Articles and Case Studies

The Perils of Warfarin

31 Mar 2011

Dr Sara Bird

by Dr Sara Bird

Mrs Antoinette Papa, 45 years of age, underwent a mechanical mitral valve replacement on 11 July 2001. Post operatively she was commenced on warfarin.

Her cardiothoracic surgeon recommended that her INR be maintained between 3.0 to 4.0. Her warfarin dosage was managed by Sullivan Nicolaides’ Warfarin Care Service (WCS).

The table below outlines the test results and instructions given to the patient with regard to her warfarin therapy between December 2001 and February 2002.

Date  INR Dose Instruction Next test
6.12.01  3.00 7.7 mg daily 3.01.02
3.01.02 2.80 Nil Thursday, Friday 6.0 mg others
10.01.02
7.01.02 2.80 5.5mg 10.01.02
10.01.02 5.70 Nil Thursday, Friday 4.0 mg others
14.01.02
14.01.02 3.40 4.0mg daily 17.01.02
17.01.02 3.80 4.0 mg Monday – Friday 3.0 mg others
24.01.02
24.01.02 6.50 Nil Thursday, Friday 26.01.02
26.01.02 3.30 3.0mg daily 30.01.02
30.01.02 2.00 4.0 mg Monday – Friday 3.0 mg Saturday, Sunday
09.02.02
09.02.02 7.00 Nil Saturday, Sunday, Monday 13.02.02
13.02.02 2.40 2.5mg daily 16.02.02
16.02.02 1.50 3.0mg daily 19.02.02
19.02.02 1.60 3.0 mg Monday, Wednesday, Friday 3.5 mg Tuesday, Thursday, Saturday, Sunday
22.02.02
22.02.02 1.50 3.5 mg Monday – Friday 4.0 mg Saturday, Sunday
25.02.02
25.02.02 1.80 4.0mg daily discharged

On 27 February 2002, the patient received a phone call from the WCS. She was told “I am sorry we are not able to manage your warfarin any more” and advised to go and see her GP.

The patient saw her GP on 28 February 2002. The GP had also received a phone call from the WCS on 27 February 2002 and notified of their decision not to monitor the patient’s warfarin any longer because her INR levels were erratic and they felt it was a consequence of non-compliance. During the consultation with the patient on 28 February 2002, the GP phoned the WCS and obtained that day’s  INR result which was 1.7. He increased her warfarin dose to 4.5 mg daily. The GP referred her to another warfarin service provider, QML, and asked her to see him again after she had her next blood test. The GP said that if her INR remained sub-therapeutic he would refer her  to a haematologist.

On 1 March 2002 the patient suffered a large right middle cerebral artery embolic infarct which left her with a dense left hemiplegia, dysarthria and cognitive impairment.

Medico-legal issues

The patient subsequently commenced legal proceedings against the WCS (the defendant). The claim proceeded to hearing in April 2009 and judgment was handed down on 24 September 2010. 1

In her Statement of Claim, the patient (now a plaintiff) alleged the WCS had breached its duty of care to her by failing to appropriately manage her warfarin therapy during the period 13 to 27 February 2002.

At the hearing, evidence was given by Dr Beverley Rowbotham, a haematologist, who managed the WCS, the patient’s GP and her cardiologist. Expert evidence was heard from four haematologists, a GP and a cardiologist.

The Judge drew the following conclusions from the expert medical evidence:

  • That the warfarin dosage management process necessarily involved the balancing of the risk of under anticoagulation and over anticoagulation;
  • That there was a known risk of thromboembolic event in the case of sub-therapeutic INR levels and that this risk magnified when the INR was at or below 1.5;
  • That there was a known risk of haemorrhagic event in the case of supra-therapeutic INR levels, and that this risk magnified when the INR was at or above 6.5;
  • That in relative terms, the risk of haemorrhagic event at  INR 6.5 or higher was significantly greater than the risk of thromboembolic event at INR 1.5 or less;
  • That the process of keeping the INR within target range, and thereby minimising these risks, necessarily involved a significant component of clinical judgement on the part of the warfarin care haematologist;
  • That it was accepted practice at the time that the anticoagulants Clexane or heparin could be used on a patient with an artificial mitral valve as “bridging therapy” by way of a substitute for, or supplement  to, warfarin in a peri-operative setting;
  • That neither the medical literature then available nor the standards of practice among warfarin care haematologists at the time support a conclusion that it was appropriate for a warfarin care haematologist, when dosing a patient with an artificial mitral valve whohad a persistent sub-therapeutic INR, to supplement the prescription of warfarin with either;
  1. The administration of Clexane or heparin; or
  2. The administration of “stat” doses of warfarin.

Accordingly, the Judge found that it had not been established that the defendant failed to exercise and observe the standards of a reasonable warfarin care haematologist  in the management of the plaintiff’s warfarin dosage level in the period 13 to 27 February 2002.

However, the separate question on liability was quite different. It went to whether, according to the relevant professional and practising standards at the time, the defendant, when presented with this patient with an artificial mitral valve and a recent history of INR instability and persistent sub-therapeutic INRs, ought to have given advice to the plaintiff, her GP or her cardiologist, and if so, what the content of that advice ought to have been and when it ought to have been given.

The defendant gave no advice to the plaintiff’s medical practitioners until Dr Rowbotham’s call to the GP on 27 February 2002. The only information being provided to the plaintiff was her dosage instructions and the dates for her next tests. She was not routinely advised of her actual INR levels.

The Judge concluded it would have been reasonably appropriate for the defendant at least to raise the prospect of investigating an alternative or supplementary form of anticoagulation and also to raise the prospect of this needing to be done in consultation with the plaintiff’s cardiologist. He then went on to find that if this had occurred, then according to the evidence of her cardiologist, he would have commenced anticoagulation with Clexane or heparin. The cardiologist would have recommended this on receipt of the second sub-therapeutic INR on 19 February 2002.

Therefore, the Judge was satisfied on the balance of probabilities that the defendant’s failure to give advice to the plaintiff and her GP by about 22 February 2002 in respect of the matters outlined above was a breach of the defendant’s duty of care to the plaintiff which was causative of her suffering the stroke on 1 March 2002. Accordingly, the plaintiff had proved her case of liability against the defendant.

The Judge awarded the plaintiff $2,201,982.00 plus legal costs. Past and future care costs formed the largest component of the award of damages

Discussion

Warfarin is one of the medications most commonly involved in claims arising out of medication errors. The Threats to Australian Patient Safety (TAPS) study collected 648 reports from GPs about threats to patient safety.2 Warfarin errors comprised 7% of these reports. Of the errors involving warfarin, over 20% resulted in hospitalisation and a further 7% resulted in the death of the patient.

Lessons from the TAPS study in preventing errors related to warfarin therapy include:

  • Patient education on commencing warfarin is an important responsibility of the clinician who initiates therapy. When therapy commences in hospital, GPs should also reinforce messages relating to safety and monitoring;
  • Clinicians and patients should clearly record warfarin dosages and INR levels in medical records (which may be electronic) and in a patient’s personal diary;
  • Details  of when the next INR is due should be discussed with the patient at the same time as the latest result and any dosage change is discussed;
  • INR results and warfarin doses  should  be communicated to the patient on the day of the testing, and patients and their carers should be educated to follow up with their treating doctor if this does not occur.

References

  1. Papa v Sullivan Nicolaides Pty Ltd [2010] QSC 364. Accessed at www.austilli.edu.au/au/cases/qld/ASC/2010/364.html
  2. Makeham M, Saltman D, Kidd M. Lessons from the TAPS study. Warfarin: a major cause of threats to patient safety. Aust Fam Physician 2008;37:817-8.
Communication with Patients, Clinical, Complaints and Adverse Events, General Practice, Intensive Care Medicine
 

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