Calcium (Ca) is one of the macro elements that have great importance in both animal and human metabolism.
In the regulation of total calcium (tCa) metabolism, mainly skin, liver, kidneys, bones and intestines at the tissue level; Parathyroid Hormone (PTH), Calcitonin (CT) and vitamin D take part at the molecular level. Calcium is the structural component of the skeletal system and has different and various functions in the organism. These include muscle contraction, blood coagulation, enzyme activity, neural stimulation, hormone release, secondary messenger, and membrane permeability.
The calcium ion concentration of the extracellular fluid in the body is vital and is always kept in balance. Parathyroid hormone (PTH), Calcitonin (CT) and Vitamin D contribute primarily to this balance. Apart from these, other hormones such as adrenal corticosteroids, estrogens, thyroxine, somatotropin and glucagon also contribute.
Calcium in plasma or serum is divided into 3 fractions. These:
- Ionized or free calcium (iCa or Ca++) (≈56%)
- Protein-bound calcium (mostly albumin) (≈34%)
- Complex or chelated calcium (transports bound to various anions with small molecular weights-phosphate, bicarbonate, citrate, lactate) (≈10%)
iCa and complexed calcium form the dispersible fraction of calcium. This fraction may also be referred to as ultrafiltrate calcium as it passes through biological membranes. iCa is the most physiologically active fraction of serum calcium. iCa is responsible for functions such as bone homeostasis, nerve conduction, blood coagulation, Vitamin D and PTH secretion, activation of metabolic and digestive enzymes, and effective use of iron, and is also a sensitive marker of pathological conditions.
About 90% of protein-bound calcium is bound to albumin and the remaining 10% is bound to various globulins. Since approximately half of the calcium is bound to proteins, the evaluation of tCa depends on serum albumin and total protein values (Figure 1).
Figure 1. While iCa normally remains in a very narrow range, the tCa concentration is affected by either bound or complex calcium. In other words, the tCa concentration may differ depending on the change in protein-bound Ca or complex Ca fractions.
Traditionally, the assessment of an animal’s calcium status has been based on the assessment of its tCa concentration. The tCa concentration is assumed to be directly proportional to the biologically active fraction and iCa, the gold standard for the determination of calcium status. However, this assumption is not valid in a variety of clinical situations. It has been suggested that tCa can be corrected or adjusted according to albumin or total protein concentration to improve the diagnostic interpretation, especially in patients with hypoalbuminemia or hypoproteinemia, when iCa measurement is not possible. Also, changes in pH change the calcium fraction bound to albumin; therefore, the iCa concentration can also change without a change in tCa. This corrected or adjustable tCa is called corrected calcium (ctCa). Evaluation of ctCa is recommended, especially when the plasma albumin concentration changes.
Concentration measurement of tCa, albumin and total protein can be done easily with in-house and laboratory-type analytical devices. Generally, Arsenazo III, Bromcresol green and Biuret methods are used in this type of device, respectively.
The measurement of iCa concentration is made with devices with ion-selective electrodes (ISE). Such devices can be mobile or bench-type POC (point-of-care) devices (Figure 2), or they can be a component of automated biochemistry analyzers. Mobile-type devices are frequently preferred in clinics and are often costly, and it is recommended to compare the results with reference laboratory results; This process is recommended in doubtful cases or for quality control purposes from time to time.
When measuring iCa concentration, sample collection and processing should be done with the utmost care and attention. Samples should be collected in an anaerobic environment (to minimize carbon dioxide loss), transported in the cold chain and processed within a few hours (to minimize lactate production). tCa concentration measurements are relatively inexpensive, readily available, and more robust to sample transport variables. For these reasons, the measurement of total calcium is frequently performed and evaluated today.
As a result, it is reported that all three parameters can be used in monitoring the body’s Ca balance. The most important thing at this point is to understand what each parameter is, its variables and what could be misleading. The use of tCa as an indicator of Ca status, especially in hypoalbuminemia cases, tends to overestimate hypocalcemia and ignore normocalcemia; Using ctCa may result in overestimating normocalcemia and ignoring hypocalcemia. Therefore, it is recommended to evaluate Ca homeostasis with iCa concentrations instead of tCa or ctCa in hypoalbuminemia cases. Thus, it can be determined whether there is true hypocalcemia.
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