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Clot formation, fibrinolysis and the citrate tube

15 min

Skills you'll build3 skills
  • Trace thrombin generation from tissue factor and factor VIIa to the platelet surface
  • Trace how plasmin forms, is inhibited, and breaks cross-linked fibrin into D-dimer
  • Reject a coagulation specimen with the wrong fill, hematocrit, handling, or timing

At the bench: Review coagulation results and identify the next laboratory check

Try first

Try first

In the body, what starts coagulation at a site of vessel injury?

Keep going to see the reasoning.

Right. Keep going to follow the reasoning.

Keep going to see the reasoning.

Keep going to see the reasoning.

Get the idea

Use the explanation to check your first decision.

Initiation of thrombin generation

Coagulation proceeds in three overlapping phases on cell surfaces. Tissue factor-bearing cells bind factor VIIa and generate small amounts of factors Xa and IXa and a small amount of thrombin. That initial thrombin activates platelets and cofactors V and VIII and supports factor XI activation. Intrinsic tenase and prothrombinase then assemble on activated platelet membranes and produce the thrombin burst that forms fibrin.1 Factors VIII and IX are required for enough thrombin to form a stable clot, which is why their deficiency causes bleeding. Factor XII, prekallikrein, and high-molecular-weight kininogen drive contact activation in the activated partial thromboplastin time (APTT) assay. Normal hemostasis in the body does not depend on them, so their deficiency can prolong the APTT markedly with intact clinical hemostasis.2

Fibrinolysis and D-dimer

Factor XIIIa cross-links fibrin as it forms. Tissue plasminogen activator, concentrated within the clot by fibrin binding, converts plasminogen to plasmin, and plasmin digests the cross-linked fibrin into fragments that include D-dimer. Plasminogen activator inhibitor-1 and α2-antiplasmin control the reaction so that lysis is limited to the clot.3 A raised D-dimer shows that cross-linked fibrin formed and was degraded. It does not confirm a thrombus or locate one, because inflammation, pregnancy, surgery, trauma, cancer, and age can all raise it.

Citrate tube fill and hematocrit

Plasma clotting assays need blood collected into buffered sodium citrate at a 9:1 blood-to-anticoagulant ratio, filled to the tube's validated volume.4 A short draw leaves excess citrate for the plasma present. The excess citrate binds part of the calcium the assay adds back and falsely prolongs clotting times. Above a hematocrit of 55%, the smaller plasma volume has the same effect in a standard tube, so the citrate volume is adjusted for the hematocrit:

Citrate volume (mL) = [(100 − hematocrit %) ÷ (595 − hematocrit %)] × intended total volume, blood plus citrate (mL)

A clot in the tube, the wrong additive, or contamination during collection also makes the specimen unacceptable.

Follow the bench demonstration

Sources for this step
References
  1. Hoffman M, Monroe DM III. A cell-based model of hemostasis. Thromb Haemost. 2001;85(6):958-965. doi:10.1055/s-0037-1615947
  2. Park S, Park JK. Back to basics: the coagulation pathway. Blood Res. 2024;59(1):35. doi:10.1007/s44313-024-00040-8
  3. Longstaff C, Kolev K. Basic mechanisms and regulation of fibrinolysis. J Thromb Haemost. 2015;13(suppl 1):S98-S105. doi:10.1111/jth.12935
  4. Clinical and Laboratory Standards Institute. Collection, Transport, and Processing of Blood Specimens for Testing Plasma-Based Coagulation Assays. 6th ed. CLSI standard H21. Clinical and Laboratory Standards Institute; 2024. Accessed September 26, 2026. https://clsi.org/shop/standards/h21/

Watch one

Read each decision and its explanation.

A citrate tube arrives for coagulation testing. It is filled to its line, drawn 25 minutes ago, and not yet centrifuged. The requisition shows the patient's hematocrit from this morning's complete blood count (CBC) is 59%.

  1. Check the fill. The tube is filled to its line, so it is not a short draw.

    A short draw leaves excess citrate for a smaller plasma volume, so the fill is checked first.

  2. Check the hematocrit. At 59% it is above the 55% threshold, so a standard tube contains excess citrate for the plasma present.

    Above a hematocrit of 55%, the plasma volume is too small for the citrate in a standard tube, even when the tube is filled to its line.

  3. Identify the effect. At this hematocrit, a standard tube gives clotting times longer than the patient's true values.

    The excess citrate binds part of the calcium the assay adds back, which falsely prolongs clotting times.

Reject the standard tube for coagulation testing. Request a specimen collected in a tube with citrate adjusted for a hematocrit of 59%, prepared and labeled under the laboratory's validated procedure.

Practice this skill

Your turn

Problem 1 of 3 · Hints available

A patient with a deficiency of factor XII has a markedly prolonged APTT and no bleeding history at any age. Which statement fits the cell-based model of coagulation?

Tissue factor and factor VIIa start coagulation at the injury. Factor XII drives the APTT reagent's contact activation, a step normal hemostasis does not depend on.

The body does not use the contact pathway to start coagulation, so hemostasis does not depend on any amount of factor XII. The APTT's sensitivity to factor XII comes from the reagent's contact-activation step.

Treated the contact pathway as the in vivo trigger

In the body, tissue factor with factor VIIa on injured tissue starts coagulation, and factors VIII and IX amplify thrombin generation on activated platelet membranes. Factor XII, prekallikrein, and high-molecular-weight kininogen (HMWK) drive the APTT reaction, which is why their deficiency prolongs the APTT without a bleeding phenotype.

The PT evaluates tissue factor and factor VII. Factor XII deficiency prolongs the APTT through the contact system the reagent uses.

Hint
  1. Consider what starts coagulation in the body, separate from what the APTT reagent adds in the tube.
  2. The APTT reagent supplies a contact activator that factor XII responds to. The body does not rely on that same trigger.

Review Classic and cell-based models

Problem 2 of 3 · Hints available

A coagulation specimen is needed from a patient with a hematocrit of 60%. Using the total-volume formula for an intended total volume of 4.5 mL (blood plus citrate), how much citrate does the adjusted tube hold?

Correct. Citrate volume = [(100 − 60) ÷ (595 − 60)] × 4.5 mL = (40 ÷ 535) × 4.5 = 0.34 mL, and the rest of the tube is filled with blood under a validated, labeled procedure.

Incorrect. That is the standard 9:1 volume. Above a hematocrit of 55%, the smaller plasma volume leaves excess citrate that binds the calcium added in the assay and falsely prolongs clotting times.

Incorrect. This scales the standard 0.45 mL up by 60 ÷ 45. A high hematocrit leaves less plasma, so the tube needs less citrate.

Hint
  1. A higher hematocrit leaves less plasma in the tube, so the tube needs a smaller citrate volume.
  2. Apply the formula with the stated hematocrit and the intended total volume of blood plus citrate.

Review Citrated plasma

Problem 3 of 3 · On your own

A postoperative patient has a D-dimer 6 times the assay's upper limit of normal. The surgical team asks whether this confirms a deep vein thrombosis. What do you report?

Surgery raises D-dimer without a thrombus, as do inflammation, pregnancy, trauma, cancer, and age. The result shows that cross-linked fibrin formed and was degraded. It does not confirm a thrombus or show its location.

D-dimer cannot confirm or locate a thrombus. Its main clinical use is helping exclude venous thromboembolism when a result falls below the cutoff in a patient with low pretest probability.

Read a raised D-dimer as evidence of thrombosis

D-dimer shows that cross-linked fibrin formed and plasmin digested it. Inflammation, pregnancy, surgery, trauma, cancer, and age also raise it, so a raised result neither confirms nor locates a thrombus. A result below the cutoff helps exclude venous thromboembolism when it is used within a validated algorithm for patients with low clinical probability.

A raised D-dimer shows that plasmin digested cross-linked fibrin. This is normal fibrinolytic activity.

Review Fibrinolysis

Use it

Read the case evidence, then make each decision. The step is complete without it.

  • A patient with cirrhosis and a hematocrit of 61% (MRN 0092247) has a coagulation panel ordered before a procedure.
  • The citrate tube is filled to about three-quarters of its line.
  • The phlebotomist's note says the draw was difficult and took several attempts.
  • No clot is visible in the tube.
Decision 1 of 2

What is the first specimen problem to address?

The fill is also short of the line. Both problems affect this tube's citrate-to-plasma ratio.

Tested a high-hematocrit specimen in a standard citrate tube

Above a hematocrit of 55%, the plasma is too small a volume for the citrate in a standard tube, and substantial underfill does the same. The excess citrate binds calcium added by the reagent and falsely prolongs clotting times, so recollect an underfilled tube at full fill and a high-hematocrit specimen with citrate adjusted for the hematocrit.

A hematocrit of 61% already calls for an adjusted tube, even at a full fill. Two separate problems affect the citrate-to-plasma ratio here.

The tube is short of its line, and a hematocrit above 55% also leaves too little plasma for a standard tube's citrate. Either one would falsely prolong clotting times, and this specimen has both.

Review Citrated plasma

Decision 2 of 2

What do you do with this specimen?

A comment does not correct the excess citrate in the plasma. The clotting times would be longer than the patient's true values.

Both defects leave excess citrate for the plasma present, and only a correctly filled, hematocrit-adjusted tube corrects that before testing.

A dilution after collection is not a validated correction for excess citrate. The citrate is adjusted when the tube is prepared, before the draw.

Review Citrated plasma

The short fill and the high hematocrit both leave more citrate in the plasma than the assay expects. Only a hematocrit-adjusted tube filled to its line corrects both problems.

For reference

Sources checked