Blood tests worth having, and how to read them

Testing beats guessing, and only for the things where a test actually tells you something.

The tests worth having

Full blood count. Detects anaemia and gives clues about its cause through red cell size — small cells suggest iron deficiency, large cells suggest B12 or folate deficiency.

Cheap, routine, and informative.

Ferritin. Reflects iron stores and is the most useful single iron measure.

The complication: ferritin is an acute phase reactant, rising with inflammation, infection and after hard exercise. A normal ferritin in the presence of inflammation can mask depleted stores.

Test when well and rested. If interpretation is unclear, transferrin saturation and inflammatory markers help.

Vitamin B12. Worth having, with an important caveat.

Serum B12 is an imperfect measure. It can appear normal in functional deficiency, and it can be falsely elevated by supplements containing analogues.

More informative: methylmalonic acid, which rises in B12 deficiency, and homocysteine. Ask about these if you have symptoms with a normal B12 result.

Vitamin D. The 25-hydroxyvitamin D test. Worth having at least once, particularly at higher latitudes and particularly in people with darker skin.

Thyroid function, where symptoms fit — fatigue, weight change, cold intolerance, hair changes. Relevant given the iodine question.

The ones worth having in specific circumstances

Zinc, though plasma zinc is a poor marker of status, affected by inflammation and by recent meals. Interpret cautiously and in the context of dietary assessment.

Selenium, in regions with low soil selenium.

Omega-3 index, which measures EPA and DHA in red cell membranes. Available privately, of interest, and not routinely necessary.

Folate, though intake is generally excellent on plant-based diets. Worth having if the full blood count shows large cells.

Lipids and glucose, as part of general health screening rather than because of diet.

The ones that are generally not useful

Food intolerance tests based on IgG antibodies, which are widely sold and which allergy and immunology bodies have repeatedly stated do not diagnose food intolerance.

IgG antibodies to food reflect exposure rather than intolerance. These tests produce long lists of foods to avoid and cause unnecessary and sometimes harmful dietary restriction.

Hair mineral analysis, which is not a validated measure of nutritional status.

Comprehensive micronutrient panels marketed direct to consumers, which test many things with poor reference ranges and which generate anxiety about minor variations.

Microbiome tests offering dietary recommendations, where the science is not yet at the point of supporting a personalised diet from a stool sample.

Reading results

Reference ranges are population-based, typically covering the central proportion of a healthy population. Being just outside a range is not necessarily abnormal, and being within it is not necessarily optimal.

Ranges differ between laboratories, so a result should be interpreted against the range printed on that report.

Trends matter more than single values. A ferritin falling over successive tests, even within range, is more informative than one value.

Context matters. Recent illness, exercise, supplements, medications and time of day all affect results.

Do not diagnose yourself from a number. Results need interpreting alongside symptoms and history, which is what the clinician is for.

Frequency

For a healthy adult on a plant-based diet: a baseline set within the first year, then every few years, or sooner if symptoms develop.

More often for: menstruating women, endurance athletes, adolescents, pregnant women, older adults, and anyone with symptoms or a condition affecting absorption.

Getting them

Availability through primary care differs by health system. Some tests are routine, some require a specific indication.

What helps when asking: describe symptoms rather than requesting a test list, mention the diet, and ask specifically about the relevant nutrients.

Private testing is available and is worth approaching carefully — the risk is over-testing, which generates incidental findings and anxiety, and results without interpretation.

If you use a private service, take the results to a clinician rather than acting on them alone.

The most useful thing testing does

Two things, both practical.

It identifies deficiencies before they cause symptoms or damage, particularly for B12, where neurological effects can become irreversible.

And it prevents unnecessary supplementation, which matters most for iron, where excess is harmful and where haemochromatosis affects people who do not know they have it.

The instruction that follows from both: test before supplementing, and do not supplement iron on the basis of feeling tired.

General information rather than individual advice. Blood results should be interpreted by a clinician alongside your symptoms and history. Do not act on test results without professional interpretation.