18 Processing Practice Questions & Answers

Every Processing practice question from the Phlebotomy Technician Practice Test, with the correct answer and a short explanation.

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1. Two specimens from the same patient are centrifuged: one was allowed to clot completely, the other was anticoagulated. What is the physical difference between the fluid layers obtained?
- A.Both tubes give the same fluid, since fibrinogen stays dissolved however blood is handled
- B.The clotted tube gives serum, which lacks fibrinogen because the protein was used in the clot✓ Answer
- C.The clotted tube gives serum, which holds more fibrinogen than the anticoagulated tube does
- D.The clotted tube gives plasma, which keeps fibrinogen because clotting removes no protein
When blood is allowed to clot, fibrinogen is converted to fibrin and trapped in the clot, so the fluid left above the cells is serum, which contains no fibrinogen or other consumed clotting factors. An anticoagulated tube blocks that conversion, so its supernatant is plasma and still contains fibrinogen.
Source: NHA CPT Test Plan, Core Knowledge, blood components: serum versus plasmaReport a problem with this question
2. A tube of anticoagulated whole blood has been centrifuged. What occupies the thin layer between the plasma above and the red cell mass below, and what does that lower layer represent?
- A.White cells and platelets form the buffy coat, and the red layer below is the hematocrit✓ Answer
- B.Fibrin strands sit in the middle layer, and the lower layer is what is called the hematocrit
- C.Serum proteins settle in the middle layer, and the lower layer is the total blood volume
- D.Reticulocytes gather in the middle layer, and the lower layer is the plasma protein share
Centrifuged anticoagulated blood separates into about 55 percent plasma on top, a buffy coat of white cells and platelets that is less than 1 percent, and roughly 45 percent packed red cells at the bottom. That packed red cell fraction is what the hematocrit reports.
Source: NHA CPT Test Plan, Core Knowledge, composition of whole bloodReport a problem with this question
3. When and where should the collected tubes be labeled?
- A.Before the venipuncture begins, so each tube is ready as it fills in sequence
- B.At the nurses' station immediately after leaving the room, from the requisition
- C.In the laboratory at accessioning, by matching the tubes against the requisition
- D.At the bedside in the patient's presence, right after the draw and before leaving✓ Answer
Labeling in front of the patient before leaving the room is what links the tube to the identity that was just verified; any step taken later allows tubes from different patients to be confused. Pre-labeling is equally unsafe, because a label applied before the draw may end up on blood from someone else.
Source: CLSI GP41 specimen labeling at the point of collection; NHA CPT Test PlanReport a problem with this question
4. Processing staff find an unlabeled lavender tube delivered in a bag with a single requisition. What should happen to it?
- A.It is rejected and recollected, since identity cannot be reconstructed after the fact✓ Answer
- B.It is labeled by the collector now, since the collector still recalls that patient
- C.It is labeled from the requisition, since only one patient's tube was in that bag
- D.It is tested and reported with a comment noting the tube arrived without a label
An unlabeled tube has no verifiable link to a patient, and a label added afterward is a guess no matter how confident the collector feels, so the specimen is rejected and a new one collected. Reporting a result from an unidentified specimen risks giving one patient another patient's result, which is why unlabeled specimens are a standing rejection criterion.
Source: NHA CPT Test Plan, specimen rejection criteria; CLSI GP41 labelingReport a problem with this question
5. The vein collapses and a light blue coagulation tube is left only half full, so the blood-to-citrate ratio is wrong. What follows?
- A.The results are unaffected, because the analyzer corrects for the volume collected
- B.The prothrombin time and aPTT come back falsely prolonged, so it is recollected✓ Answer
- C.The plasma clots inside the tube, so the analyzer reports the sample as unreadable
- D.The prothrombin time and aPTT come back falsely shortened, so the tube may be run
The citrate tube is designed for a nine-to-one ratio of blood to anticoagulant, so an underfilled tube carries excess citrate, which binds more calcium and makes clot formation look slower than it truly is. The prothrombin time, INR and aPTT are therefore falsely prolonged, and a short-drawn coagulation tube is a standing rejection criterion requiring recollection.
Source: CLSI GP41 tube fill and 9:1 citrate ratio; NHA CPT Test PlanReport a problem with this question
6. After centrifugation the serum of a chemistry specimen is distinctly pink. Which result is most affected?
- A.Sodium, falsely raised because plasma water has shifted into the red cells
- B.Potassium, falsely raised because it leaks out of the ruptured red cells✓ Answer
- C.Calcium, falsely raised because the damaged membranes bind less of it
- D.Glucose, falsely raised because the red cells release their stored sugar
Pink serum means red cells have ruptured, and potassium is roughly twenty-five times more concentrated inside the red cell than in plasma, so even mild hemolysis raises the measured value sharply. Magnesium, phosphorus, ammonia, LDH and AST rise for the same reason, and a visibly hemolyzed specimen is rejected for these analytes and recollected.
Source: NHA CPT Test Plan, specimen integrity; CLSI GP41 hemolysisReport a problem with this question
7. A tube for glucose and potassium sat on the bench for four hours before it was centrifuged. What is expected?
- A.Both unchanged, because the separator gel had already walled off the red cells
- B.Glucose falsely high and potassium falsely low, as the cells release stored sugar
- C.Glucose falsely low and potassium falsely high, as cells consume sugar and leak✓ Answer
- D.Both falsely low, because the cells keep taking up glucose and potassium alike
While cells remain in contact with serum they keep metabolizing glucose, which falls by roughly five to seven percent per hour, and potassium leaks out of the cells down its concentration gradient. That is why serum or plasma must be separated from the cells within two hours of collection, and why unspun whole blood is not refrigerated.
Source: NHA CPT Test Plan, processing; CLSI GP41 two-hour separation requirementReport a problem with this question
8. Of the specimens described below, which one is being handled correctly on the way to the laboratory?
- A.A bilirubin specimen wrapped in foil so that light cannot reach the sample✓ Answer
- B.A lactic acid specimen packed against solid ice cubes rather than a slurry
- C.A cold agglutinin specimen refrigerated for the trip to the testing bench
- D.An ammonia specimen carried at room temperature to stop the cells lysing
Bilirubin is degraded by light, so its tube travels wrapped in foil or in an amber container. Ammonia and lactic acid must instead be chilled in an ice-water slurry, which cools evenly without freezing and hemolyzing the sample, while cold agglutinins must be kept warm at body temperature until the serum has been separated.
Source: NHA CPT Test Plan, special handling; CLSI GP41 specimen transport requirementsReport a problem with this question
9. How should a labeled tube of blood be packaged for transport to an off-site reference laboratory?
- A.In a rigid sharps container, with the requisition in the same pocket
- B.In an unlabeled zip bag, with the requisition folded around the tube
- C.In a leakproof biohazard bag, with the requisition in the outer sleeve✓ Answer
- D.In a padded envelope, with the requisition taped to the tube itself
Specimens leaving the facility must go in a closable, leakproof secondary container that is labeled or color-coded for biohazard, and the requisition rides in a separate outer sleeve so paperwork stays readable if the tube leaks. A sharps container is for waste, and an unlabeled bag gives the courier no warning of the hazard.
Source: OSHA 29 CFR 1910.1030(d)(2)(xiii) and (g)(1)(i)Report a problem with this question
10. A phlebotomist collects a specimen for plasma ammonia. How should it be transported to the laboratory?
- A.Frozen on dry ice, delivered within two hours
- B.In a 37 °C heat block, delivered within an hour
- C.Wrapped in foil, delivered at room temperature
- D.In an ice-and-water slurry, delivered immediately✓ Answer
Ammonia is generated by ongoing cell metabolism after the draw, so the level rises quickly at room temperature. Chilling in a slurry of crushed ice and water (not solid ice cubes, which cool unevenly and can hemolyze cells) slows metabolism, and immediate delivery limits the time for the value to change. Lactate and arterial blood gases follow the same chilled, immediate-transport rule.
Source: CLSI GP44 (specimen handling); NHA CPT Test Plan Domain 5B, knowledge statement k89 (temperature-sensitive transport: ammonia chilled in ice slurry, immediate delivery)Report a problem with this question
11. Which specimen must be kept at body temperature (37 °C) from the time of collection until it is tested?
- A.Plasma ammonia
- B.Total bilirubin
- C.Cold agglutinins✓ Answer
- D.Arterial blood gases
Cold agglutinins are antibodies that attach to red blood cells when the specimen cools below body temperature; once bound to the cells they are removed with the clot and the measured value is falsely low. The tube is therefore pre-warmed and kept at 37 °C until testing. Ammonia and blood gases are chilled, and bilirubin is protected from light — the opposite handling.
Source: CLSI GP44; NHA CPT Test Plan Domain 5B, k89 (specimens kept at 37 °C: cold agglutinins, cryoglobulins, cryofibrinogen)Report a problem with this question
12. A capillary specimen is collected from a jaundiced newborn for total bilirubin. What special handling does this specimen require?
- A.Wrap the tube in aluminum foil right away✓ Answer
- B.Hold the tube at 37 °C in a heat block
- C.Spin the tube within 10 minutes of the draw
- D.Place the tube in an ice-and-water slurry
Bilirubin breaks down when exposed to light, so an unprotected specimen gives a falsely decreased result — a dangerous error in a newborn being evaluated for treatment. Wrapping the tube in foil (or using an amber tube) blocks light from collection until testing. B12, folate, carotene, and porphyrins are also light-sensitive.
Source: CLSI GP44; NHA CPT Test Plan Domain 5B, k89 (light-protected analytes: bilirubin falsely decreased by light exposure)Report a problem with this question
13. An SST has been centrifuged and its gel barrier has formed, but the analyzer needs more serum than remains above the gel. What should the specimen processor do?
- A.Centrifuge the same SST a second time
- B.Request that a new specimen be collected✓ Answer
- C.Rim the gel barrier and respin the tube a second time
- D.Add serum saved from an earlier draw
A gel-separator tube may be spun only once. After the barrier forms, cells trapped beneath it continue to metabolize and leak constituents; respinning forces that altered fluid up through or around the gel and contaminates the serum, most notably falsely elevating potassium. Mixing in serum from a different tube changes the specimen's collection time and integrity, so the only acceptable option is recollection.
Source: CLSI GP44 and gel-tube manufacturer instructions (e.g., BD SST): gel-separator tubes are single-spin only; recentrifugation falsely elevates potassium — NHA CPT Domain 5A, k87Report a problem with this question
14. A specimen processor has a single tube to centrifuge and no other specimens waiting. What should be done before starting the spin?
- A.Put the tube in the slot nearest the rotor's center
- B.Run the spin at half speed for double the usual time
- C.Place a water-filled tube of equal size opposite it✓ Answer
- D.Brace the centrifuge lid by hand during the whole run
Centrifuge loads must be balanced with tubes of equal size and volume placed directly opposite each other. An unbalanced rotor vibrates, which can break tubes, release biohazardous aerosols, damage the instrument, and produce poor separation. A water-filled tube matching the specimen's size and fill acts as the counterweight; changing the speed or holding the lid does nothing to correct the imbalance.
Source: CLSI GP44 centrifuge operation (balanced loads, closed lids); NHA CPT Test Plan Domain 5A, k87 (centrifuging requirements)Report a problem with this question
15. A phlebotomist prepares to draw a legal blood alcohol specimen under chain of custody. Which site preparation is acceptable?
- A.An isopropyl alcohol pad allowed to dry fully
- B.Tincture of iodine, which has an alcohol base
- C.An alcohol swab followed by a water rinse
- D.Soap and water or benzalkonium chloride✓ Answer
Any antiseptic containing alcohol can contaminate the puncture site and falsely elevate the blood alcohol result, making the specimen legally challengeable — even if the pad appears dry, and even if the site is rinsed afterward. Iodine tincture is prepared in an alcohol base, so it carries the same problem. Only non-alcohol preparations such as soap and water, benzalkonium chloride, or povidone-iodine are acceptable.
Source: NHA CPT Test Plan Domain 5C, k91 (chain of custody, blood alcohol): non-alcohol antiseptic required for legal blood alcohol collectionReport a problem with this question
16. A blood specimen is collected for a forensic drug test. Which action is required to keep the chain of custody intact?
- A.Every handler signs and dates each transfer of the specimen✓ Answer
- B.The courier logs the specimen only at its final delivery to the laboratory
- C.Only the collector and the testing analyst sign the form
- D.The specimen is sealed once it arrives at the laboratory
Chain of custody is an unbroken documentation trail: every person who takes possession of the specimen records receipt and transfer with signature, date, and time, and the specimen is sealed with a tamper-evident seal in the donor's presence at collection — not later at the lab. Any undocumented gap breaks the chain and can make the result inadmissible, requiring recollection.
Source: NHA CPT Test Plan Domain 5C, k91 (chain of custody: documented receipt/transfer by every handler; tamper-evident seal applied in donor's presence)Report a problem with this question
17. Which practice follows correct aliquoting procedure when preparing serum for multiple laboratory departments?
- A.Pool serum from two of the patient's tubes to gain volume
- B.Transfer serum by disposable pipette into a prelabeled tube✓ Answer
- C.Pour serum slowly down the inner wall of the aliquot tube
- D.Return any unused serum to the original collection tube
Aliquots are transferred with a disposable pipette into a tube already labeled with the same identifying information as the parent specimen, then capped promptly. Pouring risks splashing, aerosols, and contamination; combining serum from separate tubes — even from the same patient — mixes specimens with different additives and collection conditions; and returning serum to the original tube contaminates the remaining specimen.
Source: NHA CPT Test Plan Domain 5A, k88 (aliquoting): pipette transfer into prelabeled tubes; never pour, pool, or return aliquots — CLSI GP44Report a problem with this question
18. A CLIA-waived glucose meter in an outpatient clinic reads 32 mg/dL, which is on the facility's critical value list. What should the phlebotomist do first?
- A.Enter the value in the LIS for routine review by the provider
- B.Notify the nurse or provider at once and document the call✓ Answer
- C.Tell the patient to drink some juice and recheck in an hour
- D.Wait for a venous draw to confirm it before telling anyone
A critical (panic) value is life-threatening and must be reported immediately to a licensed caregiver — the nurse or ordering provider — with read-back verification, and the notification documented (who was told and when). The phlebotomist never interprets results or advises the patient on treatment, and routine LIS entry or waiting for confirmation delays care the patient may need within minutes.
Source: NHA CPT Test Plan Domain 5G, k93 (recognize and report critical values for POC/CLIA-waived tests); Joint Commission NPSG.02.03.01 (timely critical result reporting with documentation)Report a problem with this question

Concept-focused practice questions based on CLSI GP41 (venous blood collection), the CLSI order of draw, and the OSHA Bloodborne Pathogens Standard (29 CFR 1910.1030). Not affiliated with the NHA, ASCP, or CLSI, and not medical advice. Facility protocols and current CLSI standards take precedence — always follow your employer's procedures. About the CPT exam →

1. Two specimens from the same patient are centrifuged: one was allowed to clot completely, the other was anticoagulated. What is the physical difference between the fluid layers obtained?

2. A tube of anticoagulated whole blood has been centrifuged. What occupies the thin layer between the plasma above and the red cell mass below, and what does that lower layer represent?

3. When and where should the collected tubes be labeled?

4. Processing staff find an unlabeled lavender tube delivered in a bag with a single requisition. What should happen to it?

5. The vein collapses and a light blue coagulation tube is left only half full, so the blood-to-citrate ratio is wrong. What follows?

6. After centrifugation the serum of a chemistry specimen is distinctly pink. Which result is most affected?

7. A tube for glucose and potassium sat on the bench for four hours before it was centrifuged. What is expected?

8. Of the specimens described below, which one is being handled correctly on the way to the laboratory?

9. How should a labeled tube of blood be packaged for transport to an off-site reference laboratory?

10. A phlebotomist collects a specimen for plasma ammonia. How should it be transported to the laboratory?

11. Which specimen must be kept at body temperature (37 °C) from the time of collection until it is tested?

12. A capillary specimen is collected from a jaundiced newborn for total bilirubin. What special handling does this specimen require?

13. An SST has been centrifuged and its gel barrier has formed, but the analyzer needs more serum than remains above the gel. What should the specimen processor do?

14. A specimen processor has a single tube to centrifuge and no other specimens waiting. What should be done before starting the spin?

15. A phlebotomist prepares to draw a legal blood alcohol specimen under chain of custody. Which site preparation is acceptable?

16. A blood specimen is collected for a forensic drug test. Which action is required to keep the chain of custody intact?

17. Which practice follows correct aliquoting procedure when preparing serum for multiple laboratory departments?

18. A CLIA-waived glucose meter in an outpatient clinic reads 32 mg/dL, which is on the facility's critical value list. What should the phlebotomist do first?