Can a blood test detect vitamin deficiency?

You might think you're eating well and getting everything your body needs but tiredness, brain fog or brittle nails could be telling a different story. Vitamin deficiencies are more common than you might expect, and because the symptoms are often vague or gradual, many people live with low levels of essential nutrients without realising it.

So, can a blood test actually detect a vitamin deficiency?

The short answer is yes. A blood test is one of the most effective ways to find out whether you're low in key vitamins like B12, D, folate and others. But just as importantly, understanding which deficiencies to check for, why they matter and how to interpret the results can help you take charge of your long-term health.

Here, we explore what vitamin deficiencies look like, who's most at risk and how a simple blood test can reveal what your body's really missing.

First, what causes a vitamin deficiency?

Vitamin deficiencies can happen for a variety of reasons, not just from a poor diet. While nutrition plays a big role, it’s far from the only factor. You might become deficient if your body isn’t absorbing nutrients properly, if you’re on certain medications or even due to lifestyle choices or health conditions.

Common causes of vitamin deficiencies include:

• Restricted diets (e.g. vegan, vegetarian, low carb)

• Digestive disorders (like coeliac or Crohn’s disease)

• Age-related changes in absorption

• Heavy alcohol use

• Smoking

• Certain medications (e.g. proton pump inhibitors, metformin)

• Lack of sun exposure (for vitamin D)

Even small imbalances can lead to noticeable symptoms over time. That’s why it’s important to spot a deficiency early, before it starts affecting your day-to-day life.

What are the signs you might be deficient?

Symptoms of vitamin deficiency are often non-specific and develop gradually, making them easy to brush off. You might just think you're "a bit run down" or having an off week, when in reality, your body’s missing something vital.

Common signs include:

• Fatigue or low energy

• Brain fog or difficulty concentrating

• Frequent colds or low immunity

• Mood changes or low mood

• Brittle hair or nails

• Muscle weakness or cramps

• Pins and needles

• Pale or dry skin

• Slow wound healing

These symptoms can point to different deficiencies, so if they persist, a blood test can help clarify exactly what’s going on.

Which vitamin deficiencies can be detected by a blood test?

A range of key vitamins can be measured through a simple blood test, each with its own role in supporting your body’s health.

Vitamin D

Often called the “sunshine vitamin,” vitamin D helps your body absorb calcium and supports bone, muscle and immune health. In the UK, many people are low in vitamin D, especially in the winter months.

Deficiency symptoms: Fatigue, low mood, frequent illness, bone or muscle pain

Who’s at risk: People with darker skin, those who spend most time indoors, older adults, people who cover their skin for cultural reasons

Vitamin B12

This vitamin is crucial for nerve function and red blood cell production. B12 is mainly found in animal products, so deficiency is more common in those on vegan or vegetarian diets.

Deficiency symptoms: Tiredness, memory problems, pins and needles, pale skin, tongue soreness

Who’s at risk: Vegans, vegetarians, people over 60, those with digestive issues, people on certain medications (e.g. metformin, PPIs)

Folate (Vitamin B9)

Folate helps your body make DNA and red blood cells. It’s especially important during pregnancy, but also plays a key role in mood, energy and cognitive function.

Deficiency symptoms: Fatigue, irritability, mouth ulcers, grey hair, poor growth

Who’s at risk: Pregnant people, heavy drinkers, people with absorption issues, those on long-term medication

Iron (often measured alongside vitamin tests)

Although technically a mineral rather than a vitamin, iron is frequently tested with B12 and folate, as it’s another key player in red blood cell production and energy levels.

Deficiency symptoms: Extreme tiredness, pale skin, breathlessness, hair loss, cravings for non-food substances (e.g. ice)

Who’s at risk: Menstruating women, vegetarians, people with blood loss or digestive disorders

How does a blood test detect a deficiency?

When you take a vitamin deficiency blood test, your sample is analysed in a laboratory to measure the levels of specific vitamins and nutrients in your bloodstream.

Each vitamin has a defined “normal range” - a level considered healthy for the average adult. If your results fall below this range, it indicates a deficiency. Some tests may also highlight borderline or insufficient levels, where your vitamin levels aren’t low enough to be classed as deficient, but are still suboptimal.

This is helpful, as it allows you to take early action before a full deficiency develops.

Should you test even if you feel fine?

Not all vitamin deficiencies cause noticeable symptoms right away. Some can linger silently for months, gradually affecting your energy, mental clarity or long-term health.

It’s a good idea to check your vitamin levels if:

• You follow a restricted diet (e.g. vegan, low-dairy, low-meat)

• You feel tired all the time, even with enough sleep

• You’ve recently been unwell or recovering from illness

• You’re pregnant, breastfeeding or trying to conceive

• You’re over the age of 50

• You have a chronic condition affecting digestion

• You want a clearer picture of your baseline health

Even if you’re feeling well, a deficiency could be quietly developing in the background and early detection can help prevent complications down the line.

What happens if a vitamin deficiency is found?

If your blood test shows that you’re low in a specific vitamin, the next step is to address it, usually through supplementation, dietary changes, or both.

In most cases, deficiencies can be corrected with:

• Targeted supplements (over-the-counter or prescribed)

• Dietary tweaks to include more nutrient-rich foods

• Lifestyle changes, such as getting more sun exposure for vitamin D

Your Results Report will help you understand your levels and what to do next. At Bluecrest, you also get access to a 24/7 GP helpline, so you can talk through your results with a medical professional and create a tailored plan.

A simple way to check your levels

A Vitamin & Mineral Deficiency Test with Bluecrest is a quick, easy and accurate way to check vitamin levels and detect any mineral deficiencies. A wide range of vitamin and mineral tests can be carried out as part of these packages, giving you a clear picture of your nutritional health, without the guesswork.

Whether you're experiencing low energy, unexplained symptoms or simply want peace of mind, testing your vitamin levels is a proactive step toward feeling your best.

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