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Why would my serum calcium be high but Ionized calcium low? Does low ionized calcium mean no parathyroid issue? Should I follow-up or wait for next year annual blood work? I'm 66 years old. Recent blood work showed: Serum Calcium: 10.5 mg/dl Follow-up 1 week later (non-fasting): Ionized Calcium = 1.08 Vit D = 32 ng/ml PTH = 37.8 pg/ml

Deva Boone
Answer authored by Deva Boone
Deva Boone, MD is the founder of the Southwest Parathyroid Center. As one of the most experienced parathyroid surgeons in the U.S., she has treated thousands of patients with parathyroid conditions.

Most commonly, the total serum calcium and ionized calcium will show a similar pattern - that is, they will both be on the high end or both on the low end. But not always. There are multiple factors that could make one or more of these results less reliable. Total calcium levels depend on albumin, so a low albumin can give you a falsely low calcium level. Ionized calcium levels depend on the pH of the specimen, so any pH out of range can give you a false result on the ionized calcium. Sometimes they will give you a corrected ionized calcium based on pH - and you should use that one if it is given. (Note that you do NOT always need to correct the total calcium for albumin - see my explanation here.)

When there is a discrepancy in the labs, the best thing to do is get repeat labs. You don’t need to wait until next year. Since the total calcium was elevated for your age, you should have the calcium, ionized calcium, and PTH repeated again now. If your total calcium remains above 10.0, with a non-suppressed PTH, then you may have primary hyperparathyroidism, even if the ionized calcium is in normal range.

There is always a chance that one of your results was an error. I would repeat the labs, and also look at your prior calcium levels. If you have had past calcium levels in the 10s, then it is less likely that the calcium was an error. If all of your prior calcium levels have been in the 9s, and your repeat calcium is also in the 9s, then there is a chance that the 10.5 was a mistake. I would still want to keep an eye on your labs and make sure that the calcium remained in normal range.