Ionised calcium test explained: when it is used instead of total calcium

Calcium is needed for muscles, nerves, the heart, blood clotting and bone formation. Lab Tests Online UK says about 99% of the body's calcium is in the bones and less than 1% circulates in the blood. In the blood, calcium exists in three forms: free calcium, also called ionised or non-protein-bound calcium; calcium bound to protein, mainly albumin; and calcium complexed with phosphate and citrate.

Of these, only free calcium is physiologically active. That is the form the body's control systems, including parathyroid hormone and vitamin D, are really regulating. The routine laboratory report, though, usually shows a different figure.

Lab Tests Online UK describes two types of calcium test in the laboratory: a total calcium test, which measures both free and protein-bound calcium, and an ionised calcium test, which measures free calcium. Adjusted (corrected) calcium is a calculation, not a separate test.

Ionised calcium is measured only in some circumstances. Lab Tests Online UK lists major surgery, particularly after a blood transfusion, critically ill patients, patients with abnormal protein concentrations and patients with abnormal blood pH. In each of these, the adjusted calcium calculation is likely to be misleading, because the relationship between total and free calcium no longer holds.

Most of those situations arise in hospital, and adjusted calcium is the figure commonly reported otherwise. Lab Tests Online UK says additional tests such as phosphate, PTH, vitamin D and magnesium help to understand the underlying problem.

Ionised calcium is a blood test taken from a vein in the arm, but it is measured differently. Lab Tests Online UK says it is performed on specific analysers known as blood gas analysers or point of care analysers, so special sample handling is required. That is one reason it is not simply added to a routine tube.

Calcium testing generally needs no fasting or special preparation, but Lab Tests Online UK advises avoiding prolonged tourniquet use and fist clenching during collection because these may affect the generated result. This guidance is stated for calcium testing in general, so ask the person taking the sample whether the specific analyser or tube needs anything further.

Lab Tests Online UK says calcium results must be interpreted in relation to other tests, particularly urea, creatinine, phosphate, magnesium, alkaline phosphatase and PTH. A normal adjusted calcium with other normal results suggests no problem with calcium metabolism.

High calcium is called hypercalcaemia. It names hyperparathyroidism and cancer as two of the main causes, and also lists hyperthyroidism, sarcoidosis, tuberculosis, lithium treatment, prolonged immobilisation, acute kidney injury and excess vitamin D intake. Low calcium is hypocalcaemia, where the most common cause is low protein, especially albumin, and other causes include low vitamin D, low dietary calcium, underactive parathyroid glands, magnesium deficiency, pancreatitis, chronic kidney disease and bone disease such as osteomalacia.

The NHS notes that many people with hyperparathyroidism have no or very mild symptoms and may only find out after tests for something else, and that it can be diagnosed with blood and pee samples. Lab Tests Online UK says if calcium falls slowly many people have no symptoms at all.

Lab Tests Online UK lists the symptoms of excess calcium as thirst, frequent or excessive urination, constipation, tiredness, weakness, loss of appetite or nausea and vomiting, and those of low calcium as abdominal or muscle cramps, tingling fingers, numbness around the mouth, or convulsions. These symptoms are not specific, and they are not a reason to request an ionised calcium test yourself.

It also says the test cannot be done at home: hand-held instruments exist, but they are intended for hospital or GP surgery use and must be operated by trained personnel. If you have symptoms, see a GP, who will decide which calcium test is appropriate.