What this test measures
The Prothrombin Time (PT) is the time (in seconds) for plasma to clot after adding tissue factor (thromboplastin) and calcium. It tests the extrinsic pathway (factor VII) and the common pathway (factors X, V, II, fibrinogen). Reference range is typically 11–13.5 seconds (lab-specific). The result is also reported as INR (International Normalised Ratio) — a calculation that standardises results across different reagents and laboratories using the ISI (International Sensitivity Index) of the thromboplastin used.
INR is the universally accepted unit for monitoring warfarin therapy. Therapeutic INR for most indications (atrial fibrillation, DVT, PE) is 2.0–3.0; for mechanical heart valves, 2.5–3.5. PT/INR is also used to assess liver synthetic function (the liver makes all the clotting factors), vitamin K status, and the bleeding risk before invasive procedures.
Why it matters
PT/INR is the most-ordered coagulation test in Indian practice. It is essential for: (1) Warfarin monitoring — every adjustment, every 4–8 weeks once stable; (2) Pre-operative assessment — INR >1.5 may delay non-emergent surgery; (3) Liver disease — INR is part of MELD score (chronic liver disease severity) and Child-Pugh classification; (4) Vitamin K deficiency in newborns, malabsorption, or after broad-spectrum antibiotics; (5) DIC monitoring; (6) Workup of unexplained bleeding.
DOACs (dabigatran, rivaroxaban, apixaban, edoxaban) have variable effects on PT/INR — they may mildly prolong it but PT/INR is NOT reliable for DOAC monitoring. Specific drug-level assays are needed. For Indian patients on warfarin (still cost-effective for many indications), INR target ranges and adherence to monitoring are the single most important factors in preventing bleeding or clotting complications.
How to prepare
No fasting required. Disclose all anticoagulants (warfarin, DOACs, heparin, LMWH), liver disease, recent antibiotic use, vitamin K, and dietary green leafy vegetables (high vitamin K intake can lower INR in warfarin patients). The blood sample must be in a citrate tube and processed within 4 hours.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| Prothrombin Time (seconds)[1] | 11–13.5 seconds (lab-specific) | Shortened PT is generally not clinically significant. | Prolonged PT — warfarin therapy, vitamin K deficiency, liver disease (impaired synthesis), DIC, factor deficiency (II, V, VII, X), DOACs (variable). A prolonged PT with normal APTT typically points to factor VII deficiency or early warfarin effect. |
| INR[1] | 0.8–1.2 (untreated) | Sub-therapeutic INR in warfarin patient (<2.0) — risk of thrombosis. Causes: non-adherence, high vitamin K intake (green leafy vegetables, multivitamins), drug interactions (rifampicin, phenytoin, carbamazepine). | INR > 4.0 in warfarin patient — significantly increased bleeding risk. Causes: dose change, drug interactions (amiodarone, antibiotics — metronidazole, fluconazole, ciprofloxacin; NSAIDs, fish oil), illness, change in diet, liver dysfunction. INR > 5–8 may need vitamin K; INR > 8 with any bleeding needs urgent management. |
INR target ranges by indication
| Indication | INR target |
|---|---|
| Atrial fibrillation | 2.0 – 3.0 |
| DVT / PE treatment | 2.0 – 3.0 |
| Mechanical aortic valve | 2.5 – 3.5 |
| Mechanical mitral valve | 2.5 – 3.5 (or 3.0–4.0 depending on type) |
| Recurrent VTE despite anticoagulation | 2.5 – 3.5 |
| Antiphospholipid syndrome | 2.5 – 3.5 |
| Above target INR > 5.0 | Hold warfarin; consider oral vitamin K |
| Above target INR > 8.0 or any bleeding | Urgent reversal (vitamin K + PCC or FFP) |
Frequently asked questions
How often should I check my INR on warfarin?
After starting warfarin: every 2–3 days for the first 2 weeks, then weekly until stable, then every 4–8 weeks. Any dose change or new medication needs INR check in 5–7 days.
Do I need to fast?
No.
My INR is 5.5 — what should I do?
INR 5.5 is above the usual 2.0–3.0 target. If you have no bleeding, your doctor will hold the next 1–2 warfarin doses, recheck in 1–2 days, and resume at a lower dose. If you have any bleeding, oral vitamin K may be added.
Why does my diet affect INR?
Warfarin works by blocking vitamin K. High vitamin K intake (green leafy vegetables, broccoli, methi, palak, multivitamins) reduces warfarin's effect and lowers INR. The goal is CONSISTENT vitamin K intake, not avoidance — eat similar amounts each week.
Can DOACs (Eliquis, Xarelto, Pradaxa) be monitored with INR?
No. DOACs may mildly prolong INR but the test does not reliably reflect their effect. Specific drug-level assays (anti-Xa for apixaban/rivaroxaban; dilute thrombin time for dabigatran) are needed when monitoring is required (e.g. before urgent surgery).
What is "PT activity" on the report?
A percentage of normal — calculated from PT and reference standards. Reciprocal of INR. Many Indian labs report all three (PT seconds, INR, activity %).
Can liver disease prolong PT?
Yes — PT/INR is one of the most sensitive markers of impaired liver synthetic function. It is included in MELD score for chronic liver disease severity.
How long does the report take?
Most NABL labs report PT/INR within 2–4 hours.
Related Coagulation tests
Tests commonly ordered alongside PROTHROMBIN TIME, or that help interpret an unexpected result.
Sources & references
- NCBI StatPearls — Prothrombin Time · accessed 2026-05-30T00:00:00.000Z
- BSH — Oral Anticoagulation Guidelines · accessed 2026-05-30T00:00:00.000Z
- ISTH — INR Standardisation · accessed 2026-05-30T00:00:00.000Z
- NIH MedlinePlus — Prothrombin Time Test · accessed 2026-05-30T00:00:00.000Z
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