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Inhibitors, lupus anticoagulant and thrombophilia timing

15 min

Skills you'll build4 skills
  • Interpret a Nijmegen-Bethesda titer with the inhibitor type and its kinetics
  • Interpret lupus anticoagulant screen, mix, and confirm results from two test systems
  • Recognize when anticoagulants or acute illness make a thrombophilia result unreliable
  • Use thrombin and reptilase times to separate heparin from a fibrinogen defect

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

Try first

Try first

A patient with no personal or family bleeding history develops new bruising and an isolated prolonged APTT that loses correction on an incubated mix. Factor VIII activity is low, and a Nijmegen-Bethesda titer is run. How should the titer be interpreted?

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.

Interpreting an inhibitor titer

A Nijmegen-Bethesda titer measures how much patient plasma inhibits factor VIII in pooled normal plasma. An alloantibody in congenital hemophilia, formed after exposure to therapeutic factor, follows type I kinetics: complete, dose-proportional neutralization and a reliable titer, with 5 Nijmegen-Bethesda units per milliliter (BU/mL) separating low- from high-responding inhibitors.1 An acquired factor VIII autoantibody in a patient with no prior bleeding history follows type II kinetics: rapid early inactivation that plateaus with measurable residual factor VIII, so its titer is only semiquantitative.1

Lupus anticoagulant testing

Lupus anticoagulant (LA) testing uses two phospholipid-dependent systems, usually a dilute Russell viper venom time and an LA-sensitive APTT, each carried through a screen, a mix, and a phospholipid-rich confirm step. Shortened clotting with excess phospholipid supports phospholipid dependence.2 Heparin, vitamin K antagonists, and direct oral anticoagulants prolong these same phospholipid-dependent times, so a drug effect is excluded before LA is interpreted.2

Timing of thrombophilia testing

Warfarin lowers protein C and protein S, because both are vitamin K dependent, and acute thrombosis, liver disease, and disseminated intravascular coagulation (DIC) also change them. A low result during any of these states can be acquired, so a congenital deficiency is confirmed at a stable baseline.3 Factor V Leiden and prothrombin G20210A genotyping stay reliable during anticoagulation and acute illness, because genotyping does not depend on the current level of a protein.3

Thrombin time and reptilase time

Thrombin time adds thrombin directly to plasma and is sensitive to heparin, direct thrombin inhibitors, and low or dysfunctional fibrinogen. Reptilase time uses a snake-venom enzyme that is insensitive to heparin. Heparin or a direct thrombin inhibitor prolongs the thrombin time and leaves the reptilase time normal. Prolongation of both times points to a fibrinogen quantity or function problem.4

Follow the bench demonstration

Sources for this step
References
  1. Srivastava A, Santagostino E, Dougall A, et al. WFH guidelines for the management of hemophilia, 3rd edition. Haemophilia. 2020;26(suppl 6):1-158. doi:10.1111/hae.14046
  2. Devreese KMJ, de Groot PG, de Laat B, et al. Guidance from the Scientific and Standardization Committee for lupus anticoagulant/antiphospholipid antibodies of the International Society on Thrombosis and Haemostasis: update of the guidelines for lupus anticoagulant detection and interpretation. J Thromb Haemost. 2020;18(11):2828-2839. doi:10.1111/jth.15047
  3. Middeldorp S, Nieuwlaat R, Baumann Kreuziger L, et al. American Society of Hematology 2023 guidelines for management of venous thromboembolism: thrombophilia testing. Blood Adv. 2023;7(22):7101-7138. doi:10.1182/bloodadvances.2023010177
  4. Keohane EM, Preston MM, Mirza KM, Walenga JM, eds. Rodak's Hematology: Clinical Principles and Applications. 7th ed. Elsevier; 2025. Accessed September 26, 2026. https://www.us.elsevierhealth.com/rodaks-hematology-9780323936507.html

Watch one

Read each decision and its explanation.

A patient with a history of unprovoked deep vein thrombosis has a prolonged APTT found on a preoperative panel. She takes no anticoagulant.

  1. Review the medication record. No heparin, warfarin, or direct oral anticoagulant is documented, and drug-specific testing is negative.

    A drug effect on a phospholipid-dependent clotting time can mimic an antiphospholipid antibody, so medications are checked first.

  2. Run the LA screen. The dilute Russell viper venom time and an LA-sensitive APTT are both prolonged.

    The screen is the first of the three steps in each test system, and it is where the prolongation is first seen.

  3. Run the mix. Both systems stay prolonged on a 1:1 mix with pooled normal plasma.

    A factor deficiency corrects on mixing. A lupus anticoagulant interferes with the phospholipid the assay depends on and typically does not correct.

  4. Run the confirm step. Both systems shorten substantially with added phospholipid.

    Excess phospholipid overcomes a true lupus anticoagulant, which distinguishes it from a specific factor inhibitor.

A prolonged screen, no correction on mixing, and shortening on the phospholipid-rich confirm support a lupus anticoagulant. This fits her history of unprovoked thrombosis.

Practice this skill

Your turn

Problem 1 of 3 · Hints available

A patient with severe congenital hemophilia A begins factor VIII replacement. Two weeks later, factor VIII activity has fallen and a Nijmegen-Bethesda titer of 8 BU/mL is measured. A repeat titer after further exposure rises to 14 BU/mL. How is this titer best described?

A low-responding inhibitor stays below 5 BU/mL and does not rise on further exposure. This one is already above 5 BU/mL and rose with exposure.

An alloantibody that appears after exposure to therapeutic factor in congenital hemophilia follows type I kinetics: complete, dose-proportional neutralization. Reaching 5 BU/mL or more and rising with further exposure meets the definition of a high responder, and the titer is reliable.

This inhibitor followed exposure to therapeutic factor in a patient with known congenital hemophilia, the setting for a type I, dose-proportional alloantibody. The type II pattern is typical of an acquired autoantibody in someone with no prior deficiency.

Read an acquired factor VIII inhibitor titer as exact

Acquired factor VIII autoantibodies follow type II kinetics, rapid early inactivation that plateaus with residual factor VIII, so the titer is only semiquantitative. Alloantibodies in congenital hemophilia follow type I kinetics and give a reliable titer, with 5 Nijmegen-Bethesda units separating low from high responders.

Hint
  1. Consider whether the antibody formed after factor VIII treatment in congenital hemophilia or appeared in someone with no bleeding history.
  2. Type I kinetics give a reliable, dose-proportional titer. Type II kinetics plateau with residual factor VIII and give only a semiquantitative one.

Review Factor and inhibitor assays

Problem 2 of 3 · Hints available

A patient on warfarin for atrial fibrillation is found to have a low protein C activity on a panel ordered to investigate a family history of thrombosis. What do you do with the result?

Warfarin lowers protein C because it is vitamin K dependent, whether or not the patient also has an inherited deficiency. A low result on warfarin cannot be interpreted as congenital.

Reported protein C deficiency during warfarin therapy

Warfarin lowers protein C and protein S because both are vitamin K dependent, and acute thrombosis, liver disease, and DIC also change them. A low result in those states can be acquired, so confirm a congenital deficiency at a stable baseline. Factor V Leiden and prothrombin G20210A genotyping stays reliable during anticoagulation.

Warfarin lowers protein C and protein S together. A congenital deficiency needs confirmation once the patient is at a stable baseline, off the drug.

Repeating the same specimen reproduces the same warfarin effect. The problem is when the specimen was drawn.

Hint
  1. Consider how warfarin affects protein C, separate from any inherited deficiency.
  2. A result affected by a reversible cause needs confirmation once that cause is gone.

Review Lupus anticoagulant and thrombophilia assays

Problem 3 of 3 · On your own

A hospitalized patient on intravenous unfractionated heparin has a thrombin time of 45 seconds (reference interval, 14 to 19 seconds) and a reptilase time of 16 seconds (reference interval, 14 to 19 seconds). What does this pair of results support?

Low or dysfunctional fibrinogen prolongs both the thrombin time and the reptilase time. Here the reptilase time is normal.

Read a prolonged thrombin time as low fibrinogen

Heparin and direct thrombin inhibitors prolong the thrombin time and leave the reptilase time normal. Reporting a fibrinogen problem from the thrombin time alone misses a drug effect. Prolongation of both times points to low or dysfunctional fibrinogen.

Heparin prolongs the thrombin time and leaves the reptilase time, which is insensitive to heparin, normal. This pattern fits the patient's own heparin therapy.

A markedly prolonged thrombin time needs an explanation. With a normal reptilase time, heparin explains it.

Review Thrombin time, reptilase time, and heparin neutralization

Use it

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

  • A woman (MRN 0064128) started warfarin 5 days ago for a new, unprovoked deep vein thrombosis.
  • Her INR today is 2.4.
  • The clinical team orders a lupus anticoagulant panel and a thrombophilia panel while she is still in the hospital.
  • Her APTT is prolonged, and the laboratory is asked to interpret the panel before results are released.
Decision 1 of 3

What does warfarin do to the lupus anticoagulant panel drawn now?

Warfarin prolongs phospholipid-dependent clotting times and can distort the screen, mix, and confirm steps of lupus anticoagulant testing.

Called lupus anticoagulant with a drug still present

Heparin, vitamin K antagonists, and direct oral anticoagulants prolong phospholipid-dependent clotting times and can distort the screen, mix, and confirm steps. Interpreting the pattern while a drug is present reports an anticoagulant effect as an antiphospholipid antibody.

Warfarin, like heparin and direct oral anticoagulants, prolongs phospholipid-dependent clotting times. An interpretation made now risks reporting an anticoagulant effect as a lupus anticoagulant.

Warfarin can affect all three steps: the screen, the mix, and the confirm step.

Review Lupus anticoagulant and thrombophilia assays

Decision 2 of 3

The acute thrombosis panel also shows a low protein S activity. What do you report?

Warfarin and acute thrombosis both lower protein S, so a low result drawn now, on warfarin and during an acute event, cannot be interpreted as congenital.

Reported protein C deficiency during warfarin therapy

Warfarin lowers protein C and protein S because both are vitamin K dependent, and acute thrombosis, liver disease, and DIC also change them. A low result in those states can be acquired, so confirm a congenital deficiency at a stable baseline. Factor V Leiden and prothrombin G20210A genotyping stays reliable during anticoagulation.

Both warfarin and acute thrombosis can lower protein S. A congenital deficiency needs confirmation once she is stable and off warfarin, if it is still being considered.

A therapeutic INR reflects warfarin's intended effect on factors II, VII, IX, and X. It does not mean protein S is unaffected.

Review Lupus anticoagulant and thrombophilia assays

Decision 3 of 3

The clinical team wants an antiphospholipid syndrome classification decided quickly. What do you tell them about repeat testing?

Antiphospholipid syndrome classification requires persistent positivity, demonstrated again at least 12 weeks later.

Classification requires persistent positivity, confirmed at least 12 weeks after the first positive result, in addition to resolving the current drug interference.

Testing can be repeated once the drug effect is accounted for, at an appropriate interval off the drug or with a method chosen to avoid its interference.

Review Lupus anticoagulant and thrombophilia assays

Warfarin and the acute thrombosis both affect the results being interpreted. The lupus anticoagulant and thrombophilia panels are interpreted at a stable baseline off the drug, and any positive lupus anticoagulant is confirmed at least 12 weeks later.

For reference

Sources checked