What is a notable sign of "black blood" syndrome due to fistula stenosis?

The indication of "black blood" syndrome due to fistula stenosis is characterized by the presence of very dark blood. This phenomenon occurs because the blood returning from the dialyzer via the fistula is not adequately oxygenated due to prolonged stasis or recirculation. When the blood does not have sufficient exposure to oxygen, it appears darker than normal, leading to the description of "black blood."

This distinctive color change is an important clinical sign, as it can inform healthcare professionals about changes in blood flow or issues with the access site that require attention. Recognizing very dark blood as a symptom enables timely interventions to prevent further complications related to blood flow or oxygenation.