Why Do Some People Faint When Having Blood Drawn?

Fainting from having Blood Drawn

Every phlebotomist, sooner or later, comes across a patient who faints either in the middle of a venipuncture procedure or shortly after its completion. While having your patient pass out is far from the norm, varying estimates place the percentage of people who experience a vasovagal response (i.e. passing out) when having blood drawn between two and three percent. This means that the average phlebotomy technician will have to deal with a fainter at least a couple of times each month.

While certification courses and other types of formal phlebotomy training programs will cover how to identify a person who is losing consciousness as a result of blood loss, it may nonetheless be useful (or, at least somewhat interesting) to briefly review some of the main causes of this phenomenon.

Four Reasons People Faint During Phlebotomy Procedures

The following is by no means an all-inclusive list of the potential causes of vasovagal syndrome (fainting) experienced during venipuncture procedures, but the four reasons listed below are the most common ones that cause some patients to lose consciousness when having blood drawn.

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It may sound like a gross oversimplification, but most practicing phlebotomists will tell you that the number one cause of patients passing out is anxiety. The most-likely source of this anxiety is fear of the actual venipuncture procedure itself. As we all know, many people become squeamish at the sight of blood and many others are extremely uneasy with needles.


Individuals who are not properly hydrated are also far more likely to become light-headed and dizzy or to even pass out in the process of having blood drawn. The combination of the drop in blood pressure caused by lower-than-normal levels of body fluids and the loss of blood experienced during the procedure can lead to insufficient blood flow to the brain and a temporary vasovagal response.


A person’s diet can also contribute to fainting. If the patient comes to the phlebotomist with already-low blood sugar levels, diabetes, or some other condition that affects blood flow, the act of drawing blood can cause a sudden plummet in blood pressure that results in fainting.

Pre-existing Low Blood Pressure

Patients who, for any number of reasons, already have low blood pressure are also predisposed to passing out when blood is being taken. There are many ways to prevent this from occurring, but the phlebotomist will obviously need to be aware of the patient’s status and take appropriate measures prior to drawing blood.

How Can Fainting Be Prevented?

Because almost all incidences of fainting that occur when blood is drawn are the result of a sudden drop in blood pressure, phlebotomists are trained to look for the symptoms of vasovagal response. Preventing a loss of consciousness may be as simple as temporarily elevating the patient’s legs (e.g. having the person lie down on his or her back), or having him or her lean forward to place the head below the knees. This will allow for greater circulation to the brain and can often prevent a loss of consciousness.

Other approaches typically include restoring body fluids by giving the person something to drink, elevating blood sugar levels by providing a light snack, and using smelling salts. The Mayo Clinic’s guide to first aid for fainting contains additional helpful tips and is highly recommended.

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