Coagulation specimen collection and handling

9 DOH Abu Dhabi (HAAD) lab licensing practice questions on Coagulation specimen collection and handling, part of Coagulation and Hemostasis. The first 9 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Hard

   A 61-year-old man with polycythemia vera, awaiting venesection, has a hematocrit of 0.66 L/L (66%). His coagulation screen, from a correctly filled tube holding 0.3 mL of 3.2% citrate for 2.7 mL of blood, gives PT 16.9 s (reference 11.0-13.5 s) and APTT 43 s (25-35 s), although he has never bled abnormally. A repeat sample will be taken by syringe, adding exactly 2.7 mL of blood to an adjusted tube. Using C = 0.00185 × V × (100 − Hct), with V the blood volume in mL and Hct in %, how should the tube be prepared?

   1. AKeep the full 0.30 mL of citrate unchanged
   2. BRemove 0.13 mL of citrate, leaving 0.17 mL
   3. CRemove 0.11 mL of citrate, leaving 0.19 mL
   4. DRemove 0.17 mL of citrate, leaving 0.13 mL

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   Answer: B · Remove 0.13 mL of citrate, leaving 0.17 mL

   C = 0.00185 × 2.7 × (100 − 66) = 0.00185 × 2.7 × 34 = 0.17 mL, so 0.30 − 0.17 = 0.13 mL of citrate must be removed. With a hematocrit of 0.66 L/L there is much less plasma for the same amount of citrate, so the excess citrate binds the calcium in the reagents and falsely prolongs the PT and APTT, which explains his first results. CLSI H21 advises this adjustment when the hematocrit is above 0.55 L/L. Removing 0.17 mL confuses the citrate to keep with the citrate to remove, and leaving 0.19 mL comes from using the 3.0 mL total tube volume instead of the 2.7 mL of blood.

   • A: Above a hematocrit of 0.55 L/L, CLSI H21 advises reducing the citrate.
   • C: This uses the 3.0 mL total tube volume instead of the 2.7 mL of blood.
   • D: 0.17 mL is the citrate to keep, not the amount to remove.

2. Question 2 · Medium

   When a citrate tube is the first tube drawn with a winged (butterfly) collection set, why is a discard tube drawn first?

   1. ATo prevent carry-over of additive from the next tube
   2. BTo remove tissue factor released by the needle puncture
   3. CTo bring the blood sample to body temperature
   4. DTo fill the air in the tubing so the tube fills to 9:1

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   Answer: D · To fill the air in the tubing so the tube fills to 9:1

   The tubing of a winged set holds air, which enters the first tube and reduces the volume of blood it draws. A citrate tube drawn first would be underfilled, leaving excess citrate for the blood volume and falsely prolonged clotting times. A non-additive or citrate discard tube is drawn first to prime the tubing, and it need not fill completely. With a straight needle, CLSI H21 does not require a discard tube for routine PT and APTT, because tissue factor from the puncture has little effect on these tests.

   • A: Carry-over goes from earlier tubes to later ones; tubes drawn after the citrate tube cannot contaminate it.
   • B: Studies show that tissue factor from the puncture has little effect on PT and APTT, so this is not the reason.
   • C: Temperature does not matter here; the problem is the air in the tubing.

3. Question 3 · Easy

   In a correctly filled coagulation tube containing 3.2% sodium citrate, blood and anticoagulant are mixed in a ratio of:

   1. A1 part blood to 9 parts citrate
   2. B7 parts blood to 1 part citrate
   3. C4 parts blood to 1 part citrate
   4. D9 parts blood to 1 part citrate

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   Answer: D · 9 parts blood to 1 part citrate

   Coagulation tubes hold 3.2% (109 mmol/L) trisodium citrate, and a full draw gives 9 volumes of blood to 1 volume of citrate. Citrate binds calcium and stops clotting, and the test then adds a set amount of calcium back. CLSI H21 asks for tubes filled to at least 90% of their stated volume. The 4:1 ratio belongs to the Westergren ESR tube, not to coagulation testing.

   • A: This reverses the ratio; citrate is the small part of the mix.
   • B: About 7:1 is the ratio of blood to CPD anticoagulant in a blood donation bag.
   • C: A 4:1 ratio is used for Westergren ESR tubes, not coagulation tubes.

4. Question 4 · Medium

   Why must a citrated sample for monitoring unfractionated heparin be centrifuged within 1 hour of collection?

   1. APlatelets release PF4, which neutralizes heparin
   2. BCitrate gradually binds and removes the heparin
   3. CFactor VII becomes activated at room temperature
   4. DHeparin breaks down to inactive fragments

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   Answer: A · Platelets release PF4, which neutralizes heparin

   Platelets in standing whole blood release platelet factor 4, which binds and neutralizes heparin. The APTT and anti-Xa level then fall and underestimate the heparin effect, which could lead to an unnecessary dose increase. CLSI H21 advises centrifuging heparin samples within 1 hour of collection and testing the plasma within 4 hours. Low-molecular-weight heparin is much less affected because PF4 neutralizes it less.

   • B: Citrate binds calcium, not heparin.
   • C: Factor VII activation is linked with cold, not room temperature, and affects the PT more than heparin tests.
   • D: Heparin is stable in plasma; the loss comes from neutralization by PF4.

5. Question 5 · Easy

   When a light blue citrate tube is drawn with other tubes by evacuated-tube venipuncture, where does it come in the CLSI order of draw?

   1. AAfter EDTA tubes, before fluoride tubes
   2. BLast, after all other tubes
   3. CAfter serum tubes, before EDTA tubes
   4. DAfter blood cultures, before serum tubes

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   Answer: D · After blood cultures, before serum tubes

   The CLSI order of draw is blood culture bottles, citrate, serum tubes (with or without clot activator or gel), heparin, EDTA, then fluoride/oxalate. Citrate comes early so that clot activator from serum tubes, or heparin or EDTA from other tubes, cannot be carried over and change clotting results. Clot activator can shorten clotting times, while heparin or EDTA carry-over can prolong them. When only a citrate tube is needed and a straight needle is used, it can be drawn first without a discard tube.

   • A: EDTA carry-over binds calcium and would affect clotting results.
   • B: Drawing it last exposes it to carry-over from every other additive.
   • C: Serum tubes contain clot activator, which must not reach the citrate tube.

6. Question 6 · Medium

   A citrated sample for factor VIII and von Willebrand factor testing was kept in a refrigerator as whole blood for 4 hours before centrifugation. The most likely effect on the results is:

   1. AFalsely low factor VIII and VWF
   2. BFalsely high factor VIII and VWF
   3. CFalsely low fibrinogen only
   4. DNo change, as cooling preserves factors

   Answer and explanation in LearnFree account

7. Question 7 · Hard

   A sample is needed from a patient with a hematocrit of 70% (0.70 L/L). Using C = 0.00185 × V × (100 − Hct), where V is the volume of blood to be added (4.5 mL), how much 3.2% citrate should the tube contain?

   1. A0.58 mL
   2. B0.06 mL
   3. C0.25 mL
   4. D0.50 mL

   Answer and explanation in LearnFree account

8. Question 8 · Easy

   Why must a 3.2% sodium citrate coagulation tube be filled to the correct level?

   1. ATo keep the 9:1 ratio; underfilling prolongs PT and aPTT
   2. BOverfilling causes hemolysis of the sample
   3. CUnderfilled tubes let excess citrate activate the platelets
   4. DThe fill level does not affect coagulation tests

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9. Question 9 · Hard

   A patient has a hematocrit of 65%. How should the citrate volume in the coagulation tube be adjusted?

   1. ADouble the citrate volume
   2. BLeave it unchanged
   3. CIncrease the citrate volume
   4. DReduce the citrate volume

   Answer and explanation in LearnFree account