Calcium blood test: reading your result in pregnancy and menopause

The test measures total calcium in your blood, in mmol/l. That total has three parts: proteins, above all albumin, bind about 45 percent of it, another 45 percent is free or ionised calcium, the active form, and the remaining 10 percent is bound to other small molecules.

Because almost half rides on albumin, total calcium tracks the albumin level: when albumin falls, measured total calcium falls with it, while free calcium can stay normal. That is why laboratories sometimes report a corrected calcium. For every 10 g/l that albumin sits below 40 g/l, total calcium reads about 0.2 mmol/l lower, and a common formula is corrected calcium = measured calcium + 0.02 x (40 - albumin in g/l), with slightly different constants per laboratory. Studies show this formula often misclassifies calcium status, especially when albumin is below 30 g/l; ionised calcium is the better measurement then, and the Dutch NVKC gives 1.15 to 1.33 mmol/l for it in venous blood.

For adults the NUMBER study gives 2.18 to 2.55 mmol/l, the NVKC gives 2.10 to 2.55 mmol/l, and the laboratory that runs this test prints 2.15 to 2.55 mmol/l on your report. The limits on your own report take precedence.

The blood draw matters too. A tourniquet left on for long can raise total calcium because the blood concentrates locally under the band; within about a minute the effect is negligible, and posture during the draw also shifts the value, so a short tourniquet time is advised.

The parathyroid hormone PTH and vitamin D keep blood calcium within a narrow range, and to hold it there the body draws calcium from bone when needed. A normal result therefore says nothing about your bone density.

In pregnancy the blood is diluted, so albumin falls. Total calcium falls with it and sometimes drops below the reference range, while ionised calcium stays stable. The albumin-corrected value actually rises artificially in pregnancy. Both routine numbers give a distorted picture then. During pregnancy ionised calcium is therefore the better measurement, and a low total calcium does not automatically mean your active calcium is low.

After menopause, bone loss can occur, but blood calcium does not show it; bone density is measured with a DEXA scan. An overactive parathyroid gland, primary hyperparathyroidism, is three to four times more common in women than men and mainly affects postmenopausal women, with a peak between the ages of 50 and 60. If your calcium is raised after menopause, PTH is measured alongside it. A result alone does not make a diagnosis; your GP does that with follow-up tests.

If you take vitamin D or calcium supplements, know that too much vitamin D or calcium carbonate can raise your calcium. So discuss your supplements with your GP, especially with a high result. Never increase a supplement yourself based on a low result. A calcium level that stays abnormal is a reason to see your GP. They read it alongside albumin, PTH, vitamin D, phosphate, magnesium and kidney function.

You can have calcium measured when your symptoms fit a low or high level, and a test is also useful if you take vitamin D or calcium supplements or medicines that raise or lower calcium, such as diuretics, lithium and proton pump inhibitors.

Which measurement fits best depends on your life stage. Before pregnancy, total calcium read together with albumin against the adult range of 2.18 to 2.55 mmol/l is enough. During pregnancy, ionised calcium is the better measurement, because albumin falls, total calcium can drop below the range and corrected calcium rises artificially. After menopause, total calcium is enough again, or ionised calcium if your albumin is below 30 g/l, because the albumin correction often misclassifies at low albumin.

With a low calcium, have magnesium checked as well, because low magnesium makes the body less sensitive to PTH and can block its release, so calcium only recovers once magnesium is corrected. The complete metabolic panel brings calcium, albumin and creatinine together in one draw. You order online, without a GP referral. Blood is drawn at a partner location of a network of independent, certified laboratories, the result follows within about 120 hours, and a BIG-registered doctor assesses every result.