APTT ratio

For people on a heparin drip the doctor sets a target ratio that is deliberately above this range (often around 1.5 to 2.5); use the range your lab printed.

The APTT ratio is the APTT divided by the laboratory's own normal clotting time, giving a number around 1.0 for the general population. It is the form of the APTT that is used to monitor unfractionated heparin, in the same way that the INR is the form of the prothrombin time used to monitor warfarin.

Doctors use it mainly to monitor heparin given by drip in hospital, where the aim is to keep the ratio within a set window. It is also printed alongside the APTT in seconds on many routine clotting screens.

• Unfractionated heparin, as intended
• Inherited shortages of clotting factors
• Lupus anticoagulant
• Liver disease
• Some direct oral anticoagulant medicines
• A severe illness that uses up clotting factors

• A high factor VIII level, which rises with inflammation, pregnancy and oestrogen
• Recent surgery or injury, slightly
• A sample that has begun to clot in the tube

• In people on a heparin drip the ratio is meant to sit above the general population range
• An under-filled citrate tube or a sample from a heparin-flushed line reads high
• Different reagents behave differently, so the ratio is not comparable between laboratories

No fasting needed. On a heparin drip it is checked every few hours; otherwise a single value is usual.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.