Biomarker comparison

PT vs PTT (and INR): what each clotting test measures and when each is used

Short answer

Prothrombin time (PT) and partial thromboplastin time (PTT) each time how long plasma takes to clot after a different trigger, so between them they test most of the clotting cascade. PT covers the 'extrinsic' and common pathways (factors VII, X, V, II and fibrinogen), is prolonged by warfarin, vitamin K deficiency and liver disease, and is reported as the INR so results compare across laboratories. PTT covers the 'intrinsic' pathway (factors XII, XI, IX, VIII) plus the common pathway, is prolonged by heparin, hemophilia, von Willebrand disease and lupus anticoagulant, and is the test used to monitor heparin. Both long together points to the common pathway — liver disease, vitamin K deficiency, disseminated clotting or a high dose of anticoagulant.

Medically reviewed by Youmna DiStefano, MD · Last reviewed November 2026

Prothrombin Time vs Activated Partial Thromboplastin Time (aPTT) side by side

	Prothrombin Time	Activated Partial Thromboplastin Time (aPTT)
Pathway tested	Extrinsic and common: factors VII, X, V, II (prothrombin), fibrinogen	Intrinsic and common: factors XII, XI, IX, VIII, then X, V, II, fibrinogen
Normal range (adults)	age 18+: 11 – 13.5 seconds	age 18+: 25 – 35 seconds
Monitors which drug	Warfarin (as INR)	Unfractionated heparin; also used to screen for lupus anticoagulant
Reported as	Seconds, and as the INR (the PT ratio standardized to a reference reagent)	Seconds; sometimes a ratio to the lab's normal
Prolonged by	Warfarin, vitamin K deficiency, liver disease, factor VII deficiency, high-dose direct oral anticoagulants	Heparin, hemophilia A or B (factor VIII or IX), von Willebrand disease, factor XI or XII deficiency, lupus anticoagulant, high-dose dabigatran
Isolated prolongation suggests	Early warfarin effect or vitamin K deficiency (factor VII has the shortest half-life), early liver disease, inherited factor VII deficiency	Heparin in the sample or line, hemophilia or von Willebrand, lupus anticoagulant (which paradoxically raises clot risk), factor XII deficiency (harmless)
Before surgery	Checked when there is liver disease, anticoagulant use or a bleeding history	Checked with the PT in the same situations; a long PTT gets a mixing study to separate a deficiency from an inhibitor

Reference ranges are the reviewed catalog ranges for adults; your laboratory's printed range may differ slightly.

What each pathway means

Clotting is a chain of enzyme activations that ends in fibrin. In the laboratory the chain can be started two ways. Adding tissue factor (thromboplastin) starts the extrinsic pathway — that is the PT, and it depends on factor VII and the shared 'common' factors X, V, prothrombin and fibrinogen. Adding a surface activator and phospholipid starts the intrinsic pathway — that is the PTT, which depends on factors XII, XI, IX and VIII before joining the same common pathway. A factor shared by both (X, V, II, fibrinogen) prolongs both tests; a factor unique to one prolongs only that one.

What INR adds and when it is used

PT in seconds depends on the reagent a laboratory uses, so the same blood could read 14 seconds in one lab and 18 in another. The International Normalized Ratio corrects for that: each reagent carries a sensitivity index, and the INR is the patient's PT ratio raised to that index — so an INR of 2.5 means the same thing everywhere. INR exists for warfarin management (the usual target is 2.0–3.0; 2.5–3.5 for mechanical heart valves) and is also used in liver scoring systems (MELD, Child–Pugh). It is not a meaningful way to read the PT in someone who is not on warfarin and has normal liver function.

Reading the patterns

The combination tells you where to look:

Long PT, normal PTT — warfarin, early vitamin K deficiency (factor VII falls first), mild liver disease, or inherited factor VII deficiency.
Normal PT, long PTT — heparin (often contamination from a line), hemophilia A or B, von Willebrand disease, factor XI deficiency, lupus anticoagulant, or the harmless factor XII deficiency.
Both long — liver disease, significant vitamin K deficiency, disseminated intravascular coagulation, a high dose of any anticoagulant, dilution after massive transfusion, or a common-pathway factor deficiency.
Both normal with bleeding — platelet problems (count or function), von Willebrand disease with a normal PTT, factor XIII deficiency, or a vessel problem; the tests do not exclude a bleeding disorder.

Blood thinners and these tests

Warfarin: PT/INR, checked every one to four weeks once stable. Unfractionated heparin: PTT, or an anti-Xa level. Low-molecular-weight heparin (enoxaparin) and fondaparinux: neither test is reliable — anti-Xa levels if monitoring is needed. The direct oral anticoagulants (apixaban, rivaroxaban, edoxaban, dabigatran) prolong PT and PTT unpredictably and are not monitored with them; a normal PT or PTT does not mean the drug has worn off, and specific anti-Xa or dilute-thrombin-time assays are used when a level is needed before surgery or after bleeding.

How do you test Prothrombin Time and Activated Partial Thromboplastin Time (aPTT)?

Prices are cash-pay; every order adds one $6 lab processing fee. Physician authorization is included; no insurance or referral needed.

Test or panel	Type	Price	Fasting	Results
PT and PTT Test (Prothrombin & Partial Thromboplastin Time)	Test · 3 biomarkers	$26.99	No	1–3 business days

Common questions

What is the difference between PT, PTT and INR?

PT and PTT are clotting-time tests that start the cascade from different ends — PT the extrinsic pathway (factor VII and the common factors), PTT the intrinsic pathway (factors XII, XI, IX, VIII and the common factors). INR is not a separate test; it is the PT expressed as a standardized ratio so warfarin results mean the same thing in every laboratory.

Which test monitors warfarin and which monitors heparin?

Warfarin is monitored with the PT reported as INR, because warfarin lowers the vitamin K–dependent factors and factor VII (in the PT pathway) falls first. Unfractionated heparin is monitored with the PTT (or an anti-Xa level) because heparin acts through antithrombin on the intrinsic and common pathways. The newer oral anticoagulants are not monitored with either.

What does a high PTT with a normal PT mean?

The problem is in the intrinsic pathway or the sample. The commonest cause is heparin contamination from an IV line; the inherited causes are hemophilia A (factor VIII), hemophilia B (factor IX), von Willebrand disease and factor XI deficiency; and a lupus anticoagulant prolongs the PTT while actually increasing clot risk. A mixing study — patient plasma mixed with normal plasma — separates a missing factor (corrects) from an inhibitor (does not).

Do blood thinners like Eliquis or Xarelto change PT or PTT?

Unpredictably, which is why they are not monitored with these tests. Rivaroxaban tends to prolong the PT more than the PTT; apixaban may leave both near normal at therapeutic levels; dabigatran prolongs the PTT. A normal PT and PTT does not prove the drug has cleared — before surgery or after bleeding, a drug-specific anti-Xa or dilute thrombin time is used.

Sources

Educational content, not a diagnosis. Lab results are one input; a clinician interprets them with your history and exam.