Ionized Calcium (Ca)

Sample for Ionized Calcium

1. Collect blood anaerobically and draw without pressure or stasis.
2. The sample is stable for 6 hours at 4 °C.
3. Plasma or serum can be stored for a longer period at -20 °C.
4. The non-fasting sample is acceptable.
   1. The prolonged tourniquet should be avoided because it lowers pH and increases calcium.

Purpose of the test (Indication) for Ionized Calcium:

1. This test determines physiologically active or free Calcium in patients with altered proteins, e.g, in chronic renal failure, nephrotic syndrome, malabsorption, and multiple myeloma.
2. Ionized calcium values reflect calcium metabolism better than total calcium.
3. A significant decrease in ionized calcium, regardless of total calcium, may lead to increased neuromuscular irritability and tetany.
4. This is also advised in parathyroid diseases.
5. Patients with hypocalcemia or hypercalcemia have borderline serum calcium and altered serum proteins.
6. Patients are receiving an intravenous blood transfusion.
7. In case of major surgery.
8. In case of abnormal blood proteins.

Precautions for calcium:

1. Magnesium affects ionized calcium, so always measure magnesium levels in case of hypocalcemia.
2. A fasting specimen is preferred.
3. The prolonged tourniquet should be avoided because it lowers pH and increases calcium.
4. Serum pH can affect the calcium level. A decreased pH can cause an increased level of calcium.
5. Venous stasis or erect posture increased the calcium level by 0.6 mg/dL.
6. Diurnal variation is higher in PM (about 9 PM) than in AM (lowest level).
7. Separate immediately from RBCs to avoid calcium uptake by these cells (RBCs).
8. Excessive intake of milk leads to increased calcium levels.
9. Vitamin D intoxication also increases the calcium level.
10. Check the albumin level because hypoalbuminemia leads to an artificial decrease in the calcium level.
11. Drugs like calcium salts, alkaline antacids, thiazide diuretics, vitamin D, parathyroid and thyroid hormones, and androgens may increase the serum calcium level.
12. Drugs like aspirin, anticonvulsants, heparin, laxatives, diuretics, magnesium salts, and oral contraceptives may decrease the calcium level.

Definition of ionized calcium (Free calcium):

1. Ionized calcium (Free calcium) is the physiologically active form of calcium.
2. The parathyroid glands, bone, kidneys, and intestine regulate ionized calcium hemostasis.
3. There is a small diurnal change in ionized calcium; the peak level is around 9  P.M., and the lowest is at 9 A.M.
4. Ionized calcium level is not affected by changes in serum albumin.

Ionized Calcium (free calcium) facts:

1. The routine serum calcium measures the total serum calcium value.
2. 99% of the calcium is present in the bones.
   1. It is present in soft tissue = 1%
   2. It is present in extracellular fluid = 0.2%.
3. Calcium in serum is:
   1. 50% Nonbound calcium, also called Ionized or free dialysable calcium. Free or ionized is the active form.
   2. 40% is bound to protein. The protein-bound fraction of calcium is bound to albumin (70% to 80%).
   3. 10% is complexed with non-protein compounds (ions) like citrate, phosphate, sulfate, and bicarbonate.
   4. Only the ionized fraction is biologically active.

4. The most valuable clinical information is provided by a knowledge of the concentration of Free (ionized) Ca⁺⁺ rather than of total Ca⁺⁺ in primary hyperparathyroidism.
5. In some patients with primary hyperparathyroidism, there is an increase in ionized calcium, but total calcium is normal.
6. Ionized calcium (hypercalcemia) may be seen in:
   1. Multiple myeloma.
   2. Sarcoidosis.
   3. Hypervitaminosis D.
   4. Metastatic carcinoma infiltrates the bone.
7. Most of the labs have no facility to measure ionized calcium.
   1. The ion-selective electrode can measure ionized calcium.
   2. There are formulas by which you can calculate the ionized calcium from the total serum calcium.
8. In critically ill patients, an increased total serum calcium level usually indicates ionized hypercalcemia.
   1. Normal total serum calcium is evidence against ionized hypocalcemia.
9. Ionized calcium is preferred over total serum calcium in some of the conditions:
   1. The liver transplantation.
   2. Rapid or large transfusion of citrated blood.
   3. In these conditions, the total serum calcium interpretation is almost impossible.
10. The increase of ions to which calcium is bound is:
    1. Bicarbonate (HCO3).
    2. Citrate during a blood transfusion.
    3. Phosphate level. In the case of phosphate therapy in diabetic ketoacidosis, rhabdomyolysis, and chemotherapy (tumor lysis syndrome).
    4. Radiographic material containing calcium chelators.

Functions of Ionized calcium or free (dialysable) calcium:

1. It exerts its physiological effects upon:
   1. The neuromuscular junction.
   2. Membranes.
   3. Bone deposition.

Normal Ionized Calcium:

Source 1

• Ionized calcium:
  • Whole blood adult = 4.65 to 5.28 mg/dL (1.175 to 1.375 mmol/L)
  • Newborn = 4.20 to 5.58 mg/dL.
  • The ratio of ionized Ca⁺⁺ to total Ca⁺⁺ = 48 to 56%.

Source 2

• Newborn = 4.20 to 5.58 mg/dL  (1.05 to 1.37 mmol/L).
• 2 months to 18 years = 4.80 to 5.52 mg/dL  (1.20 to 1.38 mmol/L).
• Adult = 4.5 to 5.6 mg/dL (1.05 to 1.3 mmol/L).

• To convert into an SI unit multiplying factor is x 0.25 = mmol/L

Measurement of ionized calcium:

1. It can be measured by the ion-selective electrode method.
   1. The above method is more accurate.
2. A less accurate method to calculate serum calcium:
   1.  Adjusted calcium = (Total serum calcium – serum albumin) + 4.0
      1. (Calcium in mg/dL and albumin in g/dL).
3. In SI unit formula = Adjusted calcium = (calcium – 0.025 albumin) + 1.0
   1. (Calcium in mmol/L and albumin in g/L).
4. Ionized calcium values are affected by the blood pH level.
   1. A decrease of 0.1 pH = an increase in ionized calcium by 1.5% to 2.5%.
   2. If serum is exposed to air and stands for a long time.

The increased level of ionized calcium is seen in the following:

1. Primary hyperparathyroidism. In 25% of the patients, it is seen that total serum calcium is normal while ionized calcium is increased.
2. PTH-producing tumors.
3. Various malignancies.
4. Excess intake of vit.D.
5. Acidosis.
6. Neonatal total serum calcium associated with hypoalbuminemia may indicate ionized hypercalcemia.

The decreased level of ionized calcium is seen in the following:

1. Primary hypoparathyroidism. It is also seen in secondary hypoparathyroidism.
2. Alkalosis.
3. Vitamin D deficiency.
4. After a blood transfusion.
5. After major surgery.
6. Trauma, burns, and sepsis.
7. Fat embolism.
8. Pancreatitis.
9. After hemodialysis.
10. Multiple organ failures.
11. Toxic shock syndrome.
12. Increased calcium-binding to albumin in conditions like:
    1. Drugs like heparin, epinephrine, norepinephrine, alcohol, and isoproterenol.
    2. Acute pancreatitis.
    3. Diabetic ketoacidosis.
    4. Sepsis.
    5. AMI.

• Please see more details on the Calcium level (total) in part 1.

Critical value:

• Ionized calcium level <2 mg/dL (life-threatening).
• Ionized calcium level <3 mg/dL in multiple blood transfusions is the indication for calcium therapy.

Natural foods are a good source of calcium:

Questions and answers: