Anticoagulants are a class of medicines that are commonly identified as causing patient harm and hospital admissions (Pirmohamed 2004). When the NPSA contacted the medical and pharmacy defence organisations and the NHS Litigation Authority for available data, they were presented with 600 reported cases of patient harm from the use of anticoagulants in the UK up to the end of 2002 along with 120 reported deaths (NPSA 2006). Of these, 77% were related to warfarin and 23% related to heparin.

The NPSA undertook a literature review and completed a risk assessment on the use of anticoagulants in the NHS. This found that inadequate competencies of healthcare professionals prescribing and monitoring patients on anticoagulants and failure to implement professional guidelines contributed to the high incidence of patient harm. Do you have the knowledge and training needed to prescribe anticoagulants safely? E-learning packages are available to assist with the initiation and maintenance of anticoagulant therapy at If you haven’t used the BMJ learning areas they are worth logging into.

Oral anticoagulants

Oral anticoagulants are used to treat a number of medical conditions, including atrial fibrillation, deep vein thrombosis, pulmonary embolism, arterial thromboembolism and mitral valve replacements. The oral anticoagulant most commonly prescribed in the UK at present is warfarin. Warfarin can be difficult to prescribe as dosing is specific to each individual patient. Over anticoagulation can cause patient harm through haemorrhage and under anticoagulation can cause patient harm through thrombosis, both of which can be life threatening. Recently newer oral anticoagulants called NOACs (Novel/New Oral Anticoagulants) are commonly being used across both primary and secondary care. The advantages of these types of anticoagulants is they don't need to have their levels monitored and the dose doesn't need to be varied. Renal function should be monitored as these drugs are mostly renally excreted and there is no antidote available should a major bleed occur. A meta-analysis of over 70,000 patients showed NOACs offer better, safer anticoagulation at lower overall risk (Ruff et al 2014). Warfarin is still by far the most common anticoagulant used but use of NOACs is steadily increasing and more patients are requesting these as an alternative to warfarin and its associated monitoring.

The British Society for Haematology Committee has produced clear guidance for the management of patients on warfarin (Keeling et al 2011). It has also produced recommendations on safety indicators to complement these guidelines (Baglin et al 2006).

Read the following articles

Keeling D, Baglin T, Tait C, Watson H, Perry D, Baglin C, Kitchen S, and Makris M (2011) Guidelines on oral anticoagulation with warfarin fourth edition British Journal of Haematology, 154(3):311-324 (available from

Heart-George A (2013). Warfarin Management Pathway: A clear and safe algorithm, from admission to discharge. BMJ Qual Improv Report, 2(2). (available from

What are the key learning points for you as a future prescriber of anticoagulants?

Initiating warfarin

Before you initiate a patient on warfarin, you should be clear about;

  • The indication
  • The target International Normalised Ratio (INR)
  • The duration of treatment
  • If the patient is at high risk from warfarinisation (e.g. has congestive cardiac failure, infection, interacting drugs, liver failure, diarrhoea, a raised baseline INR)
  • If the patient has any contraindications
  • The loading regime (An age related dosing algorithm should always be followed when fast loading of warfarin is required)
  • If parenteral anticoagulation is required until oral anticoagulation with warfarin is established
  • If the patient is of childbearing potential (Warfarin is teratogenic and should not be given in the first trimester of pregnancy. It can also increase risk of haemorrhage and should be avoided in the last trimester of pregnancy)

You’ll need to carry out or order baseline tests before warfarin therapy can be started. These include a baseline INR, LFTs, Activated Partial Thromboplastin Time (APPT) and platelets. Are you confident about how to interpret each of these tests? Additional information can be found at You need to be confident that the patient is fully able to manage their warfarin therapy, as well as being able to attend frequent INR monitoring – something especially important in patients that are frail, confused, or who have chaotic lifestyles (Baglin et al, 2011).

You must give appropriate counselling to all patients who are prescribed oral anticoagulants at the start of their therapy. Anticoagulation clinics and/or GPs would usually provide the following:

  • An information booklet, providing patients with essential information about warfarin, including potential side effects and how they should be managed.
  • A record book, where all details about the patient’s dose and blood test results should be recorded.
  • A card for the patient’s purse or wallet, which alerts others to the fact that they are taking warfarin.

You must also be clear about who will be responsible for dosing the patient and monitoring them. Most patients are managed by an anticoagulant service (either based in hospital or the community). If you make a referral to an anticoagulant service, you need to make sure that you provide them with all the information they need to dose the patient effectively. This includes the diagnosis, the target INR, the planned duration of treatment, the dose of warfarin on referral and any current medication (Baglin et al, 2011)

Monitoring warfarin

If your patient is taking an anticoagulant, they must have regular INR checks and appropriate warfarin dose adjustment. Warfarin interacts with a wide range of medication (see BNF appendix 1), many producing an increase in anticoagulant effect. It can also be affected by changes in diet and alcohol consumption. As a prescriber, you should be aware of this and make appropriate prescribing decisions and dose adjustments. Many fatalities and permanent harm events with warfarin were associated with inadequate laboratory monitoring and clinically significant drug interactions, usually involving non-steroidal anti-inflammatories (NPSA, 2006).

Parenteral anticoagulants

Heparin is an injectable anticoagulant. It is used in higher doses to treat venous and arterial thromboembolism, as well as in lower doses for thromboprophylaxis. It is fast acting, so is often used until oral anticoagulation can be established, as well as times when oral anticoagulants are deemed unsuitable.

Unfractionated heparins

Therapeutic doses of sodium and calcium heparin have to be monitored on a regular basis. This ensures that the dose being given is achieving the required level of anticoagulation and the most common way to test this is with the Activated Partial Thromboplastin Time (APPT) test. Frequent dose adjustments are usually required to ensure effective anticoagulation and to prevent complications of bleeding. Both inadequate laboratory monitoring and inappropriate dosing are frequent causes of harm to patients in secondary care (NPSA, 2006).

Low molecular weight heparins (LMWH)

LMWHs are used both to treat venous and arterial thromboembolism, and for thromboprophylaxis, in the majority of hospitals within the UK. Low molecular weight heparins are prescribed according to the weight of the patient and blood tests are not generally needed to ensure effective anticoagulation. Frequent causes of patient harm include failure to weigh patients accurately, failure to identify the clinical need to initiate treatment and an inability to calculate an appropriate dose (NPSA, 2010). The renal function of the patient also needs to be taken into consideration. LMWH is renally cleared and must be used with caution in patients with renal failure due to an increased risk of bleeding. In these patients, you may need to reduce the dose or use an unfractionated heparin. You can find further dosing information in the British National Formulary.


Consult the BNF and familiarise yourself with the low molecular weight heparins currently licensed for use. What dose of dalteparin would you prescribe for a female patient with deep vein thrombosis who has normal renal function and weighs 62Kg?

Dalteparin 12,500 units should be prescribed once daily by subcutaneous injection for this patient.

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