Phlebotomy (CPT) guide · updated 2026-09-29

Specimen processing and handling: the CPT Processing domain

Processing is 14 of the 100 scored items on the NHA CPT exam. It covers letting serum tubes clot before centrifuging, separating serum or plasma on time, keeping temperature- and light-sensitive specimens in the right conditions, chain of custody, and recognizing when a specimen must be rejected and recollected.

Serum or plasma

Serum comes from a tube that is allowed to clot: red or gold tops. Plasma comes from a tube with an anticoagulant: light blue, green, lavender or gray. A serum tube needs time to clot fully before it goes in the centrifuge, commonly about 30 minutes at room temperature for tubes with a clot activator. Spin it too early and you get fibrin strands that clog analyzers.

Anticoagulated tubes can be spun right away. Whole blood tests such as a CBC are not centrifuged at all.

Centrifuging

Keep stoppers on during centrifuging to prevent aerosols.
Balance the load with tubes of equal size and volume opposite each other.
Never open the lid until the rotor has stopped.
Do not re-spin a gel separator tube once the gel barrier has formed.

Temperature and light

Always follow the laboratory test directory for a specific test. The exam uses these textbook examples.

Condition	Typical examples	Why
Chilled (ice slurry)	Ammonia, lactic acid, some blood gas and hormone tests	Slows metabolism in the tube that would change the result
Body temperature (37 C)	Cold agglutinins, cryoglobulins	These proteins drop out of solution or bind cells as the sample cools
Protected from light	Bilirubin, vitamin B12, folate, beta carotene	Light breaks down the analyte
Room temperature, prompt delivery	Most routine chemistry and hematology	Standard handling

Time limits

Serum or plasma left sitting on the cells keeps changing: glucose drops as cells use it up and potassium leaks out of cells. Separating within the time the lab sets, often within two hours of collection, protects the result. A gray top is used for glucose partly because fluoride slows that loss.

Chain of custody

Forensic specimens such as blood alcohol for legal use and workplace drug screens need a documented record of everyone who handled the specimen, from collection to testing, with signatures, dates and times, and tamper-evident seals. A break in that chain can make the result unusable in court. For blood alcohol, clean the site with something other than alcohol, such as soap and water or a non-alcohol antiseptic.

Reasons a specimen is rejected

Unlabeled or mislabeled tube, or a label that does not match the requisition.
Hemolysis, often from a small needle with strong suction, shaking, or forcing blood through a syringe needle.
Wrong tube for the test, or a light blue tube underfilled so the 9 to 1 ratio is off.
Clotted anticoagulated specimen, usually from too few or too late inversions.
Wrong temperature or too long in transit.

Data and results

The Processing domain also includes entering and retrieving specimens in the laboratory information system, recognizing critical values on point-of-care and CLIA-waived tests and reporting them right away, making sure results reach the ordering provider, and contacting the patient when a recollection is needed.

Sample Phlebotomy (CPT) questions with answers

1. A gold-top SST is collected from a patient with no anticoagulant therapy. According to CLSI GP41, what is the minimum time this tube must clot at room temperature before centrifugation?

5 minutes
15 minutes
30 minutes
60 minutes

Show answer

C. 30 minutes CLSI GP41 requires a minimum of 30 minutes of clotting time at room temperature for serum tubes before centrifugation. Centrifuging at 5 or 15 minutes risks incomplete fibrin polymerization, leaving fibrin strands in the serum that can clog analyzers or produce erroneous results. The 60-minute clotting time is reserved for patients receiving anticoagulant therapy, not for patients with normal hemostasis.

2. Which statement correctly distinguishes serum from plasma?

Serum is collected in anticoagulant-containing tubes; plasma is not.
Plasma contains fibrinogen; serum does not.
Serum is the supernatant obtained from a lavender EDTA tube after centrifugation.
Plasma and serum are interchangeable terms for the liquid portion of centrifuged blood.

Show answer

B. Plasma contains fibrinogen; serum does not. Serum is the liquid remaining after whole blood clots and the clot is removed; the clotting process consumes fibrinogen, so serum lacks it. Plasma is the supernatant from anticoagulated blood (EDTA, heparin, or citrate tubes) and retains fibrinogen because clotting is prevented. The lavender EDTA tube yields plasma, not serum, and the two fluids are not interchangeable because their protein compositions differ.

3. A phlebotomist collects a specimen for a serum bilirubin level. Which action is required immediately after the tube is filled?

Place the tube in an ice-water slurry for transport to the laboratory
Wrap the tube in aluminum foil to protect it from light
Keep the tube at 37°C until centrifugation
Invert the tube 8–10 times to activate clotting

Show answer

B. Wrap the tube in aluminum foil to protect it from light Bilirubin is photosensitive; exposure to light causes photodegradation and produces falsely low results, so the specimen must be shielded with aluminum foil or placed in an amber tube immediately after collection. Chilling in ice (A) is required for cold-labile analytes such as ammonia, not bilirubin. Warming at 37°C (C) is reserved for cold-precipitating proteins. Inversion 8–10 times (D) applies to tubes containing additives but does not address the specimen's light-protection requirement.

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Sources: NHA CPT Test Plan (2024 job analysis, exam live January 7, 2026). Independent study material. Not affiliated with or endorsed by the National Healthcareer Association.

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