I'm an adult female and have had elevated serum calcium for a few years ranging from 10.4 - 10.8 (mg/dl), however my ionized calcium from the same blood draws has been either normal or only the slightest bit above normal. My PTH ranges from normal to in the 80s (pg/ml). Which calcium is more accurate in my case - the serum total or the ionized? All other serum values are normal (metabolic, phosphorus, D.) Thank you!
Great question! Usually when we check calcium levels, we are checking the total serum calcium. This is the level that is included in a metabolic panel (basic or comprehensive), one of the standard lab tests that you will have if you are simply getting “routine labs.”
Calcium is present in your blood in two forms: 1. Ionized, or “free” calcium, that is floating around as a calcium ion. 2. Bound calcium - calcium that is bound up in something else, usually a protein like albumin, or complexed to another ion. About half of the calcium in your blood is ionized. This is the form that is available to do the things that calcium needs to do.
As mentioned above, when we check “calcium” we are usually talking about the TOTAL calcium, meaning the ionized and bound calcium levels. But it is possible to just check the ionized form. Usually, the two levels give similar information - if one is high, so is the other. But not always. There are a few scenarios that would lead you to want to use the ionized calcium rather than the total calcium.
When would the ionized calcium be more accurate than the total calcium? If your protein levels are very low, then the total calcium level is going to look low - even if the ionized “free” calcium is normal. Sometimes the total calcium will look dangerously low, but the ionized calcium is normal. Since the ionized form is the form that is available to your body, it doesn’t really matter if your bound form is low. You just need the ionized form to be normal. Note that this is pretty uncommon. Most people have normal protein levels. If you are otherwise healthy, then I would expect that your protein levels are in normal range. Note also that having “high” protein is even more rare, so “correcting down” for protein levels would only rarely be necessary.
People who have very low protein levels tend to be those in the hospital, often the ICU. These are the patients we tend to need ionized calcium levels on. For most people, it is not all that helpful.
Ionized calcium tests are also more cumbersome to process, and changes in pH of the specimen can lead to inaccuracies. These issues can make ionized calcium less reliable than the total calcium.
Back to your case: Your labs are consistent with primary hyperparathyroidism. Use the total calcium. You mentioned that the ionized is occasionally elevated, which is consistent with your elevated total calcium and consistent with primary hyperparathyroidism. You don’t need to have high calcium levels on every blood draw in order to make the diagnosis.
