Any consensus/guideline? 

INR should be measured after 3 days of starting warfarin therapy. After that, measurement is variable between institutions. It an be done every week first and if INR is in the therapeutic range continue every 2 weeks and then every 4 weeks. 

There is a guide: Evidence-Based Management of Anticoagulant Therapy

CHEST 2012; 141(2)(Suppl):e152S–e184S

Warfarin (unlike the newer oral anticoagulants) has a very slow onset of action. Therefore, checking INR before 3 days is not useful unless the patient has an inherent hepatic dysfunction such as cirrhosis.

In an otherwise healthy individual who is started on warfarin for VTE, the INR should be measured on day 3 and daily thereafter until the desired therapeutic range of 2-3 is achieved

he goal of anticoagulant therapy with warfarin is to administer the lowest effective dose of the drug to maintain the target international normalized ratio (INR). Warfarin, a vitamin K antagonist, is an oral anticoagulant indicated for the prevention and treatment of venous thrombosis and its extension and the prevention and treatment of the thromboembolic complications associated with atrial fibrillation. Warfarin has also been used to prevent recurrent transient ischemic attacks and to reduce the risk of recurrent myocardial infarction, but data supporting these indications are inconclusive at this time .

https://evidencebasedpractice.osumc.edu/Documents/Guidelines/Warfarin.pdf

On initiation of warfarin therapy, monitoring of prothrombin time/INR occurs on a daily basis until the target INR has been achieved and maintained for at least two consecutive days. Monitoring then occurs every 2–3 days for the next few weeks, and then with decreasing frequency depending on the stability of the results. If the INR remains stable, testing can be spread as far apart as every four weeks. However, as mentioned above, more frequent testing allows for more frequent dose adjustment, and means a person’s INR is more likely to stay within the therapeutic range.

Laboratory testing

Laboratory blood testing is the traditional practice in Australia. Blood is drawn from a vein, typically by a GP, and sent to the laboratory for testing. There is a time delay before results are returned to the GP, who analyses the need for dose adjustment with the help of computerised algorithms. The practitioner will then let you know whether a dose adjustment is needed. A dose adjustment will be required if your INR value for the test is outside the therapeutic range (too high or too low). As follow up is typically done by telephone, you need to take extra care that you understand and precisely follow your doctor’s instructions.

Testing using a portable device

Portable devices, introduced in the 1990s, allow you, your doctor or another health practitioner (e.g. pharmacist or nurse) to obtain blood test results on the spot, using a drop of fingertip blood as the sample. The blood drop is placed on a test strip which is inserted into the portable device so that the clotting time can be measured. The result as an INR value is then displayed on the device’s screen.

These devices have been demonstrated accurate and reliable and to provide reproducible results which are similar to those obtained through laboratory testing.

Portable devices can be used to measure blood INR either:

• In a health facility; or
• In your home.