Tuesday, November 29, 2011

Why is my finger stick often different that the laboratory glucose test?

Most of my old blogs were lost during the hacking attack and other website changes. I plan to update and repeat some of the previous blogs .
We all use home glucose test to manage our diabetes . Home glucose testing has been one of the major improvements in the day to day management of diabetes. It is not perfect but it is still a fantastic improvement when I think about the old urine glucose test that we previously used to make treatment decisions.

We assume that our home glucose test are accurate and we feel comfortable making management decisions based on those results. I believe that our home glucose test meet the requirements needed for very good glucose management. However, have you ever wondered why you finger stick capillary test is often different than the venous glucose test done in a laboratory. Lets take a little closer look at the glucose test processes.

The meter you are using is require to have an error rate of < 20% at glucose levels of 30 to 400mg/dl. so our result could be somewhere between 65 and 95 mg/dl assuming an error rate of 19%. From my own personal experience in the hospital, where I frequently compared finger stick results with hospital lab. results, I believe the error rate on most meters is small and not a major issue.

What else effects the results?

The source of the blood effects the results. Arterial blood is about 5% higher than capillary blood and 10% higher than venous blood. You are not likely to be using arterial blood but you will be comparing capillary blood ( finger stick sample) to venous blood that is usually taken from your arm by a lab. Tech. This means ,that if you did a fingerstick test( capillary blood )at the same time as a laboratory test( venous blood) , your result may be a little higher than the lab result by about 5%
Their is another interesting variable to be aware of. Your glucose meter reads” whole blood ” which is about 10 to 15 % lower than “plasma ” that the lab is using. I believed that this was a good thing when I was following hospital patients on continuous intravenous insulin infusion because the hourly finger stick test were a little higher ( capillary whole blood) and made the lab ( venous plasma )and bedside test have a little better correlation .Then the meter builders decide to convert the whole blood results into a plasma-calibrated result. For the doctor this change resulted in higher bedside result and more concern that we may miss low test results.. I had to adapt to the change.
So this is probable a lot more than you wanted to know but it may help you understand why your home test may not match the lab. results. In general our home meter are wonderful machines that have dramatically improve our lives..

ps —how about watching my App training video and let me know what you think—


David Calder, MD


  1. why capillary blood glucose is higher than venous blood glucose

    1. http://www.bestthinking.com/articles/medicine/internal_medicine/hematology/glucose-concentration-difference-between-arterial-capillary-and-venous-blood

    2. Arterial blood has the highest concentration of glucose. Glucose is remove as the blood flows through as the blood flows through the smaller vessels called capillaries before reaching the veins. The lowest concentration of glucose is in the veins.
      Blood drawn from your arm is usually venous ( lowest glucose concentration).
      Blood taken by finger stick is a combination of capillary and venous blood and will have about a 5% higher concentration of glucose than venous blood. Dr. Calder

  2. People who keep their blood sugar levels under control with diet, exercise, or oral diabetes medicines are less likely to have problems with high or low blood sugar levels.

  3. BAYSHORE CLINICAL LABORATORIES LLC http://www.bayshorelabs.com/

    Specializing in the analysis of dried blood samples collected with the patented ADx100 system. No need for traditional phlebotomy! The ADx100 system is a simple collection technique that separates serum from red blood cells using a simple finger stick. Testing is performed at Bayshore Clinical Laboratories using the same instrumentation as your hospital or commercial laboratory.

    Bayshore is nationally accredited under the Clinical Laboratory Improvement Act (CLIA) by the Federal Government #52D2053574.

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Your comments and questions are appreciated. David Calder,MD