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INR for Warfarin Monitoring

Dear Colleague

This report provides an opportunity to reflect on the frequency of INR monitoring for your patients on warfarin.

Warfarin is the most widely used anticoagulant in New Zealand and has a valuable role in the prevention of thrombosis. INR testing plays a key role in maintaining the warfarin response within the therapeutic window, however review of the NZHIS data warehouses reveals many patients are being tested more frequently than may be necessary.


What is the optimal rate of testing?1

Once a patient is stabilised on wafarin, the optimal monitoring frequency is usually between four to six weeks.

Some stable patients may be monitored with intervals of up to eight weeks or more.

More frequent monitoring will usually only be required when the the warfarin dosage is adjusted, medicines are prescribed which may interact with warfarin, or the patient has an intermittent illness.

Point-of-care testing (POCT) for INR is becoming more popular, but this data is not “captured” by the NZHIS claims data used in this report. Therefore, if your practice uses POCT, this report may underestimate the rate of testing in your patients.

Commonly used medicines which interact with warfarin
  • Erythromycin
  • Roxithromycin
  • Fluconazole
  • Miconazole oral gel
  • Metronidazole
  • Amiodarone
  • Tramadol

Increased testing may be required for patients on medicines which interact with warfarin

There are a number of medicines which can potentiate the action of warfarin (see sidebar).

Caution is required when these medicines are prescribed for patients taking warfarin as the INR may increase. Additional testing is usually required when these medicines are initiated to ensure warfarin is maintained within the therapeutic range.

For example:

  • Erythromycin or roxithromycin added to warfarin: check the INR after three days
  • Amiodarone added to warfarin: monitor INR weekly for four weeks (interaction usually seen after two weeks)
  • Tramadol added to warfarin: check INR after three days and then after one week if there is no change.

It is prudent to consider the likelihood of interaction when any medicine is introduced. Information on warfarin medicine interactions can be found in resources such as your practice management system, MIMs and BNF.


Rates of testing for your patients

The figure below shows the frequency of INR testing for your patients who have been established on warfarin for at least 15 months.

Your patients
XX patients
Testing interval greater than every six weeks Testing interval four to six weekly (optimal testing) Testing interval less than every four weeks
19% 24% 57%
9 patients per GP

The national figures show that most patients established on warfarin are receiving INR testing more frequently than four to six weekly.


  • This report shows data from 555843 tests claimed for 34137 patients which were submitted to the NZHIS Laboratory Claims Warehouse between 1 June 2009 and 31 May 2010.
  • To be shown on the report, patients needed to have had a prescription for warfarin claimed in the NZHIS Pharmaceutical Claims Warehouse in the report period (1 June 2009 to 31 May 2010), and at least one prescription for warfarin claimed in the three months preceding it (1 March 2009 to 31 May 2009). This was done to ensure that patients are on a maintenance dose of warfarin, and therefore should be monitored according to the recommendations for these patients.
  • Patients are counted on your report if you ordered an INR test for them during the report period. If another GP requested a test for the same patient, then the patient and all of their INR tests will be counted on both reports.
  • Some discrepancies will be due to rounding.


  1. bpacnz. INR Testing. 2006.