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Electrolytes
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Currently under review
Pending specialist review and validation.
Ionized calcium is the free, biologically active form of calcium in your blood. This test measures that fraction directly from a small capillary sample, such as a fingerstick or heelstick. Because it measures the unbound form, it reflects the calcium available for muscles, nerves, and the heart to function.
Capillary ionized calcium is often performed on point-of-care analyzers that also assess blood gases and pH. Unlike total calcium, it is not affected by changes in blood protein levels, so it can provide a clearer picture when albumin is low, in critical illness, or when rapid answers are needed.
Clinicians use this test to evaluate symptoms that may relate to calcium imbalance, such as tingling, muscle cramps, or irregular heartbeat, and to monitor conditions like parathyroid disorders, kidney disease, or severe illness. It helps guide urgent treatment decisions in emergency or intensive care, during major surgery, and when receiving large volume blood transfusions.
Ionized calcium is a key part of diagnosing the cause of a calcium disorder and monitoring therapy with calcium or vitamin D. It also helps interpret acid base disturbances, since blood pH can shift how much calcium is in the ionized form.
Your result represents the amount of active calcium in your blood at the time of sampling. Low values can be associated with low parathyroid hormone levels, vitamin D deficiency, kidney problems, pancreatitis, sepsis, or shifts in pH that reduce the ionized fraction. High values can occur with overactive parathyroid glands, some cancers, vitamin D excess, prolonged immobilization, or pH shifts that increase the ionized fraction.
If your result is outside the expected range, your clinician may repeat the test, confirm with a venous sample, or order related tests such as total calcium, parathyroid hormone, magnesium, phosphorus, vitamin D, renal function, and blood gases. Treatment and follow up depend on your symptoms, trend over time, and the underlying cause. Discuss any concerns with your healthcare team.
Reference intervals vary by laboratory, analyzer, methodology, population, and units. The ranges shown here are for education only. Always interpret your results against the reference interval printed on your own lab report.
Air bubbles or an uncapped sample can raise pH and lower measured ionized calcium. Cap the sample, remove air, and analyze promptly to reduce pH shifts.
Prolonged tourniquet use and repeated fist clenching can alter pH and local chemistry. Keep tourniquet time short and avoid squeezing during collection.
Excess liquid heparin can dilute or bind calcium, and the wrong heparin type can bias results. Use balanced heparin devices designed for blood gas and electrolytes.
Cooling or letting the sample sit changes pH and protein binding. Follow analyzer instructions and test the sample as soon as possible at the recommended temperature.
Calcium supplements, vitamin D, calcimimetics, bisphosphonates, diuretics, and lithium can affect calcium balance. Citrate from recent transfusions can transiently lower ionized calcium.
Alkalosis reduces ionized calcium by increasing protein binding, while acidosis increases it. Measuring pH alongside ionized calcium improves interpretation.
Chronic kidney disease, parathyroid disorders, critical illness, and pancreatitis can shift ionized calcium. In these settings, ionized calcium is preferred over total calcium for decision making.
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