CALCIUM

Calcium: serum - total and ionic, urinary.

Total serum calcium

Calcium is one of the most important minerals in the body. It is essential for the proper functioning of muscles, nerves, heart, and has an important role in coagulation and bone mineralization. Calcium is the major mineral component of bones. 99% of the body's calcium is in the bones and teeth, which is a huge reservoir for maintaining serum calcium levels, and the rest is distributed in biological fluids and soft tissues. Calcium homeostasis is maintained by parathyroid hormone (PTH). 

About half of the total amount of plasma calcium (45%) is bound to albumin (and only a small portion bound to globulins) in a non-ionized and non-diffusible form, making it a physiological inactive form. A small amount of calcium (5%) is diffusible but not ionized, being represented by citrate, phosphate and calcium bicarbonate. The rest of the plasma calcium is found in ionic or free form and constitutes the physiologically active fraction in the processes of haemostasis and regulation of neuromuscular excitability; its plasma concentration is directly related to PTH and 1,25(OH)2D. 

Calcium in the blood is tested to diagnose and monitor certain diseases related to bones, heart, kidneys and teeth. Its levels do not directly show how much calcium is in the blood, but how much calcium circulates in the blood. Calcium testing can be used if the patient has: kidney stones, bone diseases, neurological disorders.

Abnormal serum calcium levels may indicate parathyroid disfunction, bone disease, carcinoma, malnutrition and malabsorption syndrome, vitamin D deficiency, and kidney disease. Calcium ions play an important role in the transmission of nerve impulses, muscle contraction, heart function and coagulation processes. 

Biological reference range:

0 - 10 days old: 7.6-10.4 mg/dL; 10 days - 3 years old: 6.7-9.8 mg/dL; 3 - 9 years old: 8.4-10.2 mg/dL; 4 - 11 years old: 8.9-10.1 mg/dL; 11 - 13 years old: 8.8 - 10.6 mg/dL; 13 - 15 years old: 9.2-10.7 mg/dL; 15 - 18 years old: 8.4-10.7 mg/dL; adults: 8.8-10.4 mg/dL. 

Serum ionic calcium

Determination of ionic calcium provides guidance on the effect of total protein and albumin on serum calcium levels. A patient may have a high level of total calcium with a normal level of ionic calcium due to an increase in total protein and/or albumin, as is the case with dehydration or multiple myeloma. 

Ionic calcium testing is performed on patients who have a disturbed balance between bound and free calcium, and if they have transfusions, have had major surgery, or have abnormal blood protein levels, such as albumin. High fluctuations in ionic calcium can slow the heart or increase its heart rate, as well as cause muscle spasms, confusion or coma. In the case of very sick patients, it is extremely important to know the level of ionized calcium in order to intervene and prevent major complications. 

Biological reference range:

Newborns: 4.40-5.48 mg/dL; 1-18 years old: 4.80-5.52 mg/dL; adults: 4.65-5.28 mg/dL. 

Urinary calcium

Testing for calcium in the urine shows if the kidneys are excreting the right amount of calcium, and testing for vitamin, phosphorous, and/or magnesium determines what other deficiencies or excesses there are. 

Most of the calcium is excreted in the faeces and a small amount of calcium is excreted in the urine, depending on the calcium intake in the diet. Determination of urinary calcium is important in the diagnosis of hypercalcemia responsible for kidney stones. 

Biological reference range:

Normal diet: 100-300 mg/24h; low calcium diet: 50-150 mg/24h.