The Test Sample
What is being tested?
Calcium is the most abundant and one of the most important minerals in the body. It is essential for cell signaling and the proper functioning of muscles, nerves, and the heart. Calcium is needed for blood clotting and is crucial for the formation, density, and maintenance of bones. This test measures the amount of calcium in the blood or urine.
About 99% of calcium is found complexed in the bones, while the remaining 1% circulates in the blood. Calcium levels are tightly controlled; if there is too little absorbed or ingested, or if there is excess loss through the kidney or gut, calcium is taken from bone to maintain blood concentrations. Roughly half of the calcium in the blood is "free" and is metabolically active. The remaining half is "bound" to albumin, with a smaller amount complexed to anions, such as phosphate, and these bound and complexed forms are metabolically inactive.
There are two tests to measure blood calcium. The total calcium test measures both the free and bound forms. The ionized calcium test measures only the free, metabolically active form.
Some calcium is lost from the body every day, filtered from the blood by the kidneys and excreted into the urine. Measurement of the amount of calcium in the urine is used to determine how much calcium the kidneys are eliminating.
How is the sample collected for testing?
A blood sample is taken by needle from a vein in the arm. If a urine collection is required, a 24-hour urine sample or a timed collection of a shorter duration is obtained. Sometimes a random urine collection may be used, although a timed collection is preferred.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.