What is venous sampling?

What is Venous Sampling?

Venous sampling is the collection of blood from a vein for laboratory analysis, obtained either through direct venipuncture (needle stick into a vein) or from an indwelling venous catheter. 1

Primary Methods of Venous Blood Collection

Direct Venipuncture

• Direct venipuncture involves inserting a needle directly into a peripheral vein to obtain blood samples and remains the gold standard for most laboratory testing 2, 3
• Blood should be drawn using 19-21 gauge needles, with no significant difference in test results between these needle sizes 1
• The tourniquet should be applied only long enough to locate a vein—ideally 60 seconds or less—as prolonged stasis can alter coagulation assay results 1
• Blood is typically collected into vacuum tubes (such as Vacutainer or Monovette systems) containing appropriate anticoagulants like sodium citrate 1

Sampling from Indwelling Catheters

• For critically ill patients with invasive vascular monitoring, arterial catheters should be the first-choice sampling site, with central or peripheral venous catheters as the second option if arterial access is unavailable 4
• Sampling from indwelling venous catheters is common in intensive care settings where frequent blood draws make repeated venipuncture impractical 1
• When drawing from venous lines, only sodium chloride 0.9% (with or without heparin) should be used for line flush solutions to prevent contamination 4

Critical Safety Considerations

Contamination Risks

• Blood sampling from a catheter lumen that carries infusion solutions other than sodium chloride 0.9% is not recommended due to high risk of contamination and inaccurate results 4
• Glucose-containing infusions pose particular danger—even minimal contamination can produce falsely elevated glucose readings or conceal true hypoglycemia 4
• An adequate volume of dead space blood must be withdrawn and discarded before collecting the actual sample to avoid contamination from flush solutions or infused medications 1

Proper Discard Technique

• When sampling from peripheral intravenous lines, at least 5 mL of blood should be discarded before collecting the specimen to ensure accurate sodium, potassium, urea, and creatinine measurements 5
• For arterial catheters, insufficient discard volume can lead to contamination by flush solution 1

Anatomical Variations in Blood Composition

Blood glucose concentration varies significantly between vascular beds, which affects sampling site selection 1:

• Radial arterial blood glucose is approximately 0.2 mmol/L higher than peripheral venous blood 1
• Radial arterial blood glucose is 0.3-0.4 mmol/L higher than blood from the superior vena cava 1

Sampling Site Hierarchy for Critically Ill Patients

The Society of Critical Care Medicine recommends establishing a sampling site hierarchy that prioritizes arterial or venous sampling over capillary sampling in critically ill patients 6:

1. First choice: Arterial line (if present) 1, 4
2. Second choice: Central or peripheral venous catheter 1, 4
3. Avoid: Capillary (needle stick) samples in hemodynamically unstable patients, especially those on vasopressors, as these introduce large errors 1

Common Pitfalls to Avoid

• Never assume an IV catheter is equivalent to fresh venipuncture for laboratory accuracy 4
• IV catheters have significantly higher hemolysis rates compared to direct venipuncture, which can interfere with multiple laboratory tests 4
• Avoid drawing blood above a running IV infusion 4
• Any unexpectedly abnormal blood test should trigger immediate verification of the sampling source and consideration of drawing a confirmatory sample from an alternative site 4

Special Applications

Portal Venous Sampling

• EUS-guided portal venous sampling represents a specialized form of venous sampling where blood is obtained directly from the portal vein 1
• Portal blood may be "enriched" with metabolites and tumor cells not readily detectable in peripheral venous blood due to hepatic processing 1
• In patients with suspected pancreaticobiliary cancers, 100% of portal samples contained circulating tumor cells compared with <25% of peripheral blood samples 1

Documentation Requirements

Any deviation from standard venipuncture procedures must be carefully documented, including 4:

• The specific sampling site used
• Whether infusions were running or stopped
• Wait time after stopping infusions
• Type of catheter used