Why is the first drop of blood wiped away during a Capillary Blood Glucose (CBG) check?

Why the First Drop of Blood is Wiped Away During Capillary Blood Glucose Testing

The first drop of blood during capillary blood glucose (CBG) testing is wiped away to eliminate potential contaminants and tissue fluid that could interfere with measurement accuracy, with subsequent drops providing more reliable results for clinical decision-making.

Rationale for Wiping Away the First Drop

When performing capillary blood glucose testing, the practice of wiping away the first drop of blood and using subsequent drops is based on several important considerations:

Contamination Concerns

• The first drop may contain tissue fluid, alcohol residue (if used for cleaning), or other contaminants that could affect the accuracy of the reading
• These contaminants can dilute the blood sample or chemically interfere with the glucose measurement

Accuracy Considerations

• Research has shown varying results regarding which drop is most accurate:
  • A 2023 study found that while the first drop showed the lowest mean difference to plasma blood glucose (+4.92 mg/dl), there were strong correlations among all drops, making any drop suitable for monitoring 1
  • Another study indicated that accuracy may depend on the blood glucose level range, with the first drop being more accurate at levels below 10 mmol/L and the second drop more accurate at levels above 20 mmol/L 2

Clinical Practice Guidelines

• While specific guidelines about which drop to use are limited, clinical practice generally favors using the second drop for routine measurements
• For continuous glucose monitoring (CGM) systems requiring calibration, guidelines emphasize obtaining capillary blood glucose from stable periods (such as before meals) and using the same glucose meter and test strips for consistency 3

Best Practices for Capillary Blood Glucose Testing

Proper Technique

1. Wash hands thoroughly and dry completely
2. Perform finger prick with appropriate lancing device
3. Wipe away first drop of blood with clean cotton or gauze
4. Gently squeeze finger to produce second drop of blood
5. Apply second drop to test strip or meter
6. Record results and respond appropriately

Special Considerations

• In emergency situations where time is critical, research suggests the first drop may be adequate for clinical decision-making if the patient's hands are clean 4
• For patients performing self-monitoring at home, consistency in technique (always using the same drop) may be more important than which specific drop is used

Implications for Patient Care

Accuracy Impact

• Accurate blood glucose readings are essential for appropriate insulin dosing and treatment decisions
• Even small variations in readings can lead to significant differences in insulin administration

Quality Control

• Filter paper collection methods have been used to validate the accuracy of capillary blood glucose measurements, highlighting the importance of proper technique 5
• When calibrating continuous glucose monitoring systems, accuracy of capillary measurements is particularly important 3

In conclusion, while research shows strong correlations between different blood drops, the practice of wiping away the first drop remains standard to minimize potential contamination and improve measurement reliability. This practice helps ensure that treatment decisions are based on the most accurate blood glucose values possible, ultimately improving patient outcomes by preventing inappropriate insulin dosing that could lead to hypoglycemia or hyperglycemia.