What does the Calcium, Serum (Total) test measure?

Why is Calcium, Serum (Total) measured?

Serum calcium within the CMP provides the calcium homeostasis screen that triggers the PTH/vitamin D investigation pathway when abnormal. Hypercalcaemia (>10.2–10.5 mg/dL) — the most common cause of which is primary hyperparathyroidism in outpatients and malignancy in inpatients — warrants PTH measurement as the next step (see BML-THY-019). Hypocalcaemia (<8.5 mg/dL, or corrected for albumin) indicates PTH insufficiency, vitamin D deficiency, hypomagnesaemia, or hungry bone syndrome post-parathyroidectomy. In all CMP calcium interpretation, albumin must be checked simultaneously — hypoalbuminaemia lowers total calcium without affecting ionised calcium, producing false apparent hypocalcaemia that is corrected by the albumin adjustment formula.

Reference ranges

Standard reference

8.5–10.2 mg/dL (2.13–2.55 mmol/L) # REVIEWER FLAG: Confirm Quest Diagnostics reference range for serum total calcium — the upper limit varies slightly between laboratories (some use 10.5 mg/dL). Confirm critical value thresholds (<7.0 mg/dL and >12.0 mg/dL). Note that total calcium must be albumin-corrected when albumin is low — corrected calcium = measured calcium + 0.8 × (4.0 − albumin g/dL).

Functional medicine optimal

9.0–9.8 mg/dL (functional medicine mid-reference range optimal, with values consistently approaching the upper reference limit suggesting early primary hyperparathyroidism warranting PTH measurement). # REVIEWER FLAG: Not standardised as a clinical target — reviewer to confirm framing.

Units

mg/dL, mmol/L

Reference ranges vary by laboratory and assay platform. Interpret in context of the issuing laboratory's stated range.