How does hemodilution affect lab interpretation?

Impact of Hemodilution on Laboratory Test Interpretation

Hemodilution significantly affects laboratory test results by artificially lowering the concentration of measured analytes, potentially leading to misdiagnosis and inappropriate clinical management. 1

Mechanisms of Hemodilution

Hemodilution occurs when blood samples are diluted with fluids, resulting in falsely decreased concentrations of blood components. This can happen through several mechanisms:

1. Iatrogenic causes:

   • Saline or heparin contamination during blood sampling 1
   • Intravenous fluid administration prior to blood collection 1
   • Collection from sites proximal to IV fluid infusions 1
   • Blood transfusions or volume expansion therapy 1

2. Procedural causes:

   • Improper sampling technique during or after hemodialysis 1
   • Drawing blood immediately downstream from an IV site 1
   • Failure to discard adequate volume before collecting diagnostic samples 1

Effects on Specific Laboratory Parameters

Blood Urea Nitrogen (BUN) and Dialysis Adequacy Measures

• Hemodilution of predialysis BUN samples results in falsely low values 1
• Consequences include:
  • Falsely low Kt/V calculations
  • Incorrectly elevated V values
  • Falsely reduced K values
  • Inaccurate assessment of dialysis adequacy 1

Hematologic Parameters

• Falsely decreased hemoglobin and hematocrit 1
• Altered coagulation parameters:
  • Prolonged reaction times
  • Extended clot formation times 1
  • Mild hypocoagulable state 1

Other Laboratory Values

• Decreased concentration of all measured analytes
• Falsely low protein levels, electrolytes, and other solutes
• Potential underestimation of recirculation when measuring access function 1

Proper Sampling Techniques to Avoid Hemodilution

For Arteriovenous Fistula/Graft Sampling:

1. Obtain blood specimen from arterial needle before connecting arterial blood tubing 1
2. Ensure no saline/heparin is present in the needle or tubing before drawing 1
3. Never draw samples after hemodialysis has been initiated 1

For Venous Catheter Sampling:

1. Withdraw any heparin and saline from the arterial port following protocol 1
2. For adults: withdraw 10 mL of blood from the arterial port (3-5 mL for pediatric patients) 1
3. Connect a new syringe or collection device for the actual sample collection 1

For Post-Transfusion/Infusion Sampling:

1. Always attempt to obtain a pre-transfusion/infusion sample first 1
2. If unavailable, assess the sample for evidence of dilution by considering:
   • Volume of material transfused as percentage of total blood volume
   • Time between last transfusion/infusion and sample collection 1
3. Allow adequate time for reequilibration (approximately 24 hours after total blood volume exchange) 1

Clinical Implications and Pitfalls

Common Pitfalls:

• Misdiagnosis of anemia: Hemodilution may create a false impression of anemia, leading to unnecessary interventions 1
• Incorrect assessment of dialysis adequacy: Diluted BUN samples lead to underestimation of treatment effectiveness 1
• Inappropriate transfusion decisions: Falsely low hemoglobin may trigger unnecessary blood product administration 2
• Misinterpretation of coagulation status: Mild dilutional coagulopathy may be mistaken for pathological bleeding risk 1

Avoiding Misinterpretation:

1. Document fluid administration prior to laboratory testing
2. Note collection site and relationship to IV lines
3. Consider timing of samples in relation to fluid administration
4. Interpret results in context of clinical presentation
5. Repeat critical values with properly collected samples when hemodilution is suspected

Special Considerations

In Dialysis Patients:

• Predialysis samples must be drawn before any saline/heparin exposure 1
• Postdialysis samples require slow flow/stop pump technique to prevent recirculation effects 1

In Surgical Patients:

• Intentional hemodilution (target hematocrit 20-25%) may be used to reduce transfusion requirements 3, 4
• Laboratory values must be interpreted with knowledge of this intervention 4

In Critically Ill Patients:

• Aggressive fluid resuscitation significantly impacts laboratory interpretation
• Serial trending of values may be more informative than absolute numbers
• Consider the clinical context when interpreting laboratory results

By understanding the mechanisms and effects of hemodilution on laboratory values, clinicians can avoid diagnostic errors and ensure appropriate patient management.