Calcium, Ionized

Calcium, Ionized

Synonym/acronym: Free calcium, unbound calcium, Ca++, Ca2+.

Common use

To investigate various conditions related to altered levels of ionized calcium such as hypocalcemia and hypercalcemia.


Serum (1 mL) collected in a gold-, red-, or red/gray-top tube. Specimen should be transported tightly capped and remain unopened until testing. Exposure of serum to room air changes the pH of the specimen due to the release of carbon dioxide and can cause erroneous results.

Normal findings

(Method: Ion-selective electrode)
AgeConventional UnitsSI Units (Conventional Units × 0.25)
Whole blood
0–11 mo4.2–5.84 mg/dL1.05–1.46 mmol/L
1 yr-Adult4.6–5.08 mg/dL1.15–1.27 mmol/L
Adult4.12–4.92 mg/dL1.03–1.23 mmol/L
1–18 yr4.8–5.52 mg/dL1.2–1.38 mmol/L
Adult4.64–5.28 mg/dL1.16–1.32 mmol/L


Calcium, the most abundant cation in the body, participates in almost all of the body’s vital processes (see other calcium studies). Circulating calcium is found in the free or ionized form; bound to organic anions such as lactate, phosphate, or citrate; and bound to proteins such as albumin. Ionized calcium is the physiologically active form of circulating calcium. About half of the total amount of calcium circulates as free ions that participate in blood coagulation, neuromuscular conduction, intracellular regulation, glandular secretion, and control of skeletal and cardiac muscle contractility. Calcium levels are regulated largely by the parathyroid glands and by vitamin D; calcium levels are inversely proportional to PTH levels. Vitamin D enhances GI absorption of calcium. Compared to total calcium level, ionized calcium is a better measurement of calcium metabolism. Ionized calcium levels are not influenced by protein concentrations, as seen in patients with hypoalbuminemia, chronic renal failure, nephrotic syndrome, malabsorption, and multiple myeloma. Levels are also not affected in patients with metabolic acid-base balance disturbances. Elevations in ionized calcium may be seen when the total calcium is normal. Measurement of ionized calcium is useful to monitor patients undergoing cardiothoracic surgery or organ transplantation. It is also useful in the evaluation of patients in cardiac arrest.

This procedure is contraindicated for



  • Detect ectopic parathyroid hormone (PTH)–producing neoplasms
  • Evaluate the effect of protein on calcium levels
  • Identify individuals with hypocalcemia
  • Identify individuals with toxic levels of vitamin D
  • Investigate suspected hyperparathyroidism
  • Monitor patients with renal failure or organ transplantation in whom secondary hyperparathyroidism may be a complication
  • Monitor patients with sepsis or magnesium deficiency

Potential diagnosis

Increased in

  • Hyperparathyroidism (related to increased PTH)
  • PTH-producing neoplasms (PTH increases calcium levels)
  • Vitamin D toxicity (related to increased absorption of calcium)

Decreased in

    Burns, severe (related to increased amino acid release) Hypoparathyroidism (primary) (related to decreased PTH) Magnesium deficiency (inhibits release of PTH) Multiple organ failure Pancreatitis (associated with saponification or binding of calcium to fats in tissue surrounding the pancreas) The postdialysis period (result of low-calcium dialysate administration) The postsurgical period (i.e., major surgeries) (related to decreased PTH) The post-transfusion period (result of the use of citrated blood product preservative [calcium chelator]) Premature infants with hypoproteinemia and acidosis (related to alterations in transport protein levels) Pseudohypoparathyroidism (related to decreased PTH) Sepsis (related to decreased PTH) Trauma (related to decreased PTH) Vitamin D deficiency (related to decreased absorption of calcium)

Critical findings

  • Less than 3.2 mg/dL (SI: Less than 0.8 mmol/L)
  • Greater than 6.2 mg/dL (SI: Greater than 1.6 mmol/L)
  • Note and immediately report to the health-care provider (HCP) any critically increased or decreased values and related symptoms.

  • It is essential that a critical finding be communicated immediately to the requesting health-care provider (HCP). A listing of these findings varies among facilities.

  • Timely notification of a critical finding for lab or diagnostic studies is a role expectation of the professional nurse. Notification processes will vary among facilities. Upon receipt of the critical value the information should be read back to the caller to verify accuracy. Most policies require immediate notification of the primary HCP, Hospitalist, or on-call HCP. Reported information includes the patient’s name, unique identifiers, critical value, name of the person giving the report, and name of the person receiving the report. Documentation of notification should be made in the medical record with the name of the HCP notified, time and date of notification, and any orders received. Any delay in a timely report of a critical finding may require completion of a notification form with review by Risk Management.

  • Observe the patient for symptoms of critically decreased or elevated calcium levels. Hypocalcemia is evidenced by convulsions, arrhythmias, changes in electrocardiogram (ECG) in the form of prolonged ST segment and Q-T interval, facial spasms (positive Chvostek’s sign), tetany, lethargy, muscle cramps, numbness in extremities, tingling, and muscle twitching (positive Trousseau’s sign). Possible interventions include seizure precautions, increased frequency of ECG monitoring, and administration of calcium or magnesium.

  • Severe hypercalcemia is manifested by excessive thirst, polyuria, constipation, changes in ECG ( shortened QT interval due to shortening of the ST segment and prolonged PR interval), lethargy, confusion, muscle weakness, joint aches, apathy, anorexia, headache, nausea, vomiting, and ultimately may result in coma. Possible interventions include the administration of normal saline and diuretics to speed up excretion or administration of calcitonin or steroids to force the circulating calcium into the cells.

Interfering factors

  • Drugs that may increase calcium levels include antacids (some), calcitriol, and lithium.
  • Drugs that may decrease calcium levels include calcitonin, citrates, foscarnet, and pamidronate (initially).
  • Calcium exhibits diurnal variation; serial samples should be collected at the same time of day for comparison.
  • Venous hemostasis caused by prolonged use of a tourniquet during venipuncture can falsely elevate calcium levels.
  • Patients on ethylenediaminetetraacetic acid (EDTA) therapy (chelation) may show falsely decreased calcium values.
  • Specimens should never be collected above an IV line because of the potential for dilution when the specimen and the IV solution combine in the collection container, falsely decreasing the result. There is also the potential of contaminating the sample with the substance of interest if it is present in the IV solution, falsely increasing the result.

Nursing Implications and Procedure


  • Positively identify the patient using at least two unique identifiers before providing care, treatment, or services.
  • Patient Teaching: Inform the patient this test can assist in evaluating the level of blood calcium.
  • Obtain a history of the patient’s complaints, including a list of known allergens, especially allergies or sensitivities to latex.
  • Obtain a history of the patient’s cardiovascular, gastrointestinal, genitourinary, hematopoietic, hepatobiliary, and musculoskeletal systems, as well as results of previously performed laboratory tests and diagnostic and surgical procedures.
  • Note any recent procedures that could interfere with test results.
  • Obtain a list of the patient’s current medications, including herbs, nutritional supplements, and nutraceuticals (see Effects of Natural Products on Laboratory Values online at DavisPlus).
  • Review the procedure with the patient. Inform the patient that specimen collection takes approximately 5 to 10 min. Address concerns about pain and explain that there may be some discomfort during the venipuncture.
  • Sensitivity to social and cultural issues, as well as concern for modesty, is important in providing psychological support before, during, and after the procedure.
  • Note that there are no food, fluid, or medication restrictions unless by medical direction.


  • Potential complications: N/A
  • Avoid the use of equipment containing latex if the patient has a history of allergic reaction to latex.
  • Instruct the patient to cooperate fully and to follow directions. Direct the patient to breathe normally and to avoid unnecessary movement.
  • Observe standard precautions, and follow the general guidelines in Patient Preparation and Specimen Collection. Positively identify the patient, and label the appropriate specimen container with the corresponding patient demographics, initials of the person collecting the specimen, date, and time of collection. Perform a venipuncture and, without using a tourniquet, collect the specimen.
  • Remove the needle and apply direct pressure with dry gauze to stop bleeding. Observe/assess venipuncture site for bleeding or hematoma formation and secure gauze with adhesive bandage.
  • The specimen should be stored under anaerobic conditions after collection to prevent the diffusion of gas from the specimen. Falsely decreased values result from unstoppered specimens. Promptly transport the specimen to the laboratory for processing and analysis.


  • Inform the patient that a report of the results will be made available to the requesting HCP, who will discuss the results with the patient.
  • Nutritional Considerations: Patients with abnormal calcium values should be informed that daily intake of calcium is important even though body stores in the bones can be called on to supplement circulating levels. Dietary calcium can be obtained from animal or plant sources. Milk and milk products, sardines, clams, oysters, salmon, rhubarb, spinach, beet greens, broccoli, kale, tofu, legumes, and fortified orange juice are high in calcium. Milk and milk products also contain vitamin D and lactose, which assist calcium absorption. Cooked vegetables yield more absorbable calcium than raw vegetables. Patients should be informed of the substances that can inhibit calcium absorption by irreversibly binding to some of the calcium, making it unavailable for absorption, such as oxalates, which naturally occur in some vegetables (e.g., beet greens, collards, leeks, okra, parsley, quinoa, spinach, Swiss chard) and are found in tea; phytic acid, found in some cereals (e.g., wheat bran, wheat germ); phosphoric acid, found in dark cola; and insoluble dietary fiber (in excessive amounts). Excessive protein intake can also negatively affect calcium absorption, especially if it is combined with foods high in phosphorus and in the presence of a reduced dietary calcium intake.
  • Reinforce information given by the patient’s HCP regarding further testing, treatment, or referral to another HCP. Answer any questions or address any concerns voiced by the patient or family.
  • Depending on the results of this procedure, additional testing may be performed to evaluate or monitor progression of the disease process and determine the need for a change in therapy. Evaluate test results in relation to the patient’s symptoms and other tests performed.

Related Monographs

  • Related tests include albumin, ALP, calcitonin, calcium, calculus kidney stone panel, gastrin and gastrin stimulation, magnesium, PTH, parathyroid scan, phosphorus, potassium, protein total, sodium, thyroglobulin, US thyroid and parathyroid, UA, and vitamin D.
  • Refer to the Cardiovascular, Gastrointestinal, Genitourinary, Hematopoietic, Hepatobiliary, and Musculoskeletal systems tables at the end of the book for related tests by body system.
Handbook of Laboratory and Diagnostic Tests, © 2013 Farlex and Partners
References in periodicals archive ?
Serum calcium, ionized calcium, and 24-hour urinary calcium and phosphate excretion did not significantly change with lisinopril therapy (Table 3).
Measured tests include pH, PC[O.sub.2], P[O.sub.2], S[O.sub.2]%, hematocrit and hemoglobin, sodium potassium, chloride, ionized calcium, ionized magnesium, glucose, BUN, creatinine, and lactate.
Serum total calcium, ionized calcium, and total alkaline phosphatase were within the reference intervals in all patients (data not reported); interestingly, 25-hydroxyvitamin D concentrations were <12 [micro]g/L in 12 individuals (18.7%) and <15 [micro]g/L in 23 individuals (35.9%).
Analyzer-dependent differences in results for ionized calcium, ionized magnesium, sodium, and pH.

Full browser ?