Vitamin A (retinol)

Ranges vary a little between laboratories (for example 1.0 to 2.8 or 0.9 to 2.5 umol/L), are lower in children, and the value falls during any inflammation regardless of the body's stores.

Vitamin A, measured in the blood as retinol, is a fat-soluble vitamin stored in the liver and carried in the blood on a protein. It is needed for vision in dim light, the health of the skin and the lining of the airways and gut, and the immune system. The blood level is held steady by the liver and only falls once the stores are low.

It is requested when fat is poorly absorbed, for example in coeliac disease, Crohn's disease, cystic fibrosis, pancreatic conditions or after weight-loss surgery, in people on long-term intravenous feeding, and when night blindness or very dry eyes are being looked into. Occasionally it is checked when too much vitamin A is a concern.

• High-strength vitamin A supplements, cod liver oil in large amounts and eating a lot of liver
• Long-term kidney disease, where the carrier protein builds up
• Some acne and skin medicines derived from vitamin A
• High cholesterol and triglycerides, which carry more of it in the blood
• Oestrogen-containing contraception, slightly

• Any inflammation or infection, which lowers the carrier protein for a few weeks
• Poor absorption of fat: coeliac disease, Crohn's disease, cystic fibrosis, pancreatic disease, weight-loss surgery
• A diet low in vegetables, dairy and animal foods over months
• Liver disease and heavy alcohol use
• Low protein or zinc, which are needed to release it from the liver
• Long-term intravenous feeding

• A cold, an infection or any inflammation on the day of the test lowers it without touching the body's stores, so doctors often read it with a CRP
• It rises with blood fats, so a high result after a fatty meal or with high triglycerides can be misleading
• The sample has to be protected from light or the value falls

A fasting morning sample, protected from light, and ideally not during an infection. A CRP is often measured at the same time to help read a low value.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.