What is this test?
Ionized calcium is the biologically active, unbound fraction of calcium. It is useful when protein levels or critical illness make total calcium difficult to interpret.
Why might it be ordered?
- To clarify an abnormal total calcium or assess calcium in critical illness.
- To investigate parathyroid, kidney or acid-base conditions affecting active calcium.
What sample is needed?
A specially handled blood sample is required because air exposure and delays can change pH and the ionized-calcium result.
How should you prepare?
Follow the laboratory timing instructions. Tell the clinician about calcium, vitamin D and other medicines or supplements, but do not stop them on your own.
How should you understand the result?
Blood pH affects ionized calcium, so collection and prompt processing are important. Interpret it with symptoms, magnesium, phosphate and parathyroid-related tests as appropriate.
Sources and editorial status
This patient-facing text is an editorial draft. It should be approved by a laboratory physician before official publication.