Blackish Blood During Hemodialysis: Causes and Clinical Approach
Blackish-colored blood during hemodialysis most commonly indicates severe deoxygenation or hemolysis, with mechanical hemolysis being the most critical immediate concern requiring urgent intervention.
Primary Causes
Severe Hemolysis (Most Critical)
- Mechanical hemolysis from dialysis circuit malfunction is the most dangerous cause and can produce life-threatening systemic complications requiring immediate recognition and treatment 1
- Mechanical damage to red blood cells can occur from stenosis in dialysis blood lines, kinked tubing, or malfunctioning blood pumps 1
- Rapid hemolysis releases hemoglobin into plasma, creating a dark, blackish appearance to the blood 1
- This is a dialysis emergency—if suspected, immediately stop dialysis, clamp lines, and do not return blood to patient 1
Severe Deoxygenation
- Inadequate oxygenation of blood can cause darkening due to increased deoxygenated hemoglobin concentration
- May occur with respiratory compromise, severe anemia (common in ESRD), or circulatory issues during dialysis 2
- Hemodialysis itself induces significant reduction in cerebral blood flow causing hemodynamic instability 3
Uremic Coagulopathy and Blood Stasis
- ESRD patients have fundamentally altered blood composition with accumulated uremic toxins that affect red blood cell function 3
- Uremic toxins (indoxyl sulfate, acrolein, urea, p-cresol) cause accelerated eryptosis—premature red blood cell death with membrane changes 4, 3
- Blood stasis in dialysis lines combined with altered rheology from severe anemia can create darker appearance 3
Secondary Contributing Factors
Iron Overload
- Chronic IV iron supplementation (common in hemodialysis patients) can lead to hemosiderosis with iron deposits in multiple organs 4
- Between 1999-2010, hemodialysis patients received increasing IV iron doses (232-281 mg/month), with median ferritin levels rising to 450-650 µg/L 4
- Iron overload itself doesn't directly cause blackish blood but indicates altered hematologic status 5
Severe Anemia
- Most ESRD patients have significant anemia which alters blood rheology and appearance 2
- When hematocrit drops below 25%, blood appears darker and more viscous 3
- Anemia in ESRD results from decreased erythropoietin production, iron deficiency, and accelerated eryptosis 4
Immediate Clinical Actions
When blackish blood is observed during hemodialysis:
Check for hemolysis immediately 1:
- Inspect dialysis circuit for mechanical problems (stenosis, kinks, pump malfunction)
- Draw blood sample to check for free hemoglobin in plasma
- If hemolysis confirmed: stop dialysis, clamp lines, do NOT return blood to patient
Assess oxygenation status:
- Check pulse oximetry and respiratory status
- Evaluate for hemodynamic instability during dialysis 3
Evaluate for blood stasis:
Laboratory evaluation:
- Complete blood count with hemoglobin/hematocrit
- Plasma free hemoglobin (if hemolysis suspected)
- Lactate dehydrogenase (elevated in hemolysis)
- Ferritin and transferrin saturation (assess iron status) 5
Prevention Strategies
- Regular inspection and maintenance of dialysis equipment to prevent mechanical hemolysis 1
- Optimize anticoagulation during dialysis to prevent blood stasis while minimizing bleeding risk 6, 3
- Maintain adequate hemoglobin levels (though avoid excessive ESA therapy) 2
- Monitor iron status regularly (ferritin and TSAT every 3 months) to prevent both deficiency and overload 5
Critical Pitfall
Do not dismiss darkened blood as "normal" in dialysis patients—while ESRD patients have multiple hematologic abnormalities, acute changes in blood color during dialysis warrant immediate investigation, particularly to rule out life-threatening hemolysis 1.