If the following labs are ordered, which should be drawn first?

• A. CBC
• B. A non-additive serum specimen
• C. A chemistry test in a light green-topped heparin tube
• D. Glucose test in a gray topped tube

Answer: (B)

The concept being tested is preventing cross-contamination of tube additives during the draw. A non-additive serum specimen should be drawn first because it has no anticoagulant or other additive that could contaminate subsequent tubes. If a tube with an additive (such as EDTA for the CBC, heparin for chemistry, or fluoride/oxalate for glucose) is drawn first, trace amounts of that additive can linger on the needle and contaminate the next tube, potentially skewing results for the serum test. Drawing the non-additive serum tube first helps preserve the integrity of the serum sample and protects the accuracy of all other tests ordered.