Measuring calcium during a weight-loss programme

This test measures the total calcium in your blood. About 45 percent of it is bound to proteins, mainly albumin, another 45 percent circulates freely as ionised calcium, the active form, and the remaining 10 percent is attached to other small molecules. Total calcium adds those three parts together, so the result moves with your albumin.

For every 10 g/l that albumin sits below 40 g/l, total calcium falls by about 0.2 mmol/l. A common correction is measured calcium plus 0.02 times (40 minus albumin in g/l), with slightly different constants per laboratory. Below an albumin of 30 g/l, the formula often misclassifies your calcium status, and ionised calcium is then the better measurement; the NVKC gives 1.15 to 1.33 mmol/l for it in venous blood.

For adults aged 19 and over, the reference range for total calcium is 2.18 to 2.55 mmol/l; these limits come from the Dutch NUMBER study of 7.6 million GP results. The NVKC gives 2.10 to 2.55 mmol/l, and the lab that runs this test prints 2.15 to 2.55 mmol/l on your report. Three sources, three slightly different ranges: so keep to the limits on your own report.

Parathyroid hormone (PTH) and vitamin D keep calcium within a narrow range. In a weight-loss programme you therefore look beyond whether you fall inside the limits, and compare each result with your own baseline from before you started.

When you eat less, calcium in your blood usually stays within range, because parathyroid hormone and vitamin D hold it steady and your body draws calcium from bone when needed to do so. A normal result therefore does not mean your bones are fine; bone density is measured with a DEXA scan, and any concern about your bones is one for your GP, who can judge whether a scan is useful.

If you eat very little protein for a long time, your albumin can drop. Total calcium then looks low while free calcium is normal, and your result looks worse than your calcium balance really is. So always read calcium next to albumin, and next to vitamin D and magnesium as well: low magnesium makes your body less sensitive to parathyroid hormone and can block its release, so a low calcium does not recover until magnesium is corrected.

Certain medicines play a part too. Thiazide diuretics, used for high blood pressure, can raise calcium, and proton pump inhibitors, which reduce stomach acid, can lower it; if you take either, mention it when you discuss your result.

An abnormal value is no diagnosis. It can be linked to many causes, from vitamin D deficiency to an overactive parathyroid gland, and your GP assesses a persistently abnormal value together with PTH, kidney function and phosphate. A baseline from before you start makes later results easier to read, because you can see whether a shift is new or was always there; compare each result with the limits on its own report, because laboratories differ.

Ideally, measure calcium before you start a weight-loss programme, because that first result becomes your own reference point. Retest at moments you agree with your prescriber or GP, and sooner if you develop symptoms that fit an abnormal calcium: for low calcium, tingling around the mouth or in the fingers and muscle cramps, for high calcium, tiredness, nausea, constipation and thirst among others. If you take a thiazide or proton pump inhibitor, ask your doctor whether a check is useful.

Test under the same conditions each time, so differences mean something. Leaving the tourniquet on for long can raise total calcium, although within about one minute that effect is negligible, and your posture during the draw can also shift the result.

An abnormal calcium is read together with albumin, PTH, vitamin D, phosphate and magnesium, and kidney function counts too, through creatinine and eGFR. If calcium stays abnormal, see your GP rather than interpreting it yourself. You order online, without a GP referral, blood is drawn at a location in a network of independent, certified laboratories, the result follows within about 120 hours, and a BIG-registered doctor assesses every result.