Serum calcium is a routinely measured biochemical parameter that plays a central role in the assessment of metabolic, endocrine, renal, and musculoskeletal disorders. Calcium is essential for numerous physiological processes, including neuromuscular transmission, skeletal muscle contraction, cardiac function, blood coagulation, intracellular signalling, hormone secretion, and the maintenance of bone mineralisation. Measurement of serum calcium therefore forms an important component of both routine health assessment and the investigation of acute and chronic disease.
Approximately 99% of total body calcium is stored within the skeleton, with the remaining 1% distributed between the extracellular and intracellular compartments. Within the circulation, approximately half of serum calcium exists as ionised (free) calcium, the physiologically active fraction responsible for biological function. The remainder is bound primarily to albumin or complexed with circulating anions such as phosphate and citrate.
Routine laboratory testing typically measures total serum calcium, which includes both protein-bound and ionised calcium. Because total calcium concentration is influenced by serum albumin, abnormalities in albumin levels may produce misleading results despite normal physiologically active calcium concentrations. In patients with abnormal albumin levels, a corrected calcium calculation is commonly used to estimate the expected calcium concentration after adjustment for protein binding. Where precise assessment is required, particularly in critically ill patients, direct measurement of ionised calcium provides the most accurate reflection of calcium homeostasis.
Elevated serum calcium (hypercalcaemia) has a broad differential diagnosis, with primary hyperparathyroidism and malignancy-associated hypercalcaemia accounting for the majority of cases. Other causes include excessive vitamin D intake, granulomatous disease, prolonged immobilisation, endocrine disorders, familial hypocalciuric hypercalcaemia, and certain medications such as thiazide diuretics and lithium. Clinical manifestations vary according to the severity and rate of onset but may include fatigue, polyuria, polydipsia, constipation, nausea, cognitive impairment, muscle weakness, nephrolithiasis, and cardiac arrhythmias.
Reduced serum calcium (hypocalcaemia) commonly results from vitamin D deficiency, chronic kidney disease, hypoparathyroidism, hypomagnesaemia, acute pancreatitis, sepsis, or massive blood transfusion. Patients may present with paraesthesia, muscle cramps, carpopedal spasm, tetany, seizures, prolonged QT interval, or potentially life-threatening cardiac complications in severe cases.
Interpretation of serum calcium requires consideration of the wider clinical picture and associated biochemical parameters. Assessment commonly includes serum albumin, renal function, phosphate, magnesium, parathyroid hormone (PTH), vitamin D status, and acid-base balance where appropriate. Clinical history, medication review, and physical examination remain essential in determining the underlying cause of abnormal calcium concentrations.
Serum calcium is incorporated into several validated clinical calculators and decision-support tools. The most widely used application is the corrected calcium calculation, which assists clinicians in interpreting total serum calcium in patients with abnormal albumin concentrations. Although these calculators improve the interpretation of laboratory data, they should complement rather than replace clinical judgement and direct biochemical assessment where indicated.
Serum calcium remains an essential biomarker in modern clinical practice, supporting the diagnosis and management of endocrine disorders, renal disease, metabolic abnormalities, malignancy, and critically ill patients. Accurate interpretation of calcium measurements, together with comprehensive clinical assessment, contributes to timely diagnosis, appropriate treatment, and optimal patient care.