Vitamin A Testing in Clinical Practice: Detecting Deficiency and Preventing Toxicity

Vitamin A in clinical management

For clinicians, vitamin A (retinol) testing serves two distinct but equally important purposes: detecting deficiency in at-risk populations and preventing toxicity in supplemented patients. Because retinol is tightly regulated by the liver — serum levels remain relatively stable until hepatic stores are substantially depleted — by the time a blood test shows deficiency, the situation may already be clinically significant. Conversely, because vitamin A is fat-soluble and accumulates in the liver, excessive supplementation can reach toxic levels before symptoms appear.

Deficiency: recognising the at-risk patient

In high-income countries, clinical vitamin A deficiency is uncommon in the general population but remains underdiagnosed in specific groups: patients with fat malabsorption conditions (coeliac disease, Crohn's disease, cystic fibrosis, chronic pancreatitis), those with chronic liver disease (particularly alcoholic liver disease, where hepatic retinol storage and metabolism are impaired), post-bariatric surgery patients, and individuals on restrictive diets excluding all animal-source foods.

Subclinical deficiency — levels in the marginal range that don't produce obvious symptoms — is more common and can manifest as impaired dark adaptation, dry skin, reduced immune competence, and delayed wound healing. These non-specific symptoms are easily attributed to other causes without testing.

Toxicity: the supplementation safety question

Hypervitaminosis A is a real clinical concern, particularly with high-dose preformed vitamin A supplements (retinyl palmitate, retinyl acetate). Chronic intake above 10,000 IU per day can cause hepatotoxicity, bone demineralisation, headaches, and teratogenicity in pregnancy. Monitoring retinol levels in patients who supplement — especially those taking multiple products that may contain vitamin A — is a straightforward way to ensure levels remain in the safe range.

Practical testing guidance

Retinol levels below 0.70 µmol/L indicate deficiency. Levels between 0.70 and 1.05 µmol/L represent marginal status. Levels above 3.5 µmol/L suggest possible toxicity. Masdiag measures retinol from a dried blood spot using LC-MS/MS, making the test accessible for both clinical and research settings where venepuncture may not be practical.