Calcium Blood Test: Corrected Calcium, Ionised Calcium and PTH Explained
Total calcium can mislead when albumin is abnormal — always use corrected calcium. Learn hypercalcaemia and hypocalcaemia workups.
Total calcium changes with albumin. Always calculate corrected calcium: Ca + 0.02 × (40 − albumin g/L). Normal corrected calcium is 2.20–2.60 mmol/L. Corrected calcium > 2.60 mmol/L is hypercalcaemia (usually primary hyperparathyroidism or malignancy). < 2.20 is hypocalcaemia. Ionised calcium is the definitive test in acute settings.
Approximately 40% of serum calcium is albumin-bound, so total calcium misleads when albumin is abnormal. The corrected calcium formula (Ca + 0.02 × [40 − albumin]) is the standard adjustment. Persistent hypercalcaemia in ambulatory adults is 90% either primary hyperparathyroidism (PTH high or inappropriately normal) or malignancy (PTH suppressed). The PTH-first algorithm resolves 95% of cases: send PTH with calcium; if PTH raised → primary hyperparathyroidism, image parathyroids; if PTH suppressed → malignancy workup (myeloma screen, imaging, PTHrP). Hypocalcaemia most commonly reflects hypoalbuminaemia (correct first), then vitamin D deficiency, hypomagnesaemia (which blocks PTH release), and CKD. Ionised calcium is preferred over corrected calcium in acute care (critical illness, cirrhosis, dysproteinaemia). Symptoms of severe hypercalcaemia (> 3 mmol/L): "bones, stones, groans, thrones, psychiatric overtones."
- Normal corrected Ca
- 2.20 – 2.60 mmol/L (8.8–10.4 mg/dL)
- Formula
- Ca + 0.02 × (40 − albumin g/L)
- Hypercalcaemia > 3.0
- Symptomatic — urgent
- First workup
- PTH + vitamin D + phosphate
- Acute preferred test
- Ionised calcium
Hypercalcaemia: PTH-first algorithm
| Corrected Ca | PTH | Interpretation |
|---|---|---|
| High | High / inappropriately normal | Primary hyperparathyroidism → parathyroid imaging |
| High | Suppressed | Malignancy → myeloma screen, PTHrP, imaging |
| High | Suppressed + vit D high | Granulomatous disease (sarcoid, TB) |
| High + family history | Normal | Consider FHH (calcium:creatinine ratio urine) |
Hypocalcaemia workup
Confirm with corrected calcium or ionised calcium. Correct hypomagnesaemia first — low Mg blocks PTH secretion.
Check vitamin D, PTH, phosphate, magnesium, renal function.
Common causes: vitamin D deficiency, CKD, post-thyroidectomy hypoparathyroidism, magnesium depletion, acute pancreatitis.
- Corrected calcium > 3.0 mmol/L with symptoms — same-day IV hydration.
- Corrected calcium < 1.9 mmol/L with tetany or seizures — emergency IV calcium.
- Persistent hypercalcaemia + weight loss or bone pain — malignancy workup.
Abnormal calcium workup
- 1Calcium high?PTH + vit D + phosphate. High PTH → primary hyperparathyroidism. Suppressed PTH → malignancy screen.
- 2Calcium low?Magnesium, vitamin D, PTH, phosphate, renal function.
- 3Confusing result?Repeat with ionised calcium.
Related questions people ask
Frequently asked questions
- Always correct calcium for albumin.
- PTH-first algorithm resolves hypercalcaemia workup.
- Correct magnesium before treating hypocalcaemia.
- Ionised calcium is the definitive acute test.
- Ca > 3.0 mmol/L needs same-day treatment.
References
2 sources- BMJ Best PracticeHypercalcaemia
Clinical decision support.
bestpractice.bmj.com
- AACEPrimary Hyperparathyroidism Guideline
US endocrine society.
aace.com
Evidence Snapshot
This snapshot shows verified evidence records matched to this article. It is not a diagnosis or personal medical advice.
Total serum calcium is influenced by serum albumin, so a low albumin can lower total measured calcium without necessarily changing the biologically active ionised calcium fraction, which is why total calcium is often interpreted alongside albumin.
Limitation. Formulas for corrected calcium approximate but do not perfectly replace a direct ionised calcium measurement.
Symptoms of hypercalcaemia (such as fatigue, constipation, and confusion) and hypocalcaemia (such as muscle cramps and tingling) are nonspecific, so calcium abnormalities are often first detected on laboratory testing rather than from symptoms alone.
Limitation. Symptoms alone are neither sensitive nor specific for calcium abnormalities and cannot substitute for laboratory measurement.
Low serum magnesium can contribute to persistently low potassium and, in some cases, low calcium that do not correct as expected, because magnesium is required for normal handling of these other electrolytes.
Limitation. The clinical significance of borderline magnesium alterations varies by individual and situation.
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