CPHT Order of Draw & Tube Additives

Question 1

What additive is found in a lavender/purple top collection tube?

Accepted Answer

EDTA (ethylenediaminetetraacetic acid)

Answer

Lavender/purple top tubes contain EDTA, an anticoagulant that chelates calcium to prevent clotting and preserve cell morphology for hematology testing.

Question 2

Which tube color is specifically used for coagulation studies such as PT/INR and aPTT?

Accepted Answer

Light blue top (sodium citrate)

Answer

Light blue top tubes contain sodium citrate, which reversibly binds calcium to prevent clotting while allowing coagulation testing through calcium re-addition.

Question 3

What is the primary purpose of sodium fluoride in a gray top collection tube?

Accepted Answer

Inhibit glycolysis to preserve glucose levels

Answer

Sodium fluoride inhibits the glycolytic enzyme enolase, stopping red blood cells from metabolizing glucose and preserving accurate glucose and lactate levels.

Question 4

Which anticoagulant is contained in a green top tube?

Accepted Answer

Heparin

Answer

Green top tubes contain heparin (typically lithium heparin or sodium heparin), which enhances antithrombin III activity to prevent blood clotting.

Question 5

What does a gold top SST (Serum Separator Tube) contain?

Accepted Answer

Clot activator and gel separator

Answer

Gold SST tubes contain silica particles as a clot activator and a polyester gel separator that migrates between serum and clot during centrifugation.

CPHT - Certified Phlebotomist Practice Test

CPHT - Certified Phlebotomist Practice Test

CPHT Order of Draw & Tube Additives