Medservatory
USMLE Step 1Haematology and oncologyEP 08 of 14

Coagulation cascade: PT vs aPTT, pathway by pathway

PT and aPTT each test part of the coagulation cascade. Learn which factors sit behind each test, so a long result points to a pathway and not just to a bleeding risk.

  • 5:21 video
  • 3 min read
  • 9 sources
  • Published 15 Nov 2026
Video lesson · 5:21 · Watch on YouTubeSeries: Hematology: Anemia, Coagulation & Oncology Basics · @medservatory
Chapters (9)
By the end of this lesson

Relate the extrinsic, intrinsic and common pathways to PT and aPTT and predict which test is prolonged by each factor deficiency.

The short version

  1. PT tests the extrinsic and common pathways: factors VII, X, V, II and fibrinogen.
  2. aPTT tests the intrinsic and common pathways: factors XII, XI, IX and VIII, then X, V, II and fibrinogen.
  3. A long PT with a normal aPTT points to factor VII; a long aPTT with a normal PT points to the intrinsic pathway, unless heparin or an inhibitor is the cause; both long means the common pathway or several factors.
  4. Warfarin is followed with the PT (reported as the INR), unfractionated heparin with the aPTT, and factor XIII shows on neither test.

Same long aPTT, opposite stories

Two patients have the same long aPTT before surgery. One has bled into his knees since childhood. The other has never bled at all.

So what does a long aPTT really tell us? This lesson on the coagulation cascade maps the extrinsic, intrinsic and common pathways onto PT and aPTT, so you can predict which test each factor deficiency prolongs.[1][2]

How PT and aPTT are run

Both tests run on citrated plasma. Calcium is added back, and the clock stops when a fibrin clot forms. The difference between the two is the trigger.[2][9]

  • PT adds tissue factor. It tests the extrinsic and common pathways: factors VII, X, V, II and fibrinogen.[2][9]
  • aPTT adds phospholipid and a contact activator such as kaolin or silica, but no tissue factor. It tests the intrinsic and common pathways: XII, XI, IX and VIII, then X, V, II and fibrinogen.[2][3][9]

Factor VII is the only factor that the PT tests and the aPTT does not, so an isolated long PT points to factor VII.[9][2] PT reagents are also more sensitive to factor VII than to deficiencies in the common pathway.[2]

The extrinsic, intrinsic and common pathways

The coagulation cascade and the test that covers each part
PathwayWhat happensTest
ExtrinsicInjury exposes tissue factor, which binds factor VIIa and calcium to activate factor XPT
IntrinsicFactor XII, with kininogen and prekallikrein, activates XI, which activates IX. IXa and its cofactor VIII form the tenase complex that activates XaPTT
CommonXa with factor V turns prothrombin into thrombin. Thrombin cleaves fibrinogen to fibrin and activates factor XIII, which cross-links the clotPT and aPTT (XIII on neither)

Both the extrinsic and the intrinsic pathway end by activating factor X, which is why the common pathway is shared by the two tests.[1][4]

Vitamin K, warfarin and heparin: which test follows which drug

Four clotting factors need vitamin K: II, VII, IX and X. Warfarin blocks the recycling of vitamin K.[5][6]

Factor VII has the shortest half-life, so the PT climbs first. The PT is sensitive to factors VII, X and II, so warfarin is followed with the PT, reported as the INR. Unfractionated heparin is followed with the aPTT.[2][5][7][8]

Predicting the pattern: isolated long PT, isolated long aPTT, both long

Worked example: a long aPTT with a normal PT
  1. PT 13 s, control 12 s: normal
  2. aPTT 78 s, control 33 s: long
  3. Normal PT with a long aPTT points to the intrinsic pathway: VIII, IX, XI or XII, unless heparin or an inhibitor is the cause
Worked example: a long PT with a normal aPTT
  1. PT 31 s, control 12 s: long
  2. aPTT: normal
  3. Extrinsic pathway only, so factor VII
Reading PT and aPTT together
PTaPTTPoints to
LongNormalExtrinsic pathway: factor VII[2]
NormalLongIntrinsic pathway (VIII, IX, XI, XII), unless heparin or an inhibitor is the cause[2][3]
LongLongA common pathway factor, or several factors at once, as in vitamin K deficiency or liver disease[2][5]

Ranges differ between laboratories, so read each result against its own control. For that reason the worked examples above give a control beside each timed result.[3][9]

Then ask about bleeding. Into joints: factor VIII or IX, haemophilia. No bleeding at all: factor XII or the other contact factors, which prolong the test in the tube but are not needed in the body.[1][2][3]

Why haemophilia bleeds when the extrinsic pathway is intact

The intrinsic and extrinsic split describes the two tests in the tube. In the body, tissue factor starts the clot, but factors VIII and IX amplify it. That is why haemophilia bleeds even though the extrinsic pathway is intact.[1][2]

Practice question

Original · USMLE style
Your turnQB-MS-S1-HEME-004-01

A 27-year-old man has had easy bruising and prolonged bleeding after tooth extractions. His sister has similar symptoms. He eats a normal diet and takes no medications. Laboratory studies show:

Results
Platelet count310,000/uL
Prothrombin time26 seconds (control 12 seconds)
Activated partial thromboplastin time34 seconds (control 33 seconds)
Fibrinogennormal
Which of the following clotting factors is most likely deficient in this patient?
Pick one option. Your answers stay on this device.

Common questions

What is the difference between PT and aPTT?

Both run on citrated plasma with calcium added back, and differ in the trigger. PT adds tissue factor and tests the extrinsic and common pathways; aPTT adds phospholipid and a contact activator and tests the intrinsic and common pathways.

Which factors does PT measure?

Factors VII, X, V, II and fibrinogen: the extrinsic and common pathways.

Why is the aPTT long but the patient does not bleed?

Deficiency of factor XII, kininogen or prekallikrein prolongs the aPTT in the tube, but these factors are not needed in the body, so there is no bleeding.

Which test monitors warfarin and which monitors heparin?

Warfarin is followed with the PT, reported as the INR. Unfractionated heparin is followed with the aPTT.

Which factor is not measured by either PT or aPTT?

Factor XIII. Its deficiency leaves the PT and aPTT normal.

Transcript

Read the full narration, by chapter650 words

0:00Same long aPTT, opposite stories

Two patients have the same long APTT before surgery. One has bled into his knees since childhood. The other has never bled at all. Same test, opposite stories. What does a long APTT really tell us?

0:16Today: map the pathways onto PT and aPTT

Today: map the extrinsic, intrinsic and common pathways onto PT and APTT, and predict which test each factor deficiency prolongs.

0:26How PT and aPTT are run: citrated plasma, calcium, the trigger

Both tests run on citrated plasma. Calcium is added back, and the clock stops when a fibrin clot forms. The difference is the trigger. PT adds tissue factor, so it tests the extrinsic and common pathways: factors seven, 10, five, two, and fibrinogen. APTT adds phospholipid and a contact activator such as kaolin or silica, but no tissue factor, so it tests the intrinsic and common pathways: 12, 11, nine and eight, then 10, five, two and fibrinogen.

1:03The cascade: extrinsic, intrinsic and common pathways

The extrinsic pathway starts when injury exposes tissue factor, which binds factor seven A and calcium to activate factor 10. The intrinsic pathway runs from factor 12, with kininogen and prekallikrein, to 11, to nine. Nine A and its cofactor, factor eight, form the tenase complex that activates 10. In the common pathway, 10 A with factor five turns prothrombin into thrombin. Thrombin cleaves fibrinogen to fibrin and activates factor 13, which cross-links the clot. Factor 13 sits outside both tests, so its deficiency leaves PT and APTT normal.

1:47Vitamin K, warfarin (PT/INR) and heparin (aPTT)

Four factors need vitamin K: two, seven, nine and 10. Warfarin blocks the recycling of vitamin K. Factor seven has the shortest half-life, so the PT climbs first. The PT is sensitive to factors seven, 10 and two, so warfarin is followed with the PT, reported as the INR. Unfractionated heparin is followed with the APTT.

2:15Predict: isolated long PT, isolated long aPTT, both long

Now predict. PT 13 seconds, control 12: normal. APTT 78, control 33: long. With a normal PT, that points to the intrinsic pathway: eight, nine, 11 or 12, unless heparin or an inhibitor is the cause. Then bleeding. Into joints: factor eight or nine, haemophilia. No bleeding at all: factor 12 or the other contact factors, which prolong the tube test but are not needed in the body. PT 31 against a control of 12, with a normal APTT: extrinsic only, so factor seven. Both long: a common pathway factor, or several factors at once, as in vitamin K deficiency or liver disease. And why does haemophilia bleed if the extrinsic pathway is intact? In the body, tissue factor starts the clot, but factors eight and nine amplify it.

3:19Practice question

Your turn. A 27-year-old man has had easy bruising and prolonged bleeding after tooth extractions. His sister has similar symptoms. He eats a normal diet and takes no medicines. The platelet count is 310,000 per microlitre, and fibrinogen is normal. PT is 26 seconds, with a control of 12. APTT is 34, with a control of 33. Which of the following factors is most likely deficient? A, factor seven. B, factor eight. C, factor nine. D, factor 11. E, factor 12. Pause here and decide.

4:06Answer and explanation

B is out: factor eight is intrinsic, so the APTT would be long and the PT normal. C is out: factor nine, the same pattern. D is out: factor 11, the same pattern. E is out: factor 12, the same pattern, and it would not cause bleeding. So the answer is A. A long PT with a normal APTT, a normal count and normal fibrinogen means the extrinsic pathway: factor seven.

4:38Recap: three things to keep

Three things to keep. One: PT covers seven and the common pathway; APTT covers 12, 11, nine, eight and the common pathway. Two: a long PT with a normal APTT points to factor seven; a long APTT with a normal PT points to the intrinsic pathway, unless heparin or an inhibitor is the cause; both long means the common pathway or several factors. Three: warfarin is followed with the PT, heparin with the APTT, and factor 13 shows on neither. Next, von Willebrand disease versus haemophilia.

Sources

Numbers in the text, e.g. [1], point here
  1. Overview of the coagulation system. Indian J Anaesth 2014. europepmc.org/article/PMC/PMC4260295
  2. The bleeding child, Part II. Eur J Pediatr 2012. europepmc.org/article/PMC/PMC3258398
  3. Santoro et al. Isolated Prolongation of aPTT. Medicina 2023. europepmc.org/article/PMC/PMC10305031
  4. Normal Haemostasis, Inherited Bleeding Disorders and Surgery. Medicina 2025. europepmc.org/article/PMC/PMC12194927
  5. Vitamin K Deficiency: Diagnosis and Management. Ann Lab Med 2025. europepmc.org/article/PMC/PMC12187499
  6. Anticoagulation reversal. Br J Clin Pharmacol 2025. europepmc.org/article/PMC/PMC11862798
  7. Coagulation Tests and Reversal Agents in Patients Treated with Oral Anticoagulants. J Clin Med 2024. europepmc.org/article/PMC/PMC11084691
  8. Anticoagulants, Their Laboratory Monitoring, and Reversal. 2026. europepmc.org/article/PMC/PMC13600188
  9. Prothrombin time; Partial thromboplastin time. Wikipedia (secondary). en.wikipedia.org/wiki/Prothrombin_time · en.wikipedia.org/wiki/Partial_thromboplastin_time

Reference ranges differ between laboratories; the question gives its controls. The cascade and the in-body sketch on screen are schematic.

The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.

Image credits. Illustrations: Servier Medical Art (smart.servier.com), CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/), adapted (recoloured, outlined, cropped): knee, centrifuge, clock, coagulation (wound and clot). NIAID NIH BioArt Source (bioart.niaid.nih.gov), public domain: platelet aggregation (item 928), fibrinogen (item 841). The cascade, tubes, table, chart, lab slips, cards and all handwriting are drawn in code (original). No AI-generated teaching images.

Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-HEME-004.

For exam study only, not medical advice. Read the medical disclaimer.

In this series

Playlist on YouTube
← Previous · EP 07Primary haemostasis: platelet adhesion, activation and aggregation
  1. EP 01Microcytic anaemia: four causes, four iron fingerprints5:37
  2. EP 02Hepcidin: why iron falls in inflammation5:34
  3. EP 03B12 vs folate deficiency: MMA, homocysteine and the nerves5:23
  4. EP 04Intravascular vs extravascular haemolysis: site and labs5:27
  5. EP 05G6PD deficiency vs hereditary spherocytosis, by mechanism5:36
  6. EP 06Sickle cell disease pathophysiology: HbS to vaso-occlusion5:26
  7. EP 07Primary haemostasis: platelet adhesion, activation and aggregation5:53
  8. EP 08Coagulation cascade: PT vs aPTT, pathway by pathway5:21
  9. EP 09Von Willebrand disease vs haemophilia: the labs that separate them5:32
  10. EP 10Anticoagulants: target, monitoring test and reversal agent5:34
  11. EP 11Heparin-induced thrombocytopenia: why a low count clots5:29
  12. EP 12DIC vs TTP vs HUS: mechanism and the PT/PTT5:33
  13. EP 13AML vs ALL vs CML vs CLL: leukemia classification made simple5:44
  14. EP 14Oncogenes vs tumour suppressor genes: one hit or two5:17