Calcium is an important mineral that your body uses in many ways. It increases the strength of your bones and teeth and helps your muscles and nerves function. A serum calcium blood test measures the total calcium in your blood. There are several different forms of calcium in your blood. These include ionized calcium, calcium bound to other minerals called anions, and calcium bound to proteins like albumin.
Ionized calcium, also known as free calcium, is the most active form. A serum calcium test usually checks the total amount of calcium in your blood. This includes ionized calcium and calcium bound to proteins and anions. Your doctor may want to check your blood calcium levels if you have signs of kidney disease, certain kinds of cancers, or problems with your parathyroid gland. Ionized calcium levels give more information about active, ionized calcium. It may be important to know your ionized calcium levels if you have abnormal levels of proteins, such as albumin, or immunoglobins in your blood.
An imbalance can be a sign of a major health issue. Low levels of free calcium can cause your heart rate to slow down or speed up, cause muscle spasms, and even result in a coma.
Your doctor may order an ionized calcium test if you have any signs of numbness around your mouth or in your hands and feet, or if you have muscle spasms in the same areas. These are symptoms of low free calcium levels. An ionized calcium test is harder to perform than a serum calcium test. This article discusses the test used to measure the amount of ionized calcium in blood.
How the Test is Performed. How to Prepare for the Test. You should not eat or drink for at least 6 hours before the test. Many medicines can interfere with blood test results. Your health care provider will tell you if you need to stop taking any medicines before you have this test. DO NOT stop or change your medicines without talking to your provider first.
Why the Test is Performed. Results generally fall in these ranges: Children: 4. What Abnormal Results Mean. Higher-than-normal levels of ionized calcium may be due to: Decreased levels of calcium in the urine from an unknown cause Hyperparathyroidism Hyperthyroidism Milk-alkali syndrome Multiple myeloma Paget disease Sarcoidosis Thiazide diuretics Thrombocytosis high platelet count Tumors Vitamin A excess Vitamin D excess Lower-than-normal levels may be due to: Hypoparathyroidism Malabsorption Osteomalacia Pancreatitis Renal failure Rickets Vitamin D deficiency.
Thus, the main risk of systematically using the total calcium level is the overestimation of serum calcium status. The aim of this article is first to discuss about the measurement and estimation of ionized calcium in laboratories, and then to describe its applications in clinical practice. Calcium is distributed in three forms, an important fraction of it being complexed with proteins mainly albumin. As previously described, the free form of calcium is the one that plays a major role in the body 13 , 16 - This free fraction is very closely modulated by endocrine functions 1 , 8 , However, its dosage is limited by constraints which are reducing its routinely use 19 , Indeed, its precise dosage requires both strict conditions of sample handling as well as taking into account many pre-analytical conditions For an accurate determination of the calcium status based on ionized calcium level, it is essential to take all necessary precautions regarding the type of specimen collection and the choice of container collection.
The type of tube collection plays a key role in this analysis given the presence of common anticoagulants EDTA, citrate, oxalate , whose principal function is precisely the ability to complex with free calcium. Not taking this element into account would lead to a significant underestimation of ionized calcium level The International Federation of Clinical Chemistry IFCC guidelines recommend heparin as the anticoagulant of choice for the measurement of ionized calcium Heparin chelates a moderate but significant portion of ionized calcium, depending on its type and dosage quantity in the tube.
The assay can thus be performed on whole blood sample with heparin, heparinized plasma or serum. According to the Clinical Laboratory Standards Institute CLSI guidelines, measurement of ionized calcium with plasma should be avoided and use of whole blood specimens should be preferred in syringes However, evacuated blood collection tubes containing lithium heparin and gel separator material are used very frequently in clinical laboratories nowadays.
The other advantage of lithium heparin tubes is the possibility of using the entire sample volume for analysis. Although not recommended, if the sample has been taken in an evacuated tube, care should be applied to keep it sealed until analysis, in order to avoid any pH change caused by CO 2 leakage. It is also important to treat the samples as soon as possible to limit the potential effects of anaerobic metabolism-induced glycolysis on erythrocytes and leucocytes Of course, the use of a serum sample remains the ideal solution, avoiding in particular any bias related to erythrocytes or heparin but this requires a larger sample volume and a longer process time.
Although many methods, more or less complex, have been described to measure ionized calcium, it seems that its measurement has recently been democratized by the advent of ion-selective electrodes ISE 28 , Indeed, this method currently recommended by the IFCC guidelines is based on a simple principle: the measurement of a potential across a membrane only allowing selective calcium transit, this potential being mathematically correlated to the ionized calcium concentration in the sample tested.
To the best of our knowledge, there is currently no large automated instrument for the specific measurement of ionized calcium; however, the vast majority of blood-gas analyzers currently allow a direct ISE assessment of ionized calcium level. In the literature, a number of situations have been identified as potentially responsible for ionized calcium level variations 24 , We present below the details of the main factors to consider when measuring and interpreting ionized calcium levels. In conclusion, the realization and interpretation of ionized calcium measurement are challenging as the conditions and constraints that can affect its serum level before, during and after the sampling are numerous and diverse Figure 2.
These elements added to the significant cost of the technique represent all obstacles to its systematic use in current practice 41 , Although its dosage is currently more accessible thanks to the ISE technology allowing its determination on blood-gas analyzers, many institutions continue to use total calcium assay in their daily practice, test that is otherwise recommended in the international guidelines In this context, a very large number of formulas have been developed for nearly a century with two main objectives: to estimate the calcium status reported to a normal albumin level or to estimate the ionized calcium level from total calcium and a number of biological parameters such as albumin, protein, pH and anion gap.
Historically, the first estimation formula is attributed to the nomogram developed by McLean and Hastings in the s Nevertheless, its tedious application has given way to the most used estimation formula: the Payne correction equation According to data from Vancouver Coastal Health and Providence Health Care Laboratories in , total calcium and albumin were ordered concomitantly to biological samples in almost three out of four cases, suggesting that for the vast majority of clinicians, the interpretation of one cannot be fulfilled without the value of the other The literature concerning these estimation formulas is extremely rich, as illustrated by the work of Dickerson et al.
Whatever the heparin formulation used, it is essential for accurate results that the correct volume of blood is sampled to achieve correct heparin concentration and that blood and anticoagulant are well mixed immediately after sampling. Blood collected for serum estimation must be processed anaerobically. Samples should ideally be centrifuged at low temperature and the cap should not be removed prior to analysis.
Full recommendations for collection, transport and storage of specimens for ionized calcium are published [31]. The maintenance of plasma ionized calcium concentration within well-defined limits is essential for the many life-preserving physiological and cellular pathways that depend on ionized calcium.
The action of two hormones, parathyroid hormone and calcitriol, is of major importance in this regard. A range of clinical conditions — some very common — are associated with disturbance in calcium metabolism and resulting abnormality in ionized calcium concentration.
If sufficiently severe, these changes in plasma ionized calcium concentration have profound adverse effect and may actually threaten survival. Before the development of a reliable means of accurately measuring ionized calcium concentration in the mid-to-late s, the only means of assessing calcium status was to measure total calcium concentration in plasma. Because this does not accurately reflect ionized calcium concentration in some clinical situations, it is a less satisfactory alternative.
For a number of mainly logistical reasons, it continues to be used, but measurement of ionized calcium rather than total calcium is widely considered mandatory for some patient groups, most notably the critically ill. May contain information that is not supported by performance and intended use claims of Radiometer's products. See also Legal info. Radiometer and acutecaretesting. Printed from acutecaretesting. July Ionized calcium.
Introduction The adult human body contains Calcium homeostatis The concentration of calcium in blood plasma reflects a balance between the calcium absorbed from diet via the gastrointestinal tract and that lost from the body in urine.
FIGURE 2: Parathyroid control of plasma calcium Calcitriol alternative name 1,dihydroxycalciferol is produced in the kidneys from a vitamin D-derived substance, hydroxycholecalciferol. To summarize, maintenance of normal plasma calcium concentration depends on: diet containing adequate amounts of calcium exposure to sunlight for endogenous production of vitamin D plus dietary source of vitamin D both are required for optimum calcitriol production normal gastrointestinal function for absorption of dietary calcium and vitamin D normal parathyroid function for appropriate secretion of PTH normal renal function for secretion of calcitriol, and appropriate adjustment of calcium loss in urine normal bone metabolism for appropriate movement of calcium between bone and blood Assessing calcium status — the rationale for measuring ionized rather than total plasma calcium concentration.
Causes of hypocalcemia ionized calcium Hypocalcemia is a common feature of chronic renal failure, reflecting the important role of the kidneys in calcitriol production and, thereby, absorption of dietary calcium. Other causes of hypocalcemia include vitamin D deficiency and hypoparathyroidism reduced PTH secretion by parathyroid glands , due most often to inadvertent destruction of parathyroid tissue during surgery of the neck or congenital lack of parathyroid glands. Symptoms and consequences of hypercalcemia In general, the range and severity of symptoms associated with hypercalcemia reflect the severity of the increase.
Measurement of ionized calcium - sampling requirements. Summary The maintenance of plasma ionized calcium concentration within well-defined limits is essential for the many life-preserving physiological and cellular pathways that depend on ionized calcium.
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