Friday, May 31, 2013

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

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

Is there a difference between whole blood and plasma glucose test results ?

   We all put a lot of faith in our home glucose testing equipment and feel comfortable making management decisions based on the test results.  However, have you ever wondered why your finger stick capillary test is often different than the venous glucose test done in a laboratory. There are a number of variables effecting your glucose test results.
   The source of the blood sample ( arterial , capillary  or venous ) and how the blood was processed all make a difference .  Whole blood is usually removed from a vein in our arm and then put into a centrifuge and separated into plasma and red blood cells. The glucose test is then done on the plasma component of the blood. Whole blood can also be allowed to clot which separates the blood into red blood cells and serum.  Your finger stick test is measuring whole blood glucose taken from capillaries in your finger and done on a machine with it's own built in error rate. The last time I checked, meters  are  required to have an error rate of < 20% at glucose levels of 30 to 400 mg/dl.

 From my own personal experience in the hospital and office , 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.
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 ” used by the lab .. 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..

Have fun , Be smart and test your blood glucose frequently and learn how to use the results to make decisions in in your daily diabetes management decisions.
David Calder, MD

1 comment:

  1. Symptomatic indicators of the disease may only be observed at a later stage of disease.Nowdays Fasting Blood Glucose,Random Glucose Test,Oral Glucose Tolerance Test are more common diabetes test available in the market.


Your comments and questions are appreciated. David Calder,MD