Calcium:- Part 2 – Ionized (Ca), Free Calcium

Sample
- Collect blood anaerobically and draw without pressure or stasis.
- The sample is stable for 6 hours at 4 °C.
- Plasma or serum can be stored for a longer period at -20 °C.
- The non-fasting sample is acceptable.
- The prolonged tourniquet should be avoided because it will lower pH and increase calcium.
Purpose of the test (Indication):
- This test is used to determine physiologically active or free Calcium in patients with altered proteins e.g in chronic renal failure, nephrotic syndrome, malabsorption, and multiple myeloma.
- Ionized calcium values reflect calcium metabolism better than total calcium.
- A significant decrease in ionized calcium regardless of total calcium may lead to an increase in neuromuscular irritability and tetany.
- This is also advised in parathyroid diseases.
Precautions:
- Fasting specimen is preferred.
- The prolonged tourniquet should be avoided because it will lower pH and increase calcium.
- Serum pH can affect the calcium level. A decreased pH can cause an increased level of calcium.
- Venous stasis or erect posture increased the calcium level by 0.6 mg/dL.
- There is diurnal variation, higher in PM (about 9 PM) than AM (lowest level).
- Separate immediately from RBCs to avoid uptake of calcium by these cells (RBCs).
- Excessive intake of milk leads to increased calcium levels.
- Vitamin D intoxication also increases the calcium level.
- Check the albumin level because hypoalbuminemia leads to an artificial decrease in the calcium level.
- Drugs may increase the serum calcium level like calcium salts, alkaline antacids, thiazide diuretics, vitamin D, parathyroid and thyroid hormones, and androgens.
- Drugs may decrease the calcium level like aspirin, anticonvulsant, heparin, laxatives, diuretics, magnesium salts, and oral contraceptives.
Pathophysiology:
- The routine serum calcium measures total serum calcium value.
- 99% of the calcium is present in the bone structure.
- It is present in soft tissue = 1%
- It is present in extracellular fluid = 0.2%.
- Calcium in serum is:
- 50% Nonbound calcium, also called Ionized or free dialysable calcium. Free or ionized is the active form.
- 50% is bound to protein. The protein-bound fraction of calcium is bound to albumin (70% to 80%).
- 5% is complexed with non-protein compounds (ions) like citrate, phosphate, sulfate, and bicarbonate.
- Ionized calcium or free (dialysable) calcium exert its physiological effects upon:
- The neuromuscular junction.
- Membranes.
- Bone deposition.
- 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.
- In some of the patients with primary hyperparathyroidism, there is an increase in ionized calcium, but total calcium is normal.
- Ionized calcium (hypercalcemia) may be seen in:
- Multiple myeloma.
- Sarcoidosis.
- Hypervitaminosis D.
- Metastatic carcinoma infiltrates in the bone.
- Most of the labs have no facility to measure ionized calcium.
- Ionized calcium can be measured by the ion-selective electrode.
- There are formulas by which you can calculate the ionized calcium from the total serum calcium.
- Functions of the calcium:
- All body cells need calcium.
- Calcium is needed for strong muscles and bones.
- This is also important for the normal function of the heart.
- Calcium is necessary for nerve signals.
- Calcium helps in the contraction of muscles.
- Calcium maintains plasma membrane potential.
- This is a cofactor in the enzymatic reaction.
- Calcium is needed for the blood clotting mechanism.
- Calcium is needed for the blood clotting mechanism.
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).
Age | Whole blood mg/dL | Serum mg/dL | Capillary blood mg/dL |
Cord blood | 5.20 to 6.40 | ||
2 hours | 4.84 to 5.84 | ||
24 hours | 4.40 to 5.44 | 4.20 to 5.48 | |
3 days | 4.60 to 5.68 | 4.40 to 5.68 | |
5 days | 4.88 to 5.92 | 4.80 to 5.92 | |
Youth | 4.80 to 5.52 | ||
Adults | 4.64 to 5.28 | ||
18 to 60 years | 4.60 to 5.08 | ||
60 to 90 years | 4.64 to 5.16 | ||
>90 years | 4.48 to 5.28 |
- To convert into SI unit multiplying factor is x 0.25 = mmol/L
The increased level is seen in:
- Primary hyperparathyroidism.
- PTH-producing tumors.
- Various malignancies.
- Excess intake of vit.D.
The decreased level is seen in:
- primary hypoparathyroidism.
- Vitamin D deficiency.
- After blood transfusion.
- After major surgery.
- Trauma, burns, and sepsis.
- Pancreatitis.
- After hemodialysis.
- Multiple organ failures.
- Please see more details on the Calcium level (total) part 1.
Natural foods a good source of calcium:
Food | Quantity | Amount of calcium |
Kale | one cup | 245 mg |
Milk | one cup | 305 mg |
Yogurt | 6 oz | 300 mg |
Cheese | one oz | 224 mg |
Dried figs | 8 whole figs | 107 mg |
White Beans | one cup | 191 mg |
Turnip greens | one cup | 195 mg |
Black-eyed beans | 1/2 cup | 185 mg |
Canned salmon | 1/2 cup | 232 mg |
Orange juice | one cup | 500 mg |
Orange | one medium | 65 mg |
Sesame seed | one teaspoon | 88 mg |
Almond | 1/2 cup dry roasted | 72 mg |
Instant oatmeal | one cup | 187 mg |
Soy milk | one cup | 300 mg |
Firm Tofu | 1/2 cup | 861 mg |
Broccoli | one cup | 62 mg |