Research Blog

Plasma and Capillary Glucose Levels are Not Equal

Monitoring blood glucose is essential to evaluating hypoglycemia (low blood glucose), hyperglycemia (elevated blood glucose), and overall diabetes risk. The human body strives to maintain blood glucose between 72-108 mg/dL (4-6 mmol/L), utilizing glucagon and insulin to raise or reduce blood glucose levels, respectively. Insulin facilities glucose transport into most cells, though not required for glucose uptake by the brain, red blood cells, kidney, or liver. Other cells and tissues may be resistant to insulin, causing hyperglycemia and increased type 2 diabetes risk. On the other hand, lack of insulin leads to hyperglycemia and type 1 diabetes. Glucose testing methods include (Mathew 2023):

Capillary Blood Glucose (CBG) testing
- A small amount of blood is obtained using a fingerstick or skin prick from the earlobe, palm, forearm, or palm.
- Factors that decrease CBG accuracy include hypoglycemia, anemia, hematocrit alterations, hypotension, critical illness, and outdated test strips.
Venous (Plasma) Blood Sample
- Requires an inconvenient blood draw but is more accurate than CBG
Continuous Glucose Monitoring (CGM)
- A sensor attached to the upper arm or abdomen constantly measures interstitial fluid glucose levels.
- Detecting rapid glucose changes in the blood may be delayed with CGM because glucose appears in the blood before interstitial fluid.

Capillary results may be affected by older age, impaired circulation, and BMI, especially below 18 or above 30. Although capillary and venous results are concordant 84.5% of the time in those with normal glucose levels, results are not completely interchangeable (Tirimacco 2010):

Capillary and plasma/venous results are especially discordant at levels 74 mg/dL (4.1 mmol/L) and below.

A constant factor of 1.11 should be used for converting CBG results to plasma glucose when using a point-of-care glucose monitor.
Testing CBG may be appropriate for self-monitoring and for diabetes screening but not for diagnosing diabetes or hypoglycemia, which requires further lab testing, including a glucose tolerance test.
The World Health Organization suggests a cut-off of 110 mg/dL (6.1 mmol/L) or above for capillary blood glucose and a cut-off of 126 mg/dL (7.0 mmol/L) or above for fasting plasma/venous glucose when assessing diabetes risk

Postprandial capillary results can be up to 20% higher than plasma/venous levels, likely due to glucose uptake by cells and tissues. Venous plasma sampling is recommended over serum for diagnosing diabetes due to its preferred processing method (Kim 2016).

References

Kim, Hye Soon. “Blood Glucose Measurement: Is Serum Equal to Plasma?.” Diabetes & metabolism journal vol. 40,5 (2016): 365-366. doi:10.4093/dmj.2016.40.5.365

Mathew, Thomas K., et al. “Blood Glucose Monitoring.” StatPearls, StatPearls Publishing, 23 April 2023.

Tirimacco, Rosy, et al. "Should capillary blood glucose measurements be used in population surveys?." International journal of diabetes mellitus 2.1 (2010): 24-27.