Q&A column

Kristi J. Smock, MD
Professor of Pathology
University of Utah School of Medicine
Hematopathology Medical Director
Hemostasis/Thrombosis Laboratory
ARUP Laboratories
Salt Lake City, Utah
Vice Chair, CAP Hemostasis and Thrombosis Committee

Q. Obtaining an accurate blood glucose level is hindered by continued glycolysis in the evacuated tube post collection, even if a gray top tube is used. This leads to falsely low blood glucose levels. What can laboratories do to limit ex vivo glycolysis?

A.The reader has identified an important practical problem. Loss of glucose from a blood sample via glycolysis, predominantly in red and white blood cells, contributes substantially to preanalytical error when measuring glucose. Glucose decreases in whole blood at room temperature ex vivo at five to seven percent per hour. To put this in context, the reduction in glucose at one hour exceeds the desirable limit of total analytical error for glucose based on biological variation.

Some clinicians erroneously believe that a gray top tube, which contains sodium fluoride, completely prevents glycolysis. However, this is true only for long-term glycolysis, as sodium fluoride, which inhibits the glycolytic enzyme enolase, does not stop glycolysis until two to four hours after sample collection. The rate at which glucose is lost during the first 60 to 90 minutes is the same with or without sodium fluoride. After four hours, the glucose concentration is stable in whole blood for 72 hours at room temperature in the presence of sodium fluoride.

An American Association for Clinical Chemistry and American Diabetes Association guideline addresses the practice of measuring plasma glucose to diagnose diabetes. The guideline recommends removing plasma from whole blood samples immediately after blood collection or immediately immersing the blood collection tube in an ice-water slurry and removing the plasma within 30 minutes of collection. However, these methods are rarely feasible in routine patient care.

A proposed alternative is to use blood tubes containing citrate, which immediately blocks the activity of glycolytic enzymes by acidifying the blood (pH, 5.3–5.9), thereby preventing glycolysis. Several studies that validate this concept have been published over the past few years. Tubes containing granulated citrate buffer with sodium citrate and sodium EDTA are used clinically, predominantly in Europe, but are not yet available in the United States. I strongly encourage manufacturers to make these blood collection tubes available worldwide.

Sacks DB, Arnold M, Bakris GL, et al. Guidelines and recommendations for laboratory analysis in the diagnosis and management of diabetes mellitus. Clin Chem. 2011;57(6):e1–e47. Diabetes Care. 2011;34(6):e61–e99.

David B. Sacks, MBChB
Senior Investigator
Chief, Clinical Chemistry Service
NIH Clinical Center
Bethesda, Md.
Member, CAP Chemistry Resource Committee