Can the first drop of blood show hypoglycemia (low blood sugar) and the second drop show normal blood glucose levels?

Differences Between First and Second Drop Blood Glucose Measurements

Yes, the first drop of blood can show hypoglycemia (low blood glucose) while the second drop shows normal blood glucose levels in certain circumstances. This discrepancy can lead to clinical confusion and potentially unnecessary treatment.

Factors Affecting First vs. Second Drop Glucose Readings

Contamination Issues

• Hand contamination: When hands are exposed to sugar-containing substances (like fruit), the first drop can show falsely elevated readings in 88% of cases, while second drop measurements are affected in only 11% of cases 1
• Unwashed hands: Even without visible contamination, not washing hands can lead to ≥10% difference in glucose readings in 11% of first drops and 4% of second drops 1

Circulatory Factors

• Pseudohypoglycemia: This occurs when capillary blood glucose readings are falsely low compared to venous blood glucose levels
• Conditions predisposing to pseudohypoglycemia:
  • Raynaud's phenomenon
  • Peripheral vascular disease
  • Shock
  • Impaired microcirculation 2

Technical Factors

• External pressure applied around the finger during sampling can cause ≥10% differences in glucose readings in 5-13% of cases 1
• Blood glucose concentration: When blood glucose is between 10-20 mmol/L, there's no significant difference between first and second drop readings; however, differences become significant at levels <10 mmol/L or >20 mmol/L 3

Clinical Recommendations

For Accurate Blood Glucose Testing

1. Wash hands with soap and water and dry thoroughly before testing 1
2. Use the first drop of blood if hands are clean and dry 1, 4
3. If washing is not possible:
   • Wipe away the first drop of blood
   • Use the second drop for measurement
   • Only do this if hands are not visibly soiled or exposed to sugar-containing products 1

For Suspected Pseudohypoglycemia

1. Compare capillary and venous blood glucose measurements simultaneously
2. Warm the extremity before sampling if Raynaud's phenomenon or poor circulation is suspected 2
3. Avoid excessive pressure when obtaining the blood sample 1

For Hypoglycemia Management

• For confirmed hypoglycemia (blood glucose <70 mg/dL or 3.9 mmol/L):
  • Administer 15-20g of fast-acting carbohydrate to conscious patients 5, 6
  • Recheck blood glucose after 15 minutes
  • Repeat treatment if blood glucose remains <70 mg/dL
  • Once normalized, provide a meal or snack to prevent recurrence 5, 6

Important Caveats

• Pseudohypoglycemia should be suspected when there's a discrepancy between symptoms and glucose readings, especially in patients with impaired circulation
• Emergency situations: In critically ill patients or those with severe symptoms, don't delay treatment while obtaining multiple samples
• Patient education: Teach patients about proper technique for home blood glucose monitoring, including when to use first vs. second drops
• Meter accuracy: Remember that portable glucose meters have limitations in accuracy, with a goal of limiting total error to ≤15% at glucose concentrations ≥5.6 mmol/L (100 mg/dL) 5

By understanding these factors and following proper technique, healthcare providers can avoid false readings and ensure appropriate management of true hypoglycemia while preventing unnecessary treatment of pseudohypoglycemia.