Haematology• Updated 21 October 2026

Blood Clotting Tests (PT/INR and APTT)

Blood tests that check how well your blood clots, including monitoring warfarin.

Results: Usually within a day Specimen: Blood

Quick Summary

Primary Purpose

To check clotting before surgery, investigate bleeding and monitor warfarin.

What It Measures

Prothrombin time (PT), INR and activated partial thromboplastin time (APTT).

Preparation

No special preparation; tell staff about blood thinners.

Overview

INR is used to adjust warfarin doses.

APTT checks other clotting pathways and can show bleeding disorders such as haemophilia.

What It Measures

PT/INR

Clotting time, standardised as INR.

APTT

Another clotting pathway.

How It's Performed

  1. 01

    Preparation

    Usually no special preparation. Tell the staff about medicines you take.

  2. 02

    Blood sample

    A small amount of blood is taken from a vein in your arm with a thin needle.

  3. 03

    Afterwards

    Press on the spot for a minute. A small bruise is common; you can go back to normal activities.

Reference Ranges

Note: Normal ranges vary by laboratory. Consult your report for the specific reference used.

ParameterTypical Range (Adult)Units
INR (not on warfarin)0.8–1.2ratio
INR (on warfarin, usual target)2.0–3.0ratio
APTT25–35seconds

Interpreting Results & Next Steps

Discuss with Doctor

Normal ranges differ between laboratories, ages and sexes; always use the range printed on your report and ask your doctor what your results mean.

If values are Low:

A low INR on warfarin means less protection from clots.

If values are High:

A high INR or long APTT means blood takes longer to clot and bleeding risk is higher.

Sources and further reading

Last updated 21 October 2026. For more detail, see these trusted health organisations.

Related Topics

WarfarinHaemophilia