Corrected Calcium Calculator
Adjust a total serum calcium for the patient's albumin, so that a low albumin does not fool you into treating a calcium that was never actually low.
Enter the values
Normal is roughly 8.5 to 10.2 mg/dL, but use your own laboratory's range.
Normal reference albumin is 4.0 g/dL.
Corrected calcium
•
Enter valuesWhy the correction exists
About 40 percent of the calcium in your blood is bound to protein, and almost all of that is bound to albumin. Another 10 percent or so is complexed to anions like citrate and phosphate. Only the remaining half, the ionized fraction, is physiologically active. It is the only part that runs your neuromuscular junctions and your cardiac conduction.
The laboratory reports total calcium: bound plus complexed plus ionized. So in a patient with a low albumin, there is less protein to carry calcium, the total falls, and the number on the chemistry panel looks alarming while the ionized fraction, the part that matters, has not moved at all.
This is not a rare situation. Every cirrhotic, every nephrotic, every malnourished inpatient and most critically ill patients have a low albumin. Correcting is the difference between recognizing a normal calcium and treating a laboratory artifact.
A typical ward example
A 71 year old admitted with cirrhosis and ascites. Calcium 7.6 mg/dL, which flags low on the panel. Albumin 2.1 g/dL.
Corrected calcium = 7.6 + 0.8 x (4.0 - 2.1) = 7.6 + 1.52 = 9.1 mg/dL. Entirely normal. No calcium is needed, and giving it would have been treating a number rather than a patient.
Where the correction fails
The albumin correction is a rule of thumb from the 1970s, and it is genuinely unreliable in exactly the patients who are sickest. Studies in critical illness and in chronic kidney disease have repeatedly shown it misclassifies calcium status in a substantial fraction of cases.
The patient is critically ill or septic. They have chronic kidney disease, and especially if they are on dialysis. They have received a large-volume transfusion, because citrate in banked blood chelates calcium and drops the ionized fraction without touching the total. They have an acid-base disturbance, because alkalosis increases protein binding and lowers ionized calcium at any given total. They have a paraprotein. Or the corrected value does not fit the clinical picture.
An ionized calcium measures the thing you actually care about. When the answer matters, measure it rather than estimating it.
One more effect worth carrying: pH changes ionized calcium without changing total calcium. Alkalosis increases albumin binding and lowers the ionized fraction. This is why a hyperventilating patient develops perioral tingling and carpopedal spasm with a perfectly normal total calcium, and it is why correcting a respiratory alkalosis is the treatment rather than giving calcium.
Working up a genuinely low calcium
Once you have confirmed hypocalcemia is real, the differential is short and the history usually gives it away.
- Post-surgical hypoparathyroidism. Thyroidectomy or parathyroidectomy. Check the neck for a scar, check the operative date.
- Vitamin D deficiency. Common, and the most likely cause in an outpatient.
- Hypomagnesemia. Critical to catch, because you cannot correct the calcium until you correct the magnesium. Low magnesium both impairs parathyroid hormone release and blocks its effect at the receptor. Check a magnesium on every hypocalcemia.
- Chronic kidney disease, through phosphate retention and reduced calcitriol.
- Acute pancreatitis, through saponification of calcium in the retroperitoneum.
- Massive transfusion, through citrate chelation.
- Tumor lysis syndrome, through hyperphosphatemia.
Examine for Chvostek and Trousseau signs, and get an EKG looking for a prolonged QT.
Working up a high calcium
Over 90 percent of hypercalcemia is either primary hyperparathyroidism or malignancy, and the two are usually easy to separate.
| Finding | Primary hyperparathyroidism | Malignancy |
|---|---|---|
| Onset | Chronic, often incidental | Rapid, symptomatic |
| Calcium level | Usually mildly elevated | Often markedly elevated |
| PTH | High or inappropriately normal | Suppressed |
| Setting | Ambulatory, well | Known cancer, weight loss, unwell |
The first test is a parathyroid hormone level drawn at the same time as the calcium. A suppressed PTH with a high calcium sends you looking for malignancy, and then for PTH-related peptide, vitamin D metabolites, and a myeloma workup. Do not forget thiazides, lithium and excessive calcium or vitamin D supplementation, which account for most of the remainder.
When to treat, and how fast
Symptoms and rate of change matter far more than the absolute number. A calcium of 12 that arrived over a year in an asymptomatic outpatient is a clinic problem. A calcium of 12 that arrived this week in a patient who is confused is an admission.
For symptomatic hypocalcemia with tetany, seizure or a prolonged QT, intravenous calcium gluconate is the standard first move, with magnesium replaced alongside it. For hypercalcemia, the first and most important intervention is volume expansion with isotonic saline, because these patients are almost always profoundly dehydrated from a calcium-induced nephrogenic diabetes insipidus. Calcitonin works fast and wears off fast. Bisphosphonates work slowly and last. Loop diuretics are not first-line and should not be given until the patient is volume replete.
See common night calls for the overnight version of both problems, and the clinical calculators index for related tools.
This is a teaching tool, not a clinical decision system. The albumin correction is an estimate and it performs poorly in critical illness and renal disease. When the answer matters, measure an ionized calcium. Follow your own institution's protocols.