Before you use this guide
This is general information, not a diagnosis or a substitute for care from a clinician who knows your medical history. Seek urgent help for severe or rapidly worsening symptoms.
Start with risk and symptoms, not a shopping list
Tiredness, weakness, breathlessness, mouth symptoms, pins and needles or poor concentration can occur with nutritional deficiencies, but they can also have many other causes. A focused history helps decide whether testing for iron, vitamin B12, folate or another marker is appropriate.
Relevant context includes diet, heavy menstrual bleeding, pregnancy, gastrointestinal symptoms or surgery, alcohol intake, long-term illness and medicines that affect absorption.
Commonly considered blood markers
- A full blood count looks for anaemia and changes in blood cells.
- Ferritin can help assess iron stores, although inflammation can affect interpretation.
- Vitamin B12 and folate results are interpreted with symptoms and the blood count.
- Vitamin D testing is usually targeted to symptoms, high risk or another clinical reason.
- Other nutrients are tested selectively when history or symptoms provide a reason.
A low result is the start of the question
Identifying a deficiency does not explain why it developed. Causes can include limited intake, blood loss, increased requirements, malabsorption or an underlying condition. The appropriate treatment and follow-up depend on that cause.
For example, vitamin B12 deficiency can occur even when the diet appears adequate because absorption is impaired. Iron deficiency may require investigation for blood loss rather than supplementation alone.
Tell the clinic about supplements
Starting high-dose supplements before testing can change results and complicate interpretation. Some supplements also interfere with particular laboratory methods. Do not delay urgent treatment that has been clinically advised, but avoid self-prescribing large doses solely to “correct” an unconfirmed deficiency.
Bring the product names and doses you take. If repeat testing is needed, follow the interval and preparation recommended for that marker and treatment plan.
When to seek medical advice
Arrange clinical assessment for persistent symptoms, unexplained weight loss, significant breathlessness, neurological symptoms such as numbness or balance problems, blood loss, or a markedly restricted diet. Some complications of vitamin B12 deficiency can become irreversible if treatment is delayed.
A broad wellness panel should not substitute for assessment when symptoms are significant or worsening.
Common questions
Questions about this topic.
No. Some nutrients do not have a straightforward screening test, and results need to be selected and interpreted alongside symptoms, diet and medical history.
Sources and further reading
We use authoritative UK guidance and patient information. External sources may be updated after this guide is published.
