Physiology · Blood, hemostasis and basic immunity
Coagulation tests: PT/INR, aPTT and platelets !
- Measurement
- Exam priority: high
A few blood tests show which part of clotting is failing: the platelets, the extrinsic pathway (PT/INR) or the intrinsic one (aPTT).
Updated 2026-11-01
Also called: Clotting tests, Coagulogram, INR
See it in 3D: Liver
What it is
Basic hemostasis testing combines the platelet count, the prothrombin time (PT, reported also as INR), the activated partial thromboplastin time (aPTT) and fibrinogen. Each test starts clotting in a tube at a different entry point, so the pattern of abnormal results points to where the defect lies. The INR was created so that warfarin control is comparable between laboratories.
Equations
- ISI
- international sensitivity index of the thromboplastin reagent
The ISI corrects for reagent differences so results compare between laboratories.
Typical reference values (they depend on the laboratory reagent)
| Parameter | Value | Note |
|---|---|---|
| Prothrombin time (PT) | ≈11–13.5 s | |
| INR, not anticoagulated | ≈0.8–1.1 | |
| aPTT | ≈25–35 s | |
| Platelet count | ≈150 000–450 000/µL | |
| Fibrinogen | ≈200–400 mg/dL | |
| D-dimer | <0.5 µg/mL FEU | cut-off varies by method and age |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Reading the pattern
| PT/INR | aPTT | Platelets | Think of |
|---|---|---|---|
| Long | Normal | Normal | Vitamin K antagonist, early vitamin K deficiency, early liver disease, factor VII deficiency |
| Normal | Long | Normal | Unfractionated heparin, hemophilia A or B, some von Willebrand disease, factor XII, lupus anticoagulant |
| Long | Long | Normal or low | Advanced liver disease, severe vitamin K deficiency, common-pathway defect, DIC (low platelets and fibrinogen) |
| Normal | Normal | Low | Immune thrombocytopenia, dengue, marrow failure, hypersplenism |
| Normal | Normal | Normal | Mild von Willebrand disease, platelet function defect, factor XIII, vascular fragility |
Clinical notes
- Pre-analytical errors are common: an underfilled citrate tube has too much citrate and gives falsely long times; heparin from a line contaminates samples.
- In cirrhosis a long INR reflects low factor synthesis, but natural anticoagulants are also low, so the INR overstates bleeding risk.
- Dengue: falling platelets with a rising hematocrit signal plasma leakage, a warning sign of severe disease in Colombia.
Test yourself
Think of the answer, then tap the question to see it.
A patient has a long aPTT that does not correct after mixing with normal plasma. What is the likely explanation?
An inhibitor, such as a lupus anticoagulant or an acquired factor VIII antibody, neutralizes factors in the normal plasma too, so the time stays long. A deficiency would correct.
Why does an INR exist if the PT is already reported in seconds?
Thromboplastin reagents differ in sensitivity, so seconds vary between labs. The INR corrects with the reagent's ISI, making vitamin K antagonist control comparable everywhere.
Which pattern suggests disseminated intravascular coagulation?
Long PT and aPTT, low platelets, low fibrinogen and a high D-dimer: factors and platelets are consumed while fibrin is formed and broken down at the same time.
Why can a patient with von Willebrand disease have entirely normal screening tests?
The platelet count is normal and factor VIII may be only mildly low, so the aPTT can stay normal; vWF activity must be measured specifically.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- The coagulation cascade
- Primary hemostasis and platelets
- Thrombosis and bleeding: Virchow's triad and hemostatic defects
- Fibrinolysis and natural anticoagulants
Related conditions
In Biochemistry
In Pharmacology
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: Prothrombin Time Test and INR (PT/INR) (MedlinePlus (U.S. National Library of Medicine), 2024); Partial Thromboplastin Time (PTT) Test (MedlinePlus (U.S. National Library of Medicine), 2024); Platelet Tests (MedlinePlus (U.S. National Library of Medicine), 2024); 18.5 Hemostasis (OpenStax (Rice University), 2022); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 37). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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