Calcium, Ionized, Serum

Test ID: 

Calcium, Ionized, Serum

CPT code:



Calcium Filterable
Calcium Unbound
Ionized Calcium

Clinical Use:

Evaluate nonbound calcium, calcium metabolism, physiologically active calcium fraction, hyperparathyroidism, ectopic hyperparathyroidism. Occasionally useful when hypercalcemia coexists with abnormal protein state such as myeloma. Useful in assessing active calcium fraction in hypoproteinemia and acidosis when calcium is low.

Additional Test Information:

Calcium in serum exists ionized, bound to organic anions such as phosphate and citrate, and bound to proteins (mainly albumin). Of these, ionized calcium is the physiologically important form. Measurement of serum ionized calcium provides insight into the effect of total protein and albumin on serum calcium levels. A patient can have high total calcium, with normal ionized calcium and increased total protein and/or albumin, as in dehydration or in myeloma. Women have greater circadian variation of ionized calcium and intact PTH than men.1 There is an inverse relationship between ionized calcium and phosphate concentration.2

Specimen Type:


Requested Volume: 

1 mL

Container Type: 

Gel-barrier tube


Centrifuge and send unopened gel-barrier tube with vacuum intact. It is critical that the vacuum not be disrupted. Exposure of the serum to room air causes a loss of carbon dioxide to the atmosphere. This changes the sample pH and can prevent the lab from measuring the ionized calcium. To ensure that testing can be performed, send a separate tube for ionized calcium and adhere a piece of labeling tape over the top of the tube with the following written on it “Ionized Calcium − DO NOT OPEN.”

Storage Instructions:

Do not freeze.
Stability Requirements
Temperature Period
Room temperature 14 days
Refrigerated 14 days

Rejection Criteria

Hemolysis; opened gel-barrier tube; gel-barrier not completely sealing the clot; plasma specimen; gross lipemia

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MCI Diagnostic

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Mon – Sat: 7AM-11PM

Sun: 7AM-3PM

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