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Medical Author:

Siamak T. Nabili, MD, MPH

Siamak T. Nabili, MD, MPH

Dr. Nabili received his undergraduate degree from the University of California, San Diego (UCSD), majoring in chemistry and biochemistry. He then completed his graduate degree at the University of California, Los Angeles (UCLA). His graduate training included a specialized fellowship in public health where his research focused on environmental health and health-care delivery and management.

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Medical Editor:

Melissa Conrad Stöppler, MD, Chief Medical Editor

Melissa Conrad Stöppler, MD, Chief Medical Editor

Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.

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The complete blood count may be done in many different health care settings including doctors' offices, clinics, urgent care facilities, emergency rooms, hospitals, and outpatient medical laboratories. It is typically done upon an order by a physician for a variety of reasons.

The complete blood count test is performed by drawing a few milliliters (one to two teaspoons) of blood from a vein. Most commonly, the sample is obtained from a vein that is visible from the skin, such as a vein on the back of the hand or the inner angle of the elbow (antecubital fossa).

A tourniquet is usually applied to the area proximal to the vein (closer to the center of the body than the vein itself). This technique will make the vein more visible and plump by limiting the blood from the vein going back toward the heart. The tourniquet is only applied for a brief period of time (a few minutes at the most) and it is removed as soon as blood is drawn.

The skin overlying the vein is cleaned using an alcohol pad, and then a needle is inserted through the area of cleansed skin into the vein below where the tourniquet is applied. The blood is then pulled from the vein via the needle by gently pulling the plunger on the syringe or by a connection of the needle to a special vacuum vial that collects the blood.

This sample is then taken to the laboratory for analysis, and the complete blood count results may be available within hours after collection. Prompt delivery of the blood sample to the laboratory for analysis is important. A sample that is not delivered in a timely manner may yield inaccurate results.

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See the Entire Complete Blood Count (CBC) Topic Guide »