Calcium, total

Also known as Ca, serum calcium

Ca²⁺ inorganic ion

LOINC 17861-6 code verified serum mg/dL

What this substance is

formula: Ca²⁺
mass: 40.078 g/mol
monoisotopic: 39.96149
charge: 2
nucleus: 20 protons, 20 neutrons (40Ca)
electrons: 18: 1s² 2s² 2p⁶ 3s² 3p⁶
InChIKey: BHPQYMZQTOCNFJ-UHFFFAOYSA-N
ChEBI: CHEBI:29108

Drawn from ChEBI's own curated 2D drawing, accepted only after its InChIKey, formula and charge matched the ChEBI release. Nothing here is hand-entered. Two masses are given because they answer different questions: the average is natural-abundance weighted and drives unit conversion; the monoisotopic value uses only the commonest isotope of each element and is what mass spectrometry measures.

3D model · Calcium, total

A schematic atom: a nucleus of 20 protons and 20 neutrons (the most common isotope, 40Ca), inside its electron shells, 1s² 2s² 2p⁶ 3s² 3p⁶. The configuration matches the NIST Atomic Spectra Database ground state of Ca III (1S0). The cloud is drawn as nested spheres because that term is an S state: the electron density really is spherical. Not to scale: shells are spaced on a square-root scale so the inner ones stay visible, and the nucleus is magnified about ten-thousand-fold.

Reference ranges

Checking your own result against this range needs the running application; this is a static copy. Nothing you type anywhere in Empeiria is ever sent to a server.

This open edition shows a range only where a public-domain or openly licensed source states one, and none does for this marker yet. Compare your result with the range printed on your own lab report.

How it is measured

In practice

A health worker takes a small blood sample from a vein in your arm with a needle. It usually takes less than five minutes. You usually don't need to get ready, but some medicines or supplements, such as vitamin D, can matter. Most often the test measures total calcium; a separate test measures only the "free" calcium.

Total calcium is the usual test, typically within a BMP or CMP; ionized ("free") calcium is a separate test. Specimen: serum or lithium heparin plasma by venipuncture (urine also acceptable); EDTA or citrate plasma is unacceptable. Separated serum or plasma should be removed from cells within one hour. No special preparation is usually needed, though some medicines or supplements such as vitamin D may need to be stopped on the provider's advice.

Before the test

Tell your provider about all medicines and supplements you take, such as vitamin D; do not stop any unless your provider tells you to.
Fast (no food or drink) for several hours only if your provider has ordered other tests that need it.

Where: laboratory

Sources: Calcium Blood Test, MedlinePlus (U.S. National Library of Medicine); NHANES 2017-2018 Laboratory Procedure Manual, Total Calcium (CDC National Center for Health Statistics) cited · not yet checked by a person

In the lab

enzymatic colorimetric

Arsenazo III / o-cresolphthalein complexone

A dye that changes colour when it binds calcium is added, and the colour is measured.

Calcium binds a dye such as arsenazo III or o-cresolphthalein complexone to form a coloured complex, which is measured by photometry.

Why labs disagree: Total calcium depends on albumin, since about 40 percent of the calcium in blood is bound to protein. When albumin is low the total reads low even though the biologically active free calcium may be normal. Correction formulas exist but often disagree with measured free calcium, so when it matters, ionised calcium should be measured directly.

Sources: Comparison Between Measured and Calculated Free Calcium Values at Different Serum Albumin Concentrations (2016), PMC open access; NHANES 2017-2018 Laboratory Procedure Manual: Total Calcium, CDC/NCHS cited · not yet checked by a person

What it reports on

parathyroid gland regulated by
The parathyroid glands make PTH, the hormone that controls the calcium level in the blood.
bone element stored by
Only about 1% of the body's calcium is in the blood; the rest is stored in bones and teeth.
kidney regulated by
Under PTH's direction, the kidneys keep calcium in the blood instead of losing it in urine.

Associated symptoms

Conditions this marker features in

When high

high High blood calcium · defining
Defined as serum calcium above 10.5 mg/dL: mild 10.5-12, moderate 12.1-14, severe above 14 mg/dL.
high Overactive parathyroid gland · defining
Fifth International Workshop (2022) diagnostic criterion: albumin-adjusted calcium above normal with a high or inappropriately normal PTH, twice at least 2 weeks apart. In the normocalcaemic variant, calcium is normal.
high High blood sodium · supportive
Checked as part of the work-up. High calcium (above 11 mg/dL) can stop the kidneys concentrating urine, leading to water loss.

When low

low Low blood calcium · defining
Serum calcium, adjusted for albumin, below the reference range. Severity grading (Society for Endocrinology emergency guidance): severe is below 1.9 mmol/L or any low level with symptoms; mild is above 1.9 mmol/L without symptoms.
low Long-term reduced kidney function · monitoring
Checked regularly from CKD stage G3a onward. A low calcium is a classic feature of untreated CKD.
low Low blood magnesium · supportive
Low calcium is common with low magnesium. It does not respond to calcium or vitamin D alone; magnesium treatment corrects it.
low Low vitamin D · supportive
Usually kept normal by parathyroid hormone; it falls in rickets or osteomalacia from long-standing deficiency.

Ordered as part of

cmp · bmp · renal