INR / PT / PTT

The coagulation cascade has two initiation pathways — the extrinsic pathway (triggered by tissue factor) and the intrinsic pathway (triggered by contact activation) — that converge on a common pathway leading to fibrin clot formation. PT/INR and PTT measure different parts of this cascade.

The PT (prothrombin time) measures the extrinsic and common pathways: factors VII, X, V, II (prothrombin), and fibrinogen. The INR is a standardized expression of the PT ratio, developed to reduce inter-laboratory variability in warfarin monitoring. An isolated PT/INR elevation with a normal PTT points toward factor VII deficiency (the only factor unique to the extrinsic pathway) or warfarin effect (warfarin inhibits vitamin K-dependent factors II, VII, IX, and X — but factor VII has the shortest half-life and falls first).

The PTT (partial thromboplastin time, also called aPTT) measures the intrinsic and common pathways: factors XII, XI, IX, VIII, X, V, II, and fibrinogen. An isolated PTT elevation with a normal PT points toward deficiency or inhibition of factors VIII, IX, XI, or XII — or heparin effect (unfractionated heparin primarily prolongs the PTT by potentiating antithrombin inhibition of thrombin and factor Xa).

Both PT and PTT elevated together suggests a defect in the common pathway (factors X, V, II, or fibrinogen), severe liver disease (which reduces synthesis of most coagulation factors), DIC (which consumes multiple factors), or supratherapeutic anticoagulation affecting both pathways.

When the PTT is prolonged, a mixing study (1:1 mix of patient plasma with normal plasma) distinguishes a factor deficiency from an inhibitor. This distinction is critical before any invasive procedure or treatment.

If the PTT corrects with mixing (normalizes or comes close to normal), the normal plasma has supplied the missing factor — consistent with a factor deficiency. The most common causes are hemophilia A (factor VIII deficiency), hemophilia B (factor IX deficiency), factor XI deficiency, and factor XII deficiency (which prolongs the PTT but does not cause bleeding).

If the PTT does not correct with mixing (remains prolonged despite the addition of normal plasma), an inhibitor is present — something in the patient's plasma is inactivating the coagulation factors in the normal plasma. The two most important inhibitors are the lupus anticoagulant and factor-specific inhibitors.

The lupus anticoagulant is an antiphospholipid antibody that prolongs the PTT in vitro by interfering with phospholipid-dependent coagulation reactions. Paradoxically, it causes thrombosis in vivo (not bleeding) — it is associated with antiphospholipid syndrome, which causes arterial and venous thrombosis and pregnancy loss. A prolonged PTT that does not correct with mixing in a patient with unexplained thrombosis or recurrent pregnancy loss should prompt antiphospholipid antibody testing.

Factor-specific inhibitors (most commonly anti-factor VIII inhibitors, which can develop in hemophilia A patients after factor replacement or spontaneously in acquired hemophilia) cause bleeding. They are distinguished from lupus anticoagulant by specific factor assays and confirmatory testing.

The INR was developed to standardize PT reporting across different laboratory reagents for warfarin monitoring. Different thromboplastin reagents have different sensitivities to factor deficiencies, producing different PT values for the same degree of anticoagulation. The INR corrects for this by using an international sensitivity index (ISI) specific to each reagent.

INR targets for warfarin therapy depend on the indication. For most indications (atrial fibrillation, DVT/PE, mechanical aortic valve), the target is 2.0–3.0. For mechanical mitral valves and high-risk mechanical valves, the target is 2.5–3.5. For antiphospholipid syndrome with recurrent thrombosis, higher targets (3.0–4.0) are sometimes used.

Factors that affect INR stability include dietary vitamin K intake (consistent intake is more important than restriction), medications (many drugs interact with warfarin through CYP2C9 and other pathways), illness (acute illness, particularly with reduced oral intake, can cause INR fluctuation), and adherence.

The INR is not a reliable measure of bleeding risk in patients with liver disease. Liver disease reduces synthesis of both procoagulant factors (II, V, VII, IX, X) and anticoagulant proteins (protein C, protein S, antithrombin). The INR reflects only the procoagulant side — it does not account for the concurrent reduction in anticoagulant proteins, which may partially offset the procoagulant effect. Viscoelastic testing (thromboelastography, ROTEM) provides a more comprehensive assessment of hemostasis in liver disease.

Unfractionated heparin (UFH) is monitored with the PTT. The therapeutic range for most indications is a PTT of 60–100 seconds (approximately 1.5–2.5 times the upper limit of normal), though institution-specific nomograms vary. The PTT is checked 4–6 hours after initiation or dose change and adjusted accordingly.

Low-molecular-weight heparin (LMWH — enoxaparin, dalteparin) does not reliably prolong the PTT and is not monitored with PTT in most patients. Anti-Xa levels are used when monitoring is needed: in patients with renal impairment (reduced LMWH clearance), obesity, pregnancy, or when therapeutic dosing must be confirmed. The therapeutic anti-Xa range for twice-daily enoxaparin is typically 0.6–1.0 IU/mL (measured 4 hours after the dose).

Heparin resistance — a higher-than-expected heparin dose requirement to achieve therapeutic PTT — can occur with antithrombin deficiency (congenital or acquired, as in DIC or nephrotic syndrome), elevated factor VIII levels (acute-phase reactant), or elevated heparin-binding proteins. In true heparin resistance, anti-Xa monitoring is more reliable than PTT.

Direct oral anticoagulants (DOACs) — apixaban, rivaroxaban, edoxaban (factor Xa inhibitors) and dabigatran (direct thrombin inhibitor) — do not reliably prolong the PT or PTT at therapeutic doses. A normal PT and PTT does not exclude therapeutic or supratherapeutic DOAC levels. This is one of the most clinically important limitations of standard coagulation testing.

For factor Xa inhibitors (apixaban, rivaroxaban, edoxaban), drug-specific anti-Xa assays calibrated for the specific agent provide quantitative drug levels. These are not the same as the anti-Xa assay used for LMWH monitoring — the calibration is different. Standard anti-Xa assays may not accurately reflect factor Xa inhibitor levels.

For dabigatran, the dilute thrombin time (dTT) or ecarin clotting time (ECT) provides quantitative drug levels. The standard thrombin time is exquisitely sensitive to dabigatran and is qualitatively useful (a normal thrombin time essentially excludes clinically relevant dabigatran levels), but it is not quantitative.

In clinical practice, the most important implication is: when a patient on a DOAC presents with bleeding or requires urgent reversal, do not rely on PT and PTT to determine whether the drug is present or at what level. Know the last dose time, the renal function (which affects DOAC clearance), and use drug-specific assays when available and when the result will change management.