A 71-year-old man taking apixaban 5 mg twice daily for atrial fibrillation took his last dose 10 hours before admission with an acute coronary syndrome. A sample drawn on arrival, before any heparin, has a heparin-calibrated anti-Xa of 0.95 IU/mL. Unfractionated heparin is then started at the standard weight-based rate. Six hours later: heparin-calibrated anti-Xa 1.42 IU/mL (therapeutic 0.3-0.7 IU/mL), APTT 61 s (therapeutic 60-100 s for this reagent), platelets 230 × 10^3/µL (230 × 10^9/L). The sample was drawn from the arm opposite the infusion. What is the best interpretation?

Apixaban carryover; use the APTT for now

Contamination from the heparin infusion line

Heparin overdose; reduce the infusion

Heparin resistance from high factor VIII

An ICU patient with sepsis needs more than 35,000 units/day of unfractionated heparin, yet the aPTT stays at 41 s (therapeutic 65-100 s). Anti-Xa is 0.52 IU/mL (therapeutic 0.3-0.7 IU/mL), factor VIII 280% and antithrombin 90%. The best interpretation is:

High factor VIII is shortening the aPTT

Antithrombin is too low for heparin

The heparin dose is still too low

A lupus anticoagulant is present

A bleeding patient takes dabigatran. Which test gives a reliable plasma drug level?

Calibrated dilute thrombin time

Heparin-calibrated anti-Xa assay

Apixaban plasma levels are measured with:

Calibrated dilute thrombin time

Lab: anticoagulant monitoring (anti-Xa, direct thrombin inhibitors, heparin neutralization)

6 ASCP MLS practice questions on Lab: anticoagulant monitoring (anti-Xa, direct thrombin inhibitors, heparin neutralization), part of Hematology. The first 6 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Hard
A 71-year-old man taking apixaban 5 mg twice daily for atrial fibrillation took his last dose 10 hours before admission with an acute coronary syndrome. A sample drawn on arrival, before any heparin, has a heparin-calibrated anti-Xa of 0.95 IU/mL. Unfractionated heparin is then started at the standard weight-based rate. Six hours later: heparin-calibrated anti-Xa 1.42 IU/mL (therapeutic 0.3-0.7 IU/mL), APTT 61 s (therapeutic 60-100 s for this reagent), platelets 230 × 10^3/µL (230 × 10^9/L). The sample was drawn from the arm opposite the infusion. What is the best interpretation?
1. AContamination from the heparin infusion line
2. BApixaban carryover; use the APTT for now
3. CHeparin overdose; reduce the infusion
4. DHeparin resistance from high factor VIII
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Answer: B · Apixaban carryover; use the APTT for now
Heparin-calibrated anti-Xa assays measure inhibition of factor Xa by any drug, so apixaban still in the blood is read as heparin. The pre-heparin result of 0.95 IU/mL proves that anti-Xa activity was present before the infusion started, so the 1.42 IU/mL overstates the heparin effect. Apixaban has little effect on the APTT, so the APTT is the better guide to heparin dosing until the apixaban has cleared, which can take 2 to 3 days. Cutting the infusion because of the anti-Xa would leave him underdosed. The sample came from the other arm, which excludes line contamination, and high factor VIII would shorten the APTT rather than raise the anti-Xa.
- A: The sample was drawn from the arm opposite the infusion, and the APTT is not markedly prolonged.
- C: The anti-Xa was already 0.95 IU/mL before any heparin, so it overstates the heparin effect.
- D: High factor VIII shortens the APTT but does not raise the anti-Xa result.
Question 2 · Hard
An ICU patient with sepsis needs more than 35,000 units/day of unfractionated heparin, yet the aPTT stays at 41 s (therapeutic 65-100 s). Anti-Xa is 0.52 IU/mL (therapeutic 0.3-0.7 IU/mL), factor VIII 280% and antithrombin 90%. The best interpretation is:
1. AAntithrombin is too low for heparin
2. BHigh factor VIII is shortening the aPTT
3. CThe heparin dose is still too low
4. DA lupus anticoagulant is present
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Answer: B · High factor VIII is shortening the aPTT
Factor VIII and fibrinogen rise as acute-phase proteins, and a very high factor VIII shortens the aPTT, so the aPTT underestimates the heparin effect. The anti-Xa assay measures heparin activity directly and is not affected by factor VIII, and here it is already therapeutic. Raising the dose to chase the aPTT would risk bleeding. Patients with this apparent heparin resistance are best dosed by anti-Xa.
- A: Antithrombin is normal, so it does not explain the poor aPTT response.
- C: The anti-Xa level shows the heparin dose is already therapeutic.
- D: A lupus anticoagulant tends to lengthen the aPTT, not blunt its response.
Question 3 · Medium
A bleeding patient takes dabigatran. Which test gives a reliable plasma drug level?
1. AProthrombin time and INR
2. BHeparin-calibrated anti-Xa assay
3. CUndiluted thrombin time
4. DCalibrated dilute thrombin time
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Answer: D · Calibrated dilute thrombin time
Dabigatran inhibits thrombin directly, so thrombin-based tests respond to it. The standard thrombin time is so sensitive that trace amounts prolong it, which makes it useful only to show that little or no drug is present. Diluting the patient plasma in normal plasma and calibrating with dabigatran standards gives a linear dilute thrombin time that reports the level in ng/mL. Ecarin-based assays are an alternative.
- A: The PT and INR are insensitive to dabigatran.
- B: Dabigatran does not inhibit factor Xa, so anti-Xa assays are not used.
- C: The undiluted thrombin time is too sensitive to quantify the drug.
Question 4 · Easy
Apixaban plasma levels are measured with:
1. AEcarin chromogenic assay
2. BApixaban-calibrated anti-Xa
3. CCalibrated dilute thrombin time
4. DProthrombin time and INR
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Answer: B · Apixaban-calibrated anti-Xa
Apixaban, rivaroxaban and edoxaban inhibit factor Xa directly, so a chromogenic anti-Xa assay measures their effect. For a drug level in ng/mL, the assay must be calibrated with standards of that drug; a heparin-calibrated result can show that the drug is present but does not give its concentration. The PT and aPTT are too insensitive and variable to quantify apixaban. Thrombin-based tests are used for dabigatran instead.
- A: Ecarin-based assays measure direct thrombin inhibitors.
- C: A dilute thrombin time measures dabigatran, a thrombin inhibitor.
- D: The PT is insensitive to apixaban and is used for warfarin.
Question 5 · Hard
A patient on an unknown anticoagulant has these results: aPTT 84 s (reference 25-35 s), aPTT after heparinase treatment 82 s, thrombin time over 120 s (reference 15-20 s), heparin-calibrated anti-Xa below 0.10 IU/mL. Which drug is the patient most likely receiving?
1. AEnoxaparin
2. BArgatroban
3. CApixaban
4. DUnfractionated heparin
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Answer: B · Argatroban
A very long thrombin time with a prolonged aPTT points to a thrombin inhibitor. Heparinase did not shorten the aPTT and there is no anti-Xa activity, so heparin is excluded. Argatroban, a direct thrombin inhibitor, prolongs the thrombin time and aPTT without needing antithrombin and is not destroyed by heparinase. It is usually monitored with the aPTT, and it also raises the INR, which matters when switching to warfarin.
- A: Enoxaparin gives a clear anti-Xa result and is neutralized by heparinase.
- C: Apixaban shows anti-Xa activity and does not prolong the thrombin time.
- D: Heparin would be neutralized by heparinase and would show anti-Xa activity.
Question 6 · Medium
How is unfractionated heparin therapy usually monitored?
1. AWith the platelet count alone
2. BWith the D-dimer
3. CWith the PT and INR
4. DWith the aPTT or an anti-Xa assay
Answer and explanation in LearnFree account

Lab: anticoagulant monitoring (anti-Xa, direct thrombin inhibitors, heparin neutralization)

An ICU patient with sepsis needs more than 35,000 units/day of unfractionated heparin, yet the aPTT stays at 41 s (therapeutic 65-100 s). Anti-Xa is 0.52 IU/mL (therapeutic 0.3-0.7 IU/mL), factor VIII 280% and antithrombin 90%. The best interpretation is:

A bleeding patient takes dabigatran. Which test gives a reliable plasma drug level?

Apixaban plasma levels are measured with:

A patient on an unknown anticoagulant has these results: aPTT 84 s (reference 25-35 s), aPTT after heparinase treatment 82 s, thrombin time over 120 s (reference 15-20 s), heparin-calibrated anti-Xa below 0.10 IU/mL. Which drug is the patient most likely receiving?

How is unfractionated heparin therapy usually monitored?