What is the vitamin D test?

The vitamin D test measures the level of vitamin D in your blood to diagnose whether you have enough of it or whether you are deficient. Vitamin D deficiency does have adverse physical and mental health outcomes that can have serious repercussions.

Rising awareness about these deficient outcomes has led to an increase in the rate of vitamin D testing in developed countries. The cost of a single vitamin D test is reasonable.

You can get enough of your body’s vitamin D requirement from proper exposure to sunlight and even certain foods can supplemnt your need. Yet, studies indicate that approximately one billion people worldwide sufer from vitamin-D deficiency and about 50% of the world population has vitamin D insufficiency.

In the United States, approximately 42% of the population sufers from this deficiency. Because of this widespread, some researchers across the globe refer to vitamin D deficiency as an “invisible epidemic.”

Vitamin D is also known by other names:

  • Ergocalciferol Test
  • Vitamin D2 Test

Both the above tests measure vitamin D2 in your body

  • Cholecalciferol Test
  • Vitamin D3 Test

These tests measure the amount of vitamin D3 in your body

  • Calcidiol Test
  • 25-OH vitamin D test or 25-hydroxy cholecalciferol test

These tests measure the level of 25-hydroxy vitamin D in your body. Incidentally, it is this form of vitamin D that the body uses.

Vitamin D obtained from sun exposure, foods, and supplements is not biologically active. The liver converts it into 25-hydroxyvitamin D [25(OH)D], also known as “calcidiol.” Calcidiol is then converted into the physiologically active 1,25-dihydroxy vitamin D (1,25(OH)2D) in the kidneys. This form is called “calcitriol”.

Vitamin D is an essential micronutrient that our body needs for healthy bones and teeth. Besides, it also promotes the better function of your muscles, nerves, and immune system.

The most accurate way to measure the amount of vitamin D in your body is to do the 25-hydroxy vitamin D test. This is because if your body has to benefit from vitamin D, your liver must convert the acquired vitamin D into 25 hydroxyvitamin D, or 25(OH)D, which is another form of this vitamin D. The serum concentration of 25(OH)D is, therefore, the main pointer of your body’s vitamin D status.

Most vitamin D blood tests, however, measure the level of 25(OH)D in your blood because the level of 25-hydroxyvitamin D in your blood is a true indication of how much vitamin D your body has. 

How is the test done?

A vitamin D test is done using your blood. During this blood test, the pathologist will draw the blood sample from a vein in your arm, using a small needle.

He will then collect this drawn sample of blood into a test tube or vial. You will feel the prick of the needle when it is inserted and removed.

You need not be fasting for this and it can be done at any time of the day. There is no risk involved.

Why does the doctor order it? Purpose of the test

Vitamin D testing is not recommended to be done across the general population probably because of the unnecessary expense and it being not covered by health insurance.

This test is indicated only for individuals with specific conditions such as:

  • Osteoporosis
  • Rickets
  • Osteomalacia
  • Malabsorption syndromes
  • Liver and kidney disease
  • Individuals taking medications that interfere with the absorption or conversion of vitamin D in your body

Your doctor may also order this test

  • To screen for a possible deficiency or insufficiency if you have any risk factor that increases the likelihood of having a low level of vitamin D.
  • If you have symptoms that suggest low levels of this vitamin.
  • To rule out vitamin D toxicity caused by excess intake of vitamin D supplements. Symptoms of toxicity include body aches, body pain, and lack of energy.
  • To monitor your vitamin D levels if you are being treated for deficiency.

When does a doctor order a vitamin D test?

If you experience vitamin D deficiency symptoms, your doctor may order the test to confirm the same. The main symptoms include:

  • Bone pain
  • Muscle weakness, muscle pain, or muscle cramps
  • Deformed bones
  • Fractures
  • Symptoms of osteoporosis or osteopenia

Your doctor may also order this test if you exhibit certain risk factors that make you prone to developing a deficiency.

These risk factors include:

  • Age of 65 years or more when your skin makes less vitamin D
  • Factors that allow very little exposure to sunlight such
    • Staying mostly indoors
    • Using sunscreen when you venture outside
    • Living in areas where there is very little sunlight
    • People with dark skin, which makes less vitamin D from the sun than fair skin
  • Obese people might need to take more of vitamin D to achieve adequate 25(OH)D levels than people with normal weight
  • Obese individuals who have undergone weight loss surgery. In this surgical procedure, part of the upper small intestine, where vitamin D is absorbed, is bypassed, and vitamin D that is mobilized into the bloodstream from fat stores might not raise 25(OH)D to adequate levels over time.
  • Liver or kidney disease can interfere with your body’s ability to convert vitamin D into 25 hydroxyvitamin D
  • Presence of conditions that interfere with the absorption of nutrients from the gut such as Crohn’s disease, celiac disease, and ulcerative colitis
  • Intake of certain medications that affect your vitamin D levels such as phenytoin, phenobarbital, and rifampicin
  • If you are taking supplements to augment vitamin D levels, your doctor may order this test to see if your levels are improving

A lack of vitamin D can cause serious problems for babies and children. It can cause rickets (softening of the bones during childhood). In children, a health provider may order a screening test for:

  • Babies that have been are mainly fed on breast milk. This is because breast milk is low in vitamin D. Therefore, babies feeding on breast milk only, need vitamin D supplements shortly after birth. Therefore, doctors recommend they be fed a formula that contains vitamin D.
  • Children with darker skin
  • Children who live in areas with little sunlight
  • Children with diets lacking in vitamin D

Vitamin D test normal range

Levels of vitamin D are measured by the 25-hydroxyvitamin D level in nanomoles/liter (nmol/L) or nanograms/milliliter (ng/mL). The important value is the value of the total 25 hydroxy vitamin D levels in your blood because this is the form that your body uses.

Some experts recommend a level between 20 and 40 ng/mL, while some recommend a level between 30 and 50 ng/mL.

Medscape quotes the guideline published online on June 6 in the Journal of Clinical Endocrinology & Metabolism.

At that time, their experts concluded: “Based on all the evidence, at a minimum, we recommend vitamin D levels of 30 ng/mL, and because of the vagaries of some of the assays, to guarantee sufficiency, we recommend between 40 and 60 ng/mL for both children and adults.”

What Abnormal Results Mean

Abnormal levels mean either your levels are low or significantly high.  Both these conditions need to be looked after by your doctor.

In spite of differences in opinion,

  • generally, normal values are 75-100 nmol/L (30-40 ng/ml).
  • Vitamin D deficiency is defined when serum hydroxyvitamin D levels are less than 50nmol/L (20 ng/ml), and
  • you are said to have vitamin D insufficiency when your levels are at 50-75 nmol/L (20-30 ng/ml).

Lower than normal levels

Lower than normal levels can indicate vitamin D deficiency or insufficiency. Insufficiency means the levels are mildly low while a substantially low level means deficiency.

Low levels indicating deficiency can be due to several reasons:

  • Lack of skin exposure to sunlight
  • Darkly pigmented skin
  • Consistent use of high-SPF sunscreen when venturing out
  • Lack of enough vitamin D in the diet
  • Liver and kidney diseases
  • Poor food absorption such as due to malabsorption syndrome, weight loss surgery, and advanced age
  • Long-term use of certain medicines that interfere with the absorption or conversion of vitamin D. Such medicines include phenytoin, phenobarbital, and rifampin

Low levels with symptoms of bone pain are an indication for checking your bone density, which tells you how healthy your bones are. Low levels increase your risk of certain cancers, heart disease, and immunity problems.

Higher than normal levels

Higher than normal levels mean levels higher than 125 nmol/L (50 ng/mL). Such levels indicate an excess of vitamin D in your body, a condition called hypervitaminosis D.

This is usually caused by the intake of too many vitamin D supplements. You can rarely get too much vitamin D through excessive intake of vitamin D-rich foods or from too much sunlight exposure.

It can also result in the accumulation of too much calcium in the body, a condition called hypercalcemia. Untreated, this can lead to kidney damage.