Calcium is an essential mineral required by nearly every living organism. While bones and teeth contain nearly 99% of all calcium in the body, the remaining 1% found in body fluids is responsible for a myriad of important physiological effects. Abnormal calcium levels have been tied to neurological, endocrine, cardiovascular, digestive, metabolic, and pulmonary diseases, as well as cancer. Cayman's Calcium Assay provides a quick, reliable method for determining total calcium concentration in a variety of biological samples. The assay utilizes an optimized variation of the well-established o-Cresolphthalein-calcium reaction in which a vivid purple o-Cresolphthalein Complexone is formed that absorbs between 560 nm and 590 nm.