks.