Step-by-Step Walkthrough
Compare the classic screening-test map with the cell-based model, simulate PT/aPTT patterns, and apply the reasoning to clinical cases.
Click a factor to see the activation chain leading to it.
How coagulation truly happens in the body
Unlike the classic separate-pathway cascade, in the body everything happens on cell surfaces in 3 continuous phases. Step through the phases and watch factors migrate between the injured cell and the platelet.
Injury exposes Tissue Factor (TF) on the cell. TF binds FVII → the TF/VIIa complex activates small amounts of FX and generates the first thrombin ("spark").
Predict the PT and aPTT pattern
Change relative factor activity and observe which screening tests would tend to prolong. The goal is to train relationships, not manufacture results in seconds.
Classic aPTT arm
XII, XI, IX, and VIII organize contact-pathway interpretation in the in-vitro assay.
Classic PT arm
Factor VII is the main factor specific to this arm of the test.
Common assay pathway
X, V, II, and I can affect both PT and aPTT.
Fibrin stabilization
Factor XIII acts after initial fibrin formation; PT and aPTT usually remain normal.
Normal cascade versus pathological states
See how the cascade changes with factor deficiencies, systemic consumption, or anticoagulant use.
| Situation | Cascade behavior | Expected laboratory pattern | Clinical clue |
|---|---|---|---|
| Normal hemostasis | Coordinated activation of the intrinsic, extrinsic, and common pathways, with adequate thrombin generation and stable fibrin. | Normal PT/INR, normal PTT, adequate fibrinogen, and preserved platelets. | Bleeding stops after a hemostatic response proportional to the injury. |
| Factor VIII or IX deficiency | The intrinsic pathway loses efficiency, reducing Factor X activation through the tenase complex. | Normal PT with prolonged PTT; platelets are usually normal. | Hemarthrosis, deep hematomas, and post-trauma bleeding suggest Hemophilia A or B. |
| Factor VII deficiency or early warfarin effect | The extrinsic pathway is impaired first because Factor VII has a short half-life. | Prolonged PT/INR before major PTT alteration. | Bruising, mucosal bleeding, or context of low vitamin K/anticoagulation. |
| Common pathway deficiency | Convergence at X, V, II, or I is impaired, reducing thrombin or fibrin formation. | Prolonged PT and PTT, possibly low fibrinogen if Factor I is reduced. | Broader bleeding pattern, especially with liver disease or severe deficiency. |
| Heparin use | Mainly inhibits thrombin and Factor Xa through antithrombin, reducing coagulation propagation. | PTT is usually prolonged; high doses may affect other tests. | History of hospital anticoagulation or thrombosis prophylaxis/treatment. |
| DOAC use | Direct blockade of Factor Xa or thrombin interrupts central common-pathway steps. | PT/PTT may vary by drug and reagent; global tests can be unpredictable. | Use of rivaroxaban, apixaban, edoxaban, or dabigatran. |
| DIC / systemic consumption | Diffuse activation consumes platelets and factors while fibrinolysis increases. | Prolonged PT and PTT, low platelets, low fibrinogen, and elevated D-dimer. | Sepsis, trauma, shock, puncture-site bleeding, and signs of thrombosis/hemorrhage. |
| Factor XIII deficiency | Fibrin forms but does not receive enough cross-links to stabilize the clot. | PT and PTT are usually normal. | Delayed bleeding after surgery, trauma, or dental extraction. |
These patterns are educational and support initial reasoning; real results must be interpreted with clinical context, medications, and sample quality.
The case now has a sequence—not just an answer
Enter a focused experience with six progressive cases. In each case you formulate a hypothesis, choose an initial panel, interpret the pattern, and decide the next test.
Open Clinical BenchProgress and the best score for each case are saved in this browser.
- 01Phenotype and hypothesis
- 02Test selection
- 03Integrated interpretation
- 04Targeted test and synthesis