What does dark-colored blood in the blood lines indicate in a patient undergoing hemodialysis?

Dark-Colored Blood in Hemodialysis Blood Lines

Dark-colored blood in hemodialysis blood lines most commonly indicates inadequate oxygenation (venous desaturation), but can also signal inflammation, access dysfunction with reduced blood flow, or impending access thrombosis—all of which require immediate evaluation to prevent access loss and mortality.

Primary Causes and Clinical Significance

Inadequate Oxygenation (Most Common)

• Venous blood naturally appears darker than arterial blood due to lower oxygen saturation, and this is the most frequent benign explanation for dark blood in the venous return line 1
• Blood oxygen saturation is positively correlated with blood color brightness—lower saturation produces darker blood 1
• However, critically ill patients often demonstrate darkened blood color due to decreased oxygen saturation from systemic illness 1

Inflammation and Systemic Illness

• C-reactive protein and white blood cell count are negatively correlated with blood color—higher inflammation produces darker blood independent of oxygen saturation 1
• Darkened blood color is an independent significant risk factor for mortality in hemodialysis patients (comparable to low albumin and low Kt/V) 1
• Inflammation affects blood color through mechanisms beyond simple desaturation, making dark blood a marker of systemic inflammatory burden 1

Access Dysfunction and Reduced Flow

• Stenosis within the dialysis access circuit decreases blood flow and increases pressure, which can manifest as darker blood appearance due to prolonged transit time and increased oxygen extraction 2
• Up to 90% of accesses with abnormal physical examination findings (including changes in blood appearance) will have underlying clinically significant pathology on imaging 2
• Stenosis is the primary precipitator of vascular access failure and, if left untreated, progresses to thrombosis 2

Impending Access Thrombosis

• Access thrombosis represents 65-85% of all access abandonments and is often preceded by warning signs including changes in blood appearance 2
• Thrombosis typically occurs due to underlying stenosis causing low access blood flow, though it can occur in well-functioning accesses in patients with thrombophilia 3, 4
• Hemodialysis patients have altered hemostatic systems creating both prothrombotic states and bleeding diathesis 5

Immediate Assessment Algorithm

Step 1: Check Oxygen Saturation

• Measure arterial oxygen saturation immediately to rule out systemic hypoxemia 1
• If saturation is normal, proceed to evaluate other causes

Step 2: Physical Examination of Access

• Palpate for thrill and auscultate for bruit—absent or diminished findings suggest stenosis or thrombosis 2
• Check for arm swelling, which indicates venous outflow obstruction 2
• Assess for prolonged bleeding after decannulation, which suggests access dysfunction 2

Step 3: Evaluate Inflammatory Markers

• Check C-reactive protein and white blood cell count, as elevated levels correlate with darker blood and predict mortality 1
• Review recent albumin levels—low albumin combined with dark blood indicates poor prognosis 1

Step 4: Assess Access Flow

• Monitor dialysis venous pressure during the session—elevated pressure suggests outflow stenosis 2
• Consider measuring access flow rate (Qa) if available, as reduced flow predicts thrombosis 2

Diagnostic Imaging When Indicated

Doppler Ultrasound (First-Line)

• Perform duplex Doppler ultrasound when physical examination is abnormal or access dysfunction is suspected 2
• Ultrasound detects stenoses with 90% correlation to abnormal physical findings 2
• Can identify reduced flow velocity and elevated resistive indices indicating stenosis 2

Fistulography (Definitive)

• Diagnostic fistulography is indicated when endovascular intervention is planned or for surgical planning 2
• Up to 90% of dialysis access thrombosis is caused by underlying stenosis detectable on fistulography 2
• Should not be performed without intention to intervene on significant findings 2

Critical Clinical Pitfalls

Do Not Ignore Dark Blood as "Normal Venous Color"

• While venous blood is naturally darker, quantifiable darkening beyond baseline is an independent mortality predictor and warrants investigation 1
• Early detection of blood color worsening can improve patient survival 1

Do Not Wait for Complete Thrombosis

• Stenosis progresses to thrombosis if untreated, and thrombosis is the primary cause of access loss 2
• Intervene on hemodynamically significant stenoses before thrombosis occurs 2

Do Not Overlook Systemic Causes

• Dark blood may reflect systemic inflammation, infection, or critical illness requiring treatment beyond access management 1
• Check for concurrent signs of sepsis, heart failure, or respiratory failure 1

Consider Thrombophilia in Recurrent Cases

• Patients with recurrent access thrombosis despite well-functioning access require coagulation cascade evaluation 3
• Inherited thrombophilias (Factor V Leiden, prothrombin mutations) and acquired factors (elevated Factor VIII, antiphospholipid antibodies) contribute to access thrombosis 3, 4

Hematologic Factors

• Mean corpuscular hemoglobin concentration is positively correlated with blood color brightness—lower MCHC produces darker blood 1
• Serum sodium concentration also correlates positively with blood color 1
• Anemia itself does not directly cause dark blood, but the underlying inflammatory state causing anemia does 1