What color tube is used for a Glucose Tolerance Test?

• A. Red
• B. Lavender
• C. Gray
• D. Green

Answer: (C)

Glucose tolerance testing relies on preserving the glucose level from the moment of draw through all time-point measurements. If glucose is metabolized in the sample, the result will be falsely low and unreliable for diagnosing glucose intolerance. A gray-top tube contains sodium fluoride, a glycolysis inhibitor, along with potassium oxalate, an anticoagulant. The fluoride stops cells from breaking down glucose in the sample, keeping the true glucose level stable for the multiple draws required in a GTT. That makes gray the best choice for this test. Red tops lack a glycolysis inhibitor and are used for serum without preserved glucose; lavender tops are EDTA tubes used mainly for hematology; green tops contain heparin for certain chemistry tests but do not prevent glycolysis.