Taking Blood Thinners Before, During, and After Surgery

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A blood thinner is a medication that is used to prevent or treat blood clots. There are two primary types of blood thinners, anticoagulants, and antiplatelets. While they both prevent the clotting of blood, or more accurately, slow the formation of clots, they do so in different ways.


Anticoagulant medications work by interfering with the normal clotting factors that circulate in the body. This makes it harder for the blood to clot and increases the length of time that the body needs to successfully form a clot. 

Anticoagulants are more aggressive than antiplatelet medications, so this type of medication is used when a patient requires the blood to be “thinner”.


Antiplatelet medications work by interfering with the chemical “signal” that the body sends out when it needs to form a clot. Normally, the signal would activate platelets, a type of blood cell fragment, and the platelets would gather at the site of bleeding and begin to stick together to create a clot. 

With an antiplatelet medication in circulation, the broadcast of the signal is both delayed and the “volume” is turned down, so fewer platelets respond. 

Why Are Blood Thinners Used?

Surgery is a known risk factor for the formation of blood clots, as the patient is often still for extended periods of time during surgery, and for hours or days following the procedure. Being immobile is a known risk factor for the formation of blood clots, so the prevention of clots is an important part of perioperative surgical care.

For some patients, blood thinners are used to thin the blood, which makes it take longer for the blood to clot. For other patients, blood thinners are used to prevent a clot that is already present from worsening (and to prevent additional clots from forming). 

Blood tests determine if a blood thinner is needed and the dose that should be given.

Some patients will require blood thinners for extended periods of time, such as a patient who has a heart rhythm called atrial Fibrillation. For others, such as patients who recently had surgery, they may blood thinner while they are hospitalized but never need it again. 

Before Surgery

Blood thinners are tricky things prior to surgery. The surgeon must find a balance between preventing clots and having a patient bleed too much during surgery. 

For some patients who routinely take a blood thinner prior to surgery, the dose of blood thinners that are typically taken each day is stopped at least 24 hours, and up to a week, prior to the surgery. 

This short interruption is often enough to prevent excessive bleeding without dramatically increasing the risk of a blood clot. 

However, if Coumadin (warfarin) is used, it would be stopped five to seven days before surgery, with a transition to something short-acting like Lovenox. This requires a lot of planning on the part of the patient and surgeon.

The blood thinner can then be resumed the day after surgery, assuming that blood tests show that this is appropriate.

During Surgery

Blood thinners are typically not among the medications administered during a surgical procedure unless there are special circumstances that make the use of a blood thinner beneficial to the patient, such as the use of a heart-lung bypass machine. 

Blood thinners do increase bleeding during surgery, so that must be taken into account before giving this type of medication when blood loss is an expected part of the surgery.

After Surgery

Blood thinners are frequently used after surgery to prevent blood clots in the legs, called deep vein thrombosis (DVT) and other types of blood clots. Blood clots should always be taken seriously because one clot can turn into many clots, or a clot in a leg can move and become a clot in the lung. 

A heart that isn’t beating in a normal rhythm can also cause clots to form that cause a stroke, so the way that blood clot is very closely monitored for the best possible outcome.

Testing for Thin Blood

There are three blood tests that are used to test the blood for clotting. These tests are called Prothrombin Time (PT), Partial Thromboplastin Time (PTT) and the International Normalized Ratio (INR). 

You may also hear these tests referred to as “clotting studies”, “clotting times” or “PTPTTINR” as they are often ordered together. 

Blood tests for thyroid disease, including TSH, Free T4, Free T3, antibodies
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Common Blood Thinners

Among the most commonly used blood-thinning medications are the following:

  • Aspirin
  • Coumadin/Warfarin
  • Heparin
  • Lovenox
  • Plavix

The choice of a blood thinner is typically made by the surgeon, who is most likely to know how much bleeding is expected during a particular surgery. They may desire to slightly inhibit clotting, or they may need to dramatically reduce the likelihood of clotting, depending on the nature of the illness and the surgery. 

Typically, after surgery, Heparin is given as a shot in the abdomen two to three times a day. In some cases, Lovenox is used in lieu of Heparin, but in the vast majority of cases, one or the other is administered during a hospital recovery. 

For patients who are immediately discharged home after a surgical procedure, a blood thinner may or may not be prescribed as the expectation is that the patient is walking throughout the day, which dramatically decreases the risk of blood clots. 

A Word From Verywell

If you have concerns about receiving blood thinners or are unsure about why you are receiving them, it is important to speak with your healthcare team.

Issues with blood clots can be a serious risk with some surgeries and are less common with other types of surgery, which means blood thinners may or may not be essential for you depending on the nature of your procedure and your state of health.

These medications do come with risks, but the risk of a blood clot may be even higher in some situations.

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  3. Harvard Medical School. Managing your medication before a surgical procedure. Harvard Health Publications. May, 2018.

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