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Calcium Blood Test
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Lab Tests

Calcium Blood Test

A calcium blood test measures the amount of calcium in your blood to help diagnose conditions affecting your bones, kidneys, thyroid, and parathyroid glands.

Overview

Calcium is a vital mineral; while most is stored in bones and teeth, about 1% circulates in the blood. This blood calcium is essential for nerve function, muscle contraction, heart health, and hormone release. A calcium blood test helps identify imbalances that may signal underlying medical conditions.

Types of Calcium Blood Tests

  • Total calcium test: Measures all calcium in the blood, including "bound calcium" (attached to proteins) and "free calcium" (ionized). This is the most common test and is often part of a basic metabolic panel (BMP) or comprehensive metabolic panel (CMP).
  • Ionized calcium test: Measures only the "free calcium" that is not attached to proteins. This is typically ordered if total calcium results are abnormal or if the patient has a condition affecting protein-calcium balance.

Why do I need this test?

Your provider may order this test as part of a routine checkup or to investigate symptoms of abnormal calcium levels.

Symptoms of high calcium (hypercalcemia) include constipation, nausea, abdominal pain, loss of appetite, increased thirst, frequent urination, kidney stones, bone/muscle weakness, fatigue, and mental health changes like depression or confusion.

Symptoms of low calcium (hypocalcemia) include dry skin, brittle nails, muscle cramps or spasms, tingling in the extremities, irregular heartbeat, and in severe cases, seizures.

What happens during the test?

A blood sample is collected from a vein in your arm using a small needle. The procedure usually takes less than five minutes. While no special preparation is typically required, you should inform your provider of all medications and supplements you take, as some (like vitamin D) can affect results.

Understanding Results

High levels (hypercalcemia) may indicate overactive parathyroid glands (hyperparathyroidism), certain cancers, bone disorders like Paget's disease, or excessive vitamin D intake.

Low levels (hypocalcemia) may indicate low blood protein, underactive parathyroid glands (hypoparathyroidism), vitamin D or magnesium deficiency, pancreatitis, or kidney disease.

Note: This test does not measure bone density. For bone health assessment, a DEXA scan is required.

Frequently Asked Questions

Fasting is generally not required for a standard calcium blood test. You can eat and drink normally before the appointment. However, if the calcium test is part of a larger panel that includes glucose or lipid measurements, your provider may request fasting. Always follow the specific instructions given by your healthcare team regarding food and fluid intake prior to your appointment.
Total calcium measures both bound and free calcium, but only the free (ionized) portion is biologically active. Conditions that affect protein levels, such as liver disease, nephrotic syndrome, or heavy protein intake, can change how much calcium is bound to albumin. In these cases, total calcium may appear normal while the active ionized calcium is dangerously low or high. The ionized test provides a more precise picture of your physiological calcium status when symptoms persist despite normal total levels.
Dehydration can lead to a falsely elevated total calcium reading because the blood volume decreases, concentrating the minerals in the plasma. This is known as hemoconcentration. To avoid this, it is recommended to stay well-hydrated with water in the 24 hours prior to the blood draw, unless your provider has given specific fluid restriction instructions for other reasons.
Several substances can alter calcium levels. High doses of vitamin D, thiazide diuretics (like hydrochlorothiazide), and lithium can raise blood calcium, potentially mimicking hyperparathyroidism. Conversely, bisphosphonates (used for osteoporosis), certain chemotherapy drugs, and severe dehydration can lower levels. Always disclose your full medication list, including over-the-counter supplements, to your provider before the test to ensure accurate interpretation.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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