From the Guidelines
The normal calcium level in the blood is between 8.6 to 10.3 mg/dL (2.15 to 2.57 mmol/L) for adults. This range is based on the most recent and highest quality study available, which provides a clear guideline for calcium levels in adults 1. The calcium in blood exists in three distinct fractions: protein-bound calcium (40%), free (formerly called ionized) calcium (48%), and calcium complexed with various anions such as phosphate, lactate, citrate, and bicarbonate (12%).
- Key points to consider when interpreting calcium levels include:
- Albumin levels, as low albumin can lead to falsely low total calcium readings
- The need to correct total calcium for abnormal albumin or to calculate free calcium, especially in patients with low plasma levels of albumin
- The impact of pH on ionized calcium, with a fall in pH causing a rise in the concentration of ionized calcium
- It is essential to note that there are no biochemical measurements that reflect calcium nutritional status in normal subjects and in patients with kidney disease, and the major indirect measures of calcium adequacy are skeletal health assessed by risk of fractures, bone mass measurements, and desirable rates of calcium retention in bone 1.
- Calcium homeostasis is tightly regulated by parathyroid hormone, vitamin D, and other factors to maintain levels within this narrow range, as calcium is essential for numerous bodily functions including muscle contraction, nerve transmission, blood clotting, and bone formation.
From the Research
Normal Calcium Levels
The normal calcium level in the blood can be defined in several ways, depending on the source and methodology used.
- Total calcium levels are typically considered normal when they fall within the range of 8.5 to 10.5 mg/dL 2.
- Ionized calcium, which is the biologically active form of calcium, is usually considered normal when it falls within the range of 4.64 to 5.28 mg/dL 3.
- Corrected total calcium, which takes into account the level of albumin in the blood, is typically considered normal when it falls within the range of 2.11 to 2.63 mmol/L 3.
Factors Affecting Calcium Levels
Several factors can affect calcium levels in the blood, including:
- Hormonal controls, such as parathyroid hormone (PTH), 1,25-dihydroxyvitamin D, and calcitonin 4
- Dietary intake of calcium and vitamin D
- Kidney function and bone health
- Certain medical conditions, such as hyperparathyroidism or hypervitaminosis D [(2,5)]
Clinical Significance
Abnormal calcium levels can have significant clinical implications, including: