Vitamin D Testing: Who Needs It, What the Results Mean, and Why Deficiency Is So Common

The scale of deficiency

Vitamin D deficiency is one of the most common nutritional deficiencies worldwide. Depending on the threshold used, estimates suggest that 40–60% of the global population has insufficient vitamin D levels, with prevalence highest in northern latitudes, among older adults, and in populations with darker skin pigmentation.

The challenge is that deficiency is often asymptomatic until it becomes severe. By the time symptoms like fatigue, muscle weakness, or bone pain appear, levels may have been suboptimal for months or years.

What does the test measure?

The standard vitamin D test measures 25-hydroxyvitamin D, abbreviated as 25(OH)D, in the blood. This is the main circulating form of vitamin D and the most reliable indicator of overall vitamin D status, reflecting contributions from both sunlight exposure and dietary intake.

At Masdiag, we measure 25(OH)D3 and 25(OH)D2 separately using LC-MS/MS — the gold standard analytical technique — from a dried blood spot. This distinction matters because D3 (from sunlight and animal sources) and D2 (from plant sources and some supplements) have different metabolic profiles.

Interpreting the results

While thresholds vary slightly between guidelines, the general consensus is that serum 25(OH)D levels below 20 ng/mL (50 nmol/L) indicate deficiency, levels between 20–30 ng/mL (50–75 nmol/L) suggest insufficiency, and levels above 30 ng/mL (75 nmol/L) are considered sufficient for most individuals. Some experts advocate for optimal levels of 40–60 ng/mL, particularly for immune function and disease prevention.

Who should be tested?

While routine screening isn't recommended for every individual, testing is strongly indicated for those with risk factors: people living at higher latitudes with limited sun exposure, older adults (who synthesise vitamin D less efficiently), individuals with darker skin, those with malabsorption conditions such as coeliac disease or inflammatory bowel disease, people with obesity (vitamin D is sequestered in fat tissue), and anyone on medications that affect vitamin D metabolism.

For practitioners and researchers, dried blood spot testing removes the barrier of clinic-based venepuncture, making population screening and longitudinal monitoring practical at scale.

Beyond bone health

The role of vitamin D has expanded far beyond its classical association with calcium metabolism and bone health. Research now links vitamin D status to immune regulation, cardiovascular health, mood disorders, autoimmune conditions, and even pregnancy outcomes. Masdiag's own research team has contributed significantly to this evidence base, with 15 published papers on vitamin D methodology and clinical applications.