Blood Sciences Test


Vitamin D (25 OH)

Specimen

Serum

Units

nmol/L

Reference Range

Test Usage

Vitamin D is the hormone that enhances intestinal absorption of calcium and insures healthy bone formation. The best way to obtain vitamin D is through direct exposure of the skin to sunlight, because ultraviolet B rays stimulate the skin to synthesize vitamin D3. Some people do not have adequate exposure to sunlight and are at higher risk of developing vitamin D deficiency. Examples include individuals who:

Vitamin D is inert and must be converted to its active form. The liver metabolizes it to 25-hydroxyvitamin D (25-OHD), which is the major storage and circulating form of the hormone. The kidney converts 25-OHD to 1,25-dihydroxyvitamin D (1,25-OHD) under the regulation of parathyroid hormone. Therefore, normal vitamin D metabolism is dependent on sunlight exposure, intestinal absorption, and liver and kidney function. Vitamin D malabsorption may be associated with several GI disorders including Crohn disease, celiac disease, and pancreatic insufficiency.

As vitamin D levels fall, the initial compensatory mechanism is increased PTH secretion, which stimulates the kidneys to increase phosphate excretion and decrease the calcium excretion. Blood calcium levels remain normal until the very late stages of vitamin D deficiency. Alkaline phosphatase is usually elevated in response to the effect of PTH on calcium absorption from bone. The combination of a normal serum calcium, low phosphate, and elevated alkaline phosphatase is suggestive of disturbed vitamin D metabolism. If creatinine and Urea are also increased, the problem probably lies with the renal production of 1,25-OHD. If liver function tests are abnormal or serum albumin is low, the problem may be a low 25-OHD level due to liver disease or malnutrition.

When 1,25-OHD levels are low, either due to deficient vitamin D stores or renal disease, the parathyroid glands release more PTH to try to synthesize more 1,25-OHD. Since this cannot happen, increased PTH promotes calcium absorption from skeletal bone, which may result in severe bone disease. Children develop rickets, while adults develop osteomalacia. Osteomalacia may present as a diffuse, dull, aching pain affecting many areas of the body including ribs and sternum. Vitamin D deficiency is a major risk factor for bone loss, weakness and fracture in the elderly.

Some patients taking long-term antiepileptic drug therapy develop a syndrome of low plasma 25-OHD, intestinal malabsorption of calcium, slight decrease in plasma calcium, secondary hyperparathyroidism and cortical osteopenia. In one study of inpatients on a medical ward, the most common disorders associated with vitamin D deficiency were anticonvulsant therapy, renal dialysis, nephrotic syndrome, and winter season. Cirrhosis, malabsorption and glucocorticoid therapy were also contributory factors (NEJM 1998; 338:777).

Both 25-OHD and 1,25-OHD can be measured to assess vitamin D status. 25-OHD is the preferred test for patients with normal renal function. 1,25-OHD should be measured in patients with renal disease.

LIMITATIONS OF THIS TEST

Ergocalciferol (D2) can be measured

Colecalciferol (D3) can be measured

Calcitriol (1, 25 dihydroxy vitamin D) CANNOT be measured

Alfacalcidol (1 alpha hydroxycholecalciferol) CANNOT be measured

Turnaround time

7 days

Availability

Local test

Can be added on to an existing request up to 4 days following sample receipt

Assayed

Locally

Minimum Repeat Interval

3 months

Specimen Labelling Procedure