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Applying a tourniquet increases the amount of blood in the vein to make them stand out more. The tourniquet should not be so tight that it cuts off the circulation.  The tourniquet should be put on the arm about four inches above the vein. A blood pressure cuff that is inflated to 40–60 mm Hg also works well. Warmth will make the patient's veins dilate and expand, making them easier to see. Contrary to popular culture, you should palpate the arm, rather than slapping it. Slapping the skin is poor technique that may result in a hematoma. Use your index finger to look for a vein, which feels soft and spongy. Don't use your thumb, as it contains its own pulse.  The warm pack or water bottle should be put on the area before it is disinfected. Nothing more should touch the area after it is disinfected. Do not apply the warm pack or water bottle directly to the skin. Wrap it in a thin towel to prevent burns. If it hurts, it is too hot. Many people have needle phobias and nervousness and apprehension is a normal response. Stress not only makes the veins hard to hit, but it could also negatively affect the test results (particularly for biochemistry panels). Reassure your patient and explain that the pain is very brief and minor.  Tell your patient to try visualization and deep breathing. Observe your patient and have them lie down on their back if you think they might faint. This will improve the blood flow to their head. It also reduces their chances of falling and injuring themselves if they do pass out.
Make sure your tourniquet has been applied properly. Put a warm pack or water bottle over the area. Use proper palpation techniques. Tell the patient to relax.