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Things worth knowing about "Calcium absorption test"

The calcium absorption test measures how well the body absorbs calcium from the intestine. It helps diagnose disorders of calcium metabolism.

What is the Calcium Absorption Test?

The calcium absorption test is a diagnostic procedure used to assess how efficiently the human body absorbs calcium from the gastrointestinal tract into the bloodstream. Calcium is an essential mineral required for bone strength, muscle contraction, nerve transmission, and blood clotting. Impaired absorption can lead to calcium deficiency even when dietary intake appears adequate.

When is the Test Performed?

The calcium absorption test is indicated when impaired intestinal calcium uptake is suspected. Common indications include:

Recurrent or unexplained low blood calcium levels (hypocalcaemia)
Suspected malabsorption syndromes, such as coeliac disease or Crohn's disease
Evaluation of osteoporosis or reduced bone density
Monitoring after gastrointestinal surgery
Suspected vitamin D deficiency, as vitamin D is the key regulator of calcium absorption
Assessment of parathyroid disorders (hypoparathyroidism, hyperparathyroidism)

How Does the Test Work?

The calcium absorption test is typically performed as a fasting oral calcium loading test. The patient is given a defined oral dose of calcium, and the subsequent rise in blood calcium levels or urinary calcium excretion is measured.

Test Procedure

Preparation: The patient must fast for 8 to 12 hours before the test. Certain medications that may affect results should be paused or noted by the treating physician.
Baseline measurement: A blood sample is taken to determine the fasting calcium level.
Oral calcium administration: The patient consumes a standardised dose of calcium, usually as a calcium solution or tablet.
Follow-up measurements: Blood samples are taken at defined intervals (e.g., 1, 2, and 4 hours after ingestion) to track the rise in calcium. A 24-hour urine collection may also be used to assess renal calcium excretion.

Test Variants

Isotope-labelled calcium absorption test: This specialised variant uses radioactively or stably labelled calcium to measure absorption with high precision. It is mainly used in research settings and specialised centres.
Indirect absorption assessment: Blood and urine markers (e.g., serum calcium, parathyroid hormone, vitamin D metabolites) are evaluated to draw conclusions about absorptive capacity without a direct oral loading procedure.

Interpreting the Results

A normal result shows a clear rise in blood calcium after the oral dose, indicating intact intestinal absorption. An absent or minimal rise may point to the following causes:

Vitamin D deficiency: The active form of vitamin D (calcitriol) is essential for activating calcium transport proteins in the intestinal wall.
Malabsorption syndromes: Conditions such as coeliac disease, Crohn's disease, or short bowel syndrome reduce the absorptive surface area of the intestine.
Hypoparathyroidism: Deficiency of parathyroid hormone impairs the activation of vitamin D and subsequently reduces calcium absorption.
Drug interactions: Certain medications (e.g., proton pump inhibitors, corticosteroids, some antibiotics) can inhibit calcium absorption.
Chronic kidney disease: As the kidney is involved in activating vitamin D, renal insufficiency can impair calcium absorption.

Clinical Significance

The calcium absorption test provides valuable information that goes beyond a routine blood test. It allows for targeted investigation of calcium metabolism disorders and helps guide the most appropriate treatment – whether that is vitamin D supplementation, dietary adjustment, or treatment of an underlying disease. In patients with osteoporosis or increased fracture risk, the test can be particularly helpful in identifying the root cause of inadequate calcium supply to the bones.

References

Peacock, M. - Calcium Metabolism in Health and Disease. Clinical Journal of the American Society of Nephrology, 2010; 5 (Suppl 1): S23–S30.
Holick, M. F. - Vitamin D Deficiency. New England Journal of Medicine, 2007; 357: 266–281.
National Institutes of Health (NIH) Office of Dietary Supplements - Calcium: Fact Sheet for Health Professionals. NIH, 2023.

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