Can drawing blood from the same site as Total Parenteral Nutrition (TPN) administration affect laboratory test results?

Blood Drawing from TPN Administration Sites: Impact on Laboratory Results

Yes, drawing blood from the same site as Total Parenteral Nutrition (TPN) administration can significantly affect laboratory test results and should be avoided whenever possible.

Why This Matters

Drawing blood from a catheter used for TPN infusion risks contamination of the blood sample with TPN solution components, which can falsely elevate multiple laboratory values and lead to incorrect clinical decisions 1. The TPN solution contains high concentrations of glucose, amino acids, lipids, electrolytes, vitamins, and trace elements that will artificially alter test results if mixed with the blood sample 1.

Specific Recommendations for Blood Collection

When Drawing from a Catheter is Unavoidable

If you must draw blood from a catheter that contains or has been used for TPN or other infusions:

• Flush the catheter thoroughly prior to blood collection 1
• Discard the first 5 mL of blood to avoid contamination from residual TPN or heparin in the catheter 1
• This discard volume is critical because it clears the catheter dead space of any infused solutions 1

Preferred Blood Collection Methods

The hierarchy for blood sampling should prioritize:

1. Peripheral venipuncture from a separate site - This is the gold standard to avoid any contamination 1
2. Arterial sampling from a dedicated arterial line (not used for infusions) 1
3. Venous sampling from a separate venous catheter (not used for TPN) 1
4. Only as last resort: sampling from the TPN catheter with proper flushing and discard technique 1

Technical Considerations

Blood Collection Technique

• Use a straight needle over butterfly needles when possible, as butterfly needles may cause contact activation and hemolysis 1
• Draw blood directly into collection tubes rather than using a syringe first, as syringes lack anticoagulant and can activate coagulation 1
• Ensure proper tube fill to maintain correct blood-to-anticoagulant ratios 1
• Avoid tourniquets or use only briefly for vein location, as prolonged use causes blood cell activation and hemolysis 1

Multilumen Catheters

When using a multilumen central venous catheter where one lumen is dedicated to TPN:

• Draw blood from a different lumen than the one used for TPN infusion 1
• Even with separate lumens, be cautious as cross-contamination can occur
• The discard volume recommendation still applies 1

Common Pitfalls to Avoid

Laboratory Values Most Affected by TPN Contamination

Blood samples contaminated with TPN solution will show falsely elevated:

• Glucose levels (TPN contains 50% dextrose solutions) 1
• Triglycerides and lipids (from lipid emulsions in TPN) 1
• Electrolytes (sodium, potassium, calcium, magnesium, phosphate) 1
• Amino acid levels and protein markers 1

Critical Monitoring Situations

For patients on long-term TPN requiring frequent laboratory monitoring:

• Establish a dedicated peripheral access site for blood draws separate from the TPN catheter 1
• For home TPN patients, coordinate blood draws during clinic visits using peripheral venipuncture 1
• Laboratory monitoring should occur at least 3 times yearly for stable patients on TPN 1

Special Populations

Pediatric Patients

In pediatric populations where blood volume conservation is critical:

• The first milliliter(s) may be discarded rather than the full 5 mL in adults, but some discard is still mandatory 1
• Consider using devices that minimize blood waste with catheter sampling 1

Patients with Limited Venous Access

For patients with thrombosed central veins or limited access (common complications of long-term TPN):

• Plan blood draws strategically during TPN cycling periods when infusion is paused 1
• Consider timing laboratory tests when TPN can be temporarily discontinued
• Document the sampling method and any potential contamination risk in the medical record

Documentation Requirements

Always document:

• The site of blood collection (peripheral vs. catheter) 1
• Whether blood was drawn from a TPN line and the discard volume used 1
• Any preanalytical conditions that may influence results 1

This documentation allows laboratory personnel and clinicians to interpret results appropriately and identify potential contamination issues.