Warfarin

The vitamin K antagonists (VKAs) are a group of anticoagulants. Most vitamin K antagonists are coumarins, so the terms are often used interchangeably. However, there exist some VKAs which are not coumarins. The most commonly used VKA by far is warfarin (Marevan®, Coumadin®). These drugs are administered orally.

Monitoring

The problem with coumarins is that people respond differently to the same dose. This is because polymorphisms in the gene for vitamin K epoxide reductase change the affinity of the VKAs to the enzyme and differences in vitamin K content of diet.

To monitor the anticoagulant effect of warfarin, the international normalised ratio (INR) is used. INR is a standardised form of prothrombin time which is normalised so that the result is similar between different laboratory methods and equipments. The prothrombin time is first measured and then normalised by a specific equation and factor.

It’s important to start with a low dose and continuously test the INR of the patient to make sure that they’re not receiving too much (and therefore bleed too easily) or too little (and therefore have suboptimal anticoagulant effect). The dose should always be adjusted so that the INR of the patient is between 2 and 3 (between 2,5 – 3,5 in case of mechanical heart valves). INR should be measured often, initially daily but later more and more rarely (but still regularly).

Heparin

Heparin, more specifically unfractionated heparin (UFH) to distinguish them from low-molecular-weight heparin (LMWH) is an anticoagulant. It's administered by intravenous or subcutaneous injection. Nowadays LMWH have replaced many use cases of UFH, as it's as efficacious and more predictable anticoagulant effect.

Monitoring

Like for warfarin, patients receiving heparins may measure its effect frequently to ensure that they’re not overtreated or undertreated.

Heparin effect can be measured either by anti-factor Xa activity (preferred) or activated partial thromboplastin time (aPTT). One should aim for the anti-factor Xa activity to be 0.3 - 0.7 or the aPTT to be 1.5 – 2.5 times that of a control subject.

LMWH

The low-molecular-weight heparins (LMWH) are commonly used anticoagulants. They are fractionated heparins as opposed to unfractionated heparin. These drugs are just fragments of the unfractionated heparin. They’re more predictable and have longer half-life than unfractionated heparin and are therefore preferred in most cases.There is no need to monitor the effect of LMWH, as the anticoagulant effect is much more predictable based on the dose than for UFH and warfarin. One can measure anti-factor Xa activity, but there is no established therapeutic range anyway.

DOAC

Direct oral anticoagulants (DOACs) are commonly used anticoagulants. As the name suggests, they act directly on one of the clotting factors and because they’re taken orally (except argatroban).There is no routine monitoring of DOAC efficacy, as they have a predictable dose-effect relationship. Many labs can measure the serum level of the specific DOAC or the anti-factor Xa assay (a different assay from the one used for UFH/LMWH), but this does not correlate with therapeutic efficacy, only drug absorption. As such, monitoring is rarely indicated.

Antiplatelets

Acetylsalicylic acid (ASA) is technically an NSAID, although it’s more frequently used as an antiplatelet rather than for its anti-inflammatory effects. It’s a very frequently used antiplatelet.One can monitor ASA therapeutic effect with platelet aggregometry with arachidonic acid, but this is not routinely performed. There are no known uses for this, as a therapeutic reference range has not been established.

P2Y₁₂ inhibitors are antiplatelet drugs. They are are often used in combination with acetylsalicylic acid, which is called dual antiplatelet therapy (DAPT). However, they may also be used alone.One can monitor P2Y₁₂ therapeutic effect with platelet aggregometry with ADP, but this is not routinely performed. There are no known uses for this, as a therapeutic reference range has not been established.