Calcium blood level: normal range and what it means if you train hard

This test measures total calcium in your blood serum, in mmol/l. That total has three parts: about 45 percent is bound to proteins, mainly albumin, about 45 percent is free or ionised calcium, the active form, and the remaining 10 percent is bound to other small molecules.

For adults aged 19 and over the reference range is 2.18 to 2.55 mmol/l; for 13- to 19-year-olds it is 2.23 to 2.57 and for 6- to 13-year-olds 2.29 to 2.56 mmol/l. These limits come from the Dutch NUMBER study, based on 7.6 million GP results. The Dutch clinical chemistry society NVKC gives 2.10 to 2.55 mmol/l for total calcium, and the laboratory that runs this test prints 2.15 to 2.55 mmol/l on your report. Reference values therefore differ per source and per laboratory, and the limits on your own report apply.

Because much of it is bound to albumin, total calcium moves with albumin. For every 10 g/l that albumin sits below 40 g/l, total calcium falls about 0.2 mmol/l, and laboratories sometimes correct for this with a formula, using slightly different constants. That correction often misclassifies calcium status, most of all when albumin is below 30 g/l, and ionised calcium is then the better measurement; the NVKC gives 1.15 to 1.33 mmol/l for it in venous blood.

Calcium is regulated by parathyroid hormone (PTH) and vitamin D. When blood calcium threatens to fall, the body draws calcium from bone, which is why a normal blood value says nothing about your bone density.

Serum calcium does not measure bone stress or bone density, and for an athlete that is the first thing to know. The body keeps your calcium blood level steady by drawing calcium from bone when needed, so a normal result after a stress fracture or hard training block says nothing about your bones. Bone density is measured with a DEXA scan.

If you take supplements, the value can be informative. Too much vitamin D can raise blood calcium, and so can too much calcium carbonate or antacids. Excess calcium in the blood is called hypercalcaemia. Medicines can shift the value too: thiazide diuretics and lithium can raise calcium, loop diuretics, proton pump inhibitors, bisphosphonates and denosumab can lower it, and long immobility is among the possible causes of a high value.

With a low value, magnesium matters. Low magnesium makes the body less sensitive to PTH and can block its release, so low calcium does not recover until magnesium is corrected. That is why calcium and magnesium are read together when calcium is low.

If you want to compare two results, for example before and after a training block, look at how the blood was drawn. A long tourniquet can raise measured total calcium because the blood becomes more concentrated, and your posture during the draw also shifts the value; within about one minute of tourniquet use the effect is negligible. So have both samples taken the same way, same posture and shortest possible tourniquet, use the same laboratory where you can, and read each result against the limits on that report. A persistently high or low value is a reason to see your GP.

Have your calcium blood level measured if symptoms could fit a high or low value: persistent tiredness, strong thirst, frequent urination or constipation, and also tingling around the mouth or in the fingers and muscle cramps. A test also makes sense if you take vitamin D, calcium carbonate or antacids, or thiazide diuretics, lithium or proton pump inhibitors, because these can raise or lower the value.

After an earlier abnormal result, a repeat test under the same conditions is useful. An abnormal result is read together with albumin, PTH, vitamin D, phosphate and magnesium, and kidney function counts too, through creatinine and eGFR. After a stress fracture or with questions about your bones, this is not the right test; bone density is measured with a DEXA scan, which you discuss with your GP.

You order the test online, without a GP referral. Blood is drawn at a location within a network of independent, certified laboratories, the result follows within about 120 hours, and a BIG-registered doctor assesses every result.