Can dialysis cause a decrease in hemoglobin (Hgb) of 2 units after the first use?

Can Dialysis Cause a 2-Point Hemoglobin Drop After First Use?

A 2 g/dL hemoglobin decrease after the first dialysis session is unlikely to be caused by dialysis-related blood loss alone, but can occur through hemodilution from fluid shifts and should prompt investigation for other causes.

Blood Loss from First Dialysis Session

The actual blood loss from a single hemodialysis session is minimal and cannot account for a 2 g/dL hemoglobin drop:

• Modern dialysis membranes cause blood loss of only 0.3-0.9 mL per session, with blood-line losses adding another 0.2 mL, totaling approximately 1.1 mL per session 1
• Residual blood in the dialyzer tubing and membrane averages 0.978 mL (range 0.01-23.9 mL) per session, which is insufficient to cause significant hemoglobin decline 2
• To put this in perspective, annual blood loss from the dialysis technique itself is only 165 mL per year (assuming 150 sessions), which translates to barely over 1 mL per session 1

Hemodilution as the Primary Mechanism

The more likely explanation for an apparent hemoglobin drop after first dialysis is hemodilution from fluid redistribution:

• Hemoglobin levels typically decline before dialysis initiation due to overhydration and fluid accumulation, then rapidly increase after dialysis as excess fluid is removed 3
• Overhydrated patients show lower hemoglobin levels along with higher inflammatory markers before dialysis initiation, suggesting hemodilution is masking the true hemoglobin concentration 3
• The timing of blood sampling matters critically—if pre-dialysis blood is drawn after fluid removal has begun, it will show falsely elevated hemoglobin, making the post-dialysis value appear lower by comparison 1

Vascular Access-Related Blood Loss

Catheter use significantly increases blood loss compared to arteriovenous fistulas, but this is primarily relevant for chronic dialysis:

• Double-lumen catheters cause 2.7 L of blood loss annually (versus 165 mL from the dialysis technique itself) due to purge protocols that discard 7-10 mL of blood from each catheter branch at each session 1
• For a first dialysis session with a catheter, this would represent approximately 14-20 mL of blood loss from the purge alone—still insufficient to cause a 2 g/dL hemoglobin drop
• Excessive bleeding from cannulation sites (≥4 mL/session) occurs in 39.6% of chronic dialysis patients and is associated with arteriovenous grafts, longer dialysis vintage, and pre-existing anemia 4

Alternative Explanations to Investigate

When encountering a 2 g/dL hemoglobin drop after first dialysis, consider:

• Laboratory error or sampling technique issues: Dilution of the pre-dialysis sample with saline, or drawing the post-dialysis sample before adequate time has elapsed for equilibration 1
• Hemolysis during dialysis: More negative arterial chamber pressures (more negative than -350 mm Hg) cause increased hemolysis, though this typically doesn't require increased erythropoietin dosing 5
• Occult bleeding: Uremic enteropathy causes occult gastrointestinal bleeding averaging 2,257 mL per year in chronic dialysis patients 1, but acute bleeding should be ruled out
• Pre-existing anemia progression: Hemoglobin levels gradually decrease in the months before dialysis initiation due to worsening kidney function 3

Clinical Approach

For a 2 g/dL hemoglobin drop after first dialysis:

1. Repeat the hemoglobin measurement using proper sampling technique (slow flow/stop pump method 15 seconds after reducing blood flow to 50-100 mL/min) 1
2. Check for signs of acute bleeding: examine the access site, check stool for blood, assess for hemodynamic instability
3. Review the dialysis record for excessive blood loss from the circuit, catheter purge volume, or equipment malfunction
4. Consider hemolysis: check lactate dehydrogenase, haptoglobin, and review arterial chamber pressures during the session 5
5. Assess volume status: the hemoglobin may actually be higher post-dialysis if adequate ultrafiltration occurred, suggesting the pre-dialysis value was falsely elevated by hemodilution 3

Common Pitfalls

• Don't assume dialysis-related blood loss caused the drop—the volumes are too small for a single session 1, 2
• Don't overlook sampling errors—timing and technique dramatically affect measured hemoglobin values 1
• Don't ignore fluid status—overhydration before dialysis can mask true hemoglobin levels 3