Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins to fill with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore blood flow. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, the area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

No special preparation is necessary.

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Normal results vary, but in general are:

• Male: 13.8 to 17.2 gm/dL
• Female: 12.1 to 15.1 gm/dL

Note: gm/dL = grams per deciliter

Lower-than-normal hemoglobin may be due to:

• Anemia (various types)
• Bleeding
• Erythropoietin deficiency (from kidney disease )
• Lead poisoning
• Malnutrition
• Nutritional deficiencies of iron, folate , vitamin B12 , vitamin B6
• Overhydration
• Red blood cell destruction associated with transfusion reaction

Higher-than-normal hemoglobin may be due to:

• Congenital heart disease
• Cor pulmonale
• Increased red blood cells due to too much erythropoietin
• Pulmonary fibrosis
• Polycythemia vera

Additional conditions under which the test may be performed:

• Anemia of chronic disease
• Clinical hemoglobin C
• Diabetes
• Drug-induced immune hemolytic anemia
• Giant cell (temporal, cranial) arteritis
• Hemolytic anemia due to G6PD deficiency
• Idiopathic aplastic anemia
• Idiopathic autoimmune hemolytic anemia
• Immune hemolytic anemia
• Iron deficiency anemia
• Paroxysmal cold hemoglobinuria (PCH)
• Paroxysmal nocturnal hemoglobinuria (PNH)
• Pernicious anemia
• Placenta abruptio
• Polymyalgia rheumatica
• Rhabdomyolysis
• Secondary aplastic anemia

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.