What Is a Blood Test? A blood test is when a sample of blood is taken from the body to be tested in a lab. Doctors order blood tests to check things such as the levels of glucose, hemoglobin, or white blood cells. This can help them find problems like a disease or medical condition. Sometimes, blood tests can help them see how well an organ (such as the liver or kidneys) is working. What Is a Phosphorus Test? A phosphorus test measures the amount of phosphorus in the blood. Phosphorus is a mineral that helps in the formation of bones and teeth. It also helps make protein and energy, and helps keep muscles and nerves working the way they should. Why Are Phosphorus Tests Done? A phosphorus test might be done if someone has a medical condition that makes high or low phosphorus levels more likely, such as a kidney problem. How Should I Prepare for a Phosphorus Test? You should be able to eat and drink normally unless you're also getting other tests that require fasting beforehand. Tell your doctor about any medicines you take because some drugs might affect the test results. It can help to wear a T shirt or other short-sleeve top on the day of the test to make things faster and easier for the technician who will be drawing the blood. How Is a Phosphorus Test Done? Usually, the blood is drawn from a vein in your arm — most often on the inside of the elbow, but sometimes on the back of the hand. To do that, a health professional will: clean the skin put an elastic band (tourniquet) above the area to get the veins to swell with blood insert a needle into a vein (usually in the arm inside of the elbow or on the back of the hand) pull the blood sample into a vial or syringe take off the elastic band and remove the needle from the vein It's best to try to relax and stay still during the procedure because tensing muscles can make it harder and more painful to draw blood. And if you don't want to watch the needle being inserted or see the blood collecting, you don't have to. Look the other way and maybe relax by focusing on saying the alphabet backwards, doing some breathing exercises, thinking of a place that makes you happy, or listening to your favorite music. How Long Does a Phosphorus Test Take? Most blood tests take just a few minutes. Occasionally, it can be hard to find a vein, so the health professional may need to try more than once. What Happens After a Phosphorus Test? The health professional will remove the elastic band and the needle and cover the area with cotton or a bandage to stop the bleeding. Afterward, there may be some mild bruising, which should go away in a few days. When Are Phosphorus Test Results Ready? Blood samples are processed by a machine, and it may take a few hours to a day for the results to be available. If the test results show signs of a problem, the doctor might order other tests to figure out what the problem is and how to treat it. Are There Any Risks From Phosphorus Tests? A phosphorus test is a safe procedure with minimal risks. Some people might feel faint or lightheaded from the test. A few teens have a strong fear of needles. If you're anxious, talk with the doctor before the test about ways to make the procedure easier. A small bruise or mild soreness around the blood test site is common and can last for a few days. Get medical care if the discomfort gets worse or lasts longer. If you have questions about the phosphorus test, speak with your doctor or the health professional doing the blood draw. Back to Articles Related Articles Blood Test (Video) These videos show what's involved in getting a blood test and what it's like to be the person taking the blood sample. Read More Kidneys and Urinary Tract The kidneys perform several functions that are essential to health, the most important of which are to filter blood and produce urine. Read More Kidney Disease Sometimes, the kidneys can't do their job properly. In teens, kidney disease is usually due to infections, structural issues, glomerulonephritis, or nephrotic syndrome. Read More Blood Test: Magnesium A magnesium test looks at levels of the mineral magnesium in a person's blood. Find out why doctors do this test and what's involved for teens. Read More Blood Test: Complete Blood Count This common blood test helps doctors gather information about a person's blood cells and how they're working. Find out why doctors do this test and what's involved for teens. Read More Blood Find out about the mysterious, life-sustaining fluid called blood. Read More Diabetes Center Our Diabetes Center provides information and advice for teens about treating and living with diabetes. Read More Note: All information is for educational purposes only. For specific medical advice, diagnoses, and treatment, consult your doctor. © 1995-2021 KidsHealth®. All rights reserved. Images provided by The Nemours Foundation, iStock, Getty Images, Veer, Shutterstock, and Clipart.com.