Calcium just outside the range: reading your result with albumin

A standard test measures the total calcium in your blood. That total has three parts: roughly 45 percent sticks to proteins, mostly albumin, another 45 percent or so is free, and that free part is called ionised calcium, the active form; the last 10 percent is bound to other small molecules. Your result is the sum of all three.

The reference values for total calcium come from a Dutch study of 7.6 million GP test results, and they depend on your age: 2.18 to 2.55 mmol/l from 19, 2.23 to 2.57 mmol/l from 13 to 19 and 2.29 to 2.56 mmol/l from 6 to 13. The NVKC, the Dutch society for clinical chemistry, gives 2.10 to 2.55 mmol/l for adults, and the lab that runs this test prints 2.15 to 2.55 mmol/l on your report. For free calcium in blood from a vein, the NVKC gives 1.15 to 1.33 mmol/l. Reference values therefore differ per source and per laboratory, and the limits on your own report are the ones that count.

Your body keeps blood calcium within narrow limits. Parathyroid hormone (PTH) and vitamin D control this together, and when needed your body takes calcium from your bones to do it. So a normal result tells you nothing about your bone density.

A calcium result that is slightly off is not always a calcium problem. Almost half of your calcium rides on proteins, mostly albumin, so your total calcium moves up and down with your albumin, and a low albumin makes your calcium read lower while your free calcium can still be normal.

Corrected calcium is a way to estimate this. For every 10 g/l that albumin sits below 40 g/l, total calcium drops by about 0.2 mmol/l, and a common formula is measured calcium + 0.02 x (40 - albumin in g/l). An example: your calcium is 2.12 mmol/l and your albumin is 34 g/l, so you work out 2.12 + 0.02 x (40 - 34) = 2.24 mmol/l, which falls inside the adult reference range. Laboratories use slightly different numbers in the formula, and the formula is only an estimate: studies show it often puts people in the wrong category, most likely when albumin is below 30 g/l. In that situation, measuring free (ionised) calcium directly is more reliable, because the lab then measures the part that is not bound to protein.

A slightly high result can also come from the blood draw. If the tourniquet stays on for a long time, the blood in your arm becomes more concentrated and total calcium reads higher; within about one minute the effect is negligible, and your posture during the draw can also shift the value. So a value just outside the range is often retested, ideally with a calm draw and a short tourniquet. If it is still off after that repeat test, it is time to see your GP, who reads the result together with other blood values.

You can have calcium measured as part of a general health check, ideally together with albumin, because without albumin a slightly abnormal calcium is hard to read. The complete metabolic panel measures calcium together with albumin and creatinine.

If your calcium was slightly high or low on an earlier test, a repeat test makes sense; ask at the draw whether the tourniquet can stay on briefly, because a long tourniquet time can push calcium up. If your calcium is low, have your magnesium checked too, because with low magnesium your body responds less well to PTH and calcium only recovers once magnesium is back to normal.

If your calcium stays abnormal, see your GP, who will look at albumin, PTH, vitamin D, kidney function, phosphate and magnesium; an abnormal calcium is not a value to interpret on your own. You order the blood test online, without a referral from your GP, your blood is drawn at a partner location in a network of independent, certified laboratories, the result follows within about 120 hours, and a BIG-registered doctor assesses every result.