How to prepare for your blood test

A blood test measures what is in your blood at one moment. Some markers barely move. Others shift with what you ate last night, how hard you trained, the time of day you gave the sample, or a supplement you took that morning. Preparation exists so that a short-term swing is not recorded as your baseline.

Most of the instructions are simple, and plenty of tests need none at all. What matters is knowing which ones apply to you and following those few properly. Preparation protects the quality of the result. It does not guarantee a particular number.

This page is educational information, not medical advice. It does not replace guidance from the clinician who knows your history, and nothing here is a reason to start, stop or change a prescribed medicine.

You do not need to work out which instructions apply to you. Each test's page lists them before you buy, your basket carries them through checkout, and your order repeats them once it is placed. This page is the full detail behind those instructions, with one section for each.

Every instruction below is reproduced as our reviewing clinician confirmed it, in clinical language rather than ours. We do not rewrite clinical wording to make it sound more like the rest of the site, because a reworded instruction is a different instruction.

A few sections are not preparation steps at all. Referral turnaround explains why some results arrive later, and the two availability sections describe who a test is for. They are here because the detail is useful, not because there is anything to do beforehand.

Where several tests are taken from one sample, the strictest instruction that applies is the one to follow. If two instructions look as though they disagree, or you are not sure an instruction is possible for you, contact us before your sample is taken.

Where lipids, fasting insulin or fasting glucose are being assessed, patients should fast for 12 hours before the blood sample.

During the fasting period:

Plain water is permitted.

Avoid all food and caloric drinks.

Avoid coffee, tea, milk, juice and other beverages containing calories.

Avoid alcohol during the fasting period.

HbA1c does not require fasting and can be measured regardless of when the patient last ate.

Where several tests are being performed together, the most restrictive fasting requirement should be followed.

Patients should continue prescribed medication unless specifically instructed otherwise. Any medication or supplement taken during the fasting period should be documented where relevant.

Avoid alcohol for at least 24 hours before blood testing.

Where liver function, GGT or a lipid profile is being assessed, a longer period of 48 hours without alcohol is preferable, particularly where an accurate baseline is required or previous results have been abnormal.

Alcohol consumption can transiently affect GGT and other liver enzymes, as well as triglyceride levels, and may therefore influence interpretation of the results.

For repeat monitoring, patients should ideally undergo testing under consistent conditions, including a similar period of alcohol avoidance before each test.

Patients should disclose any significant alcohol consumption in the days preceding the blood test.

The appropriate timing of female sex-hormone testing depends on the hormone being measured, the patient's menstrual-cycle pattern and whether hormonal medication is being used.

Oestradiol (E2), FSH and LH

In patients with regular menstrual cycles, testing is generally most informative when performed during the early follicular phase, typically days 1–3 of the cycle, where assessment of baseline reproductive hormones is required.

For patients with irregular or absent cycles, cycle-day timing may not be appropriate. The clinical context and reason for testing should be considered when determining the most appropriate timing.

Progesterone

Progesterone is cycle-dependent and should generally be measured approximately 7 days before the expected next menstrual period, rather than automatically on day 21.

For a typical 28-day cycle, this corresponds approximately to day 21. For longer, shorter or irregular cycles, the appropriate testing day should be adjusted accordingly.

Where ovulation can be identified, progesterone is ideally measured approximately 7 days after ovulation.

Cortisol

Cortisol should ideally be measured between 08:00 and 09:00, when levels are normally at their highest.

Cortisol testing does not need to be scheduled around a particular menstrual-cycle day, including where cycles are irregular.

For repeat testing, samples should be collected at approximately the same time of day to allow meaningful comparison.

Testosterone

For assessment of testosterone status, blood samples should preferably be collected in the morning, ideally before 11:00, when testosterone concentrations are generally at their highest.

Where possible, testing should be performed fasted and when the patient is clinically well. A low or borderline testosterone result should generally be confirmed with a second morning measurement before a diagnosis of testosterone deficiency is established.

For patients receiving testosterone replacement therapy, the timing of blood sampling relative to testosterone administration is important. Please seek advice from your prescribing clinician regarding the appropriate timing of the blood test.

For all testosterone monitoring, the following should be recorded:

Date and time of blood collection

Date and time of the most recent testosterone administration

Testosterone dose administered

Route/formulation of testosterone, where relevant

DHEA-S

DHEA-S does not require menstrual-cycle timing and can be measured at any time of day in both male and female patients.

Where DHEA-S is included as part of a comprehensive hormone panel, morning sampling is preferred for consistency with the other endocrine markers being assessed.

For repeat monitoring, samples should ideally be collected at approximately the same time of day.

Where the patient is taking DHEA or another androgenic supplement, the dose, timing of the most recent dose and duration of use should be documented, as supplementation can significantly influence DHEA-S concentrations.

DHEA-S should be interpreted in conjunction with age, sex, clinical presentation and other relevant androgen markers, rather than as an isolated measure of androgen status.

In female patients, DHEA-S may be used in the assessment of hyperandrogenism, including suspected PCOS or adrenal androgen excess. Interpretation should take into account menstrual history, hormonal contraception or HRT, and clinical features of androgen excess such as hirsutism, acne or androgenic alopecia.

DHEA-S does not require the specific 08:00–09:00 sampling window used for cortisol.

The salivary cortisol day curve requires four saliva samples collected at specific times:

On waking: within 30 minutes of getting up

Midday: 12:00

Afternoon: 16:00

Before bed: immediately before going to sleep

The exact collection time of each sample should be recorded accurately. It is important to follow the specified timings because cortisol levels vary substantially throughout the day and the laboratory uses the collection time when interpreting the results.

Avoid sexual intercourse or ejaculation for at least 48 hours before PSA blood testing, as recent ejaculation may temporarily increase PSA levels and potentially affect the result.

To minimise transient factors that may affect PSA results, patients should: Avoid vigorous exercise, including heavy resistance training/weight training, for 48 hours before the blood test.

Avoid cycling and other activities that place significant pressure on the perineum for at least 48 hours before the blood test. If PSA is unexpectedly elevated, repeat testing may be appropriate after potential transient causes have resolved.

Avoid strenuous or unusually intense exercise for at least 24 hours before blood testing, particularly when CRP, FBC and liver-function markers are being assessed.

Intense exercise can cause temporary changes in inflammatory markers and blood-cell parameters and can increase muscle-derived enzymes, particularly CK and AST, which may complicate interpretation of liver-function results. Exercise can also produce transient changes in ALT and other biochemical markers.

Patients should therefore undertake their blood test following their usual level of activity, rather than immediately following unusually strenuous exercise or a hard training session.

Where accurate baseline or treatment-monitoring results are required, 48 hours without strenuous exercise may be preferable, particularly following exceptionally intense or prolonged exercise.

Normal light activity, such as routine walking, is generally acceptable.

Biotin (vitamin B7) can interfere with certain laboratory immunoassays and may produce falsely abnormal results. This is particularly relevant to thyroid function tests, AMH and other hormone assays.

Biotin supplements should not be taken for 48 hours (2 days) before blood testing.

Biotin may be present in multivitamins and, particularly, hair, skin and nail supplements.

If biotin has been prescribed, do not stop it without first discussing this with the prescribing clinician.

Patients should disclose all supplements being taken when completing their clinical information.

Homocysteine and insulin require a suitable venous blood sample and should not be collected using a capillary/finger-prick testing method.

Where these markers are required as part of the clinical assessment, patients should attend a venous blood draw rather than selecting a finger-prick collection option.

This is particularly important because the laboratory methodology, sample type and processing requirements need to be appropriate for accurate measurement of these analytes.

Saliva kit included. For each sample, avoid food, drink, smoking and tooth brushing for at least 60 minutes beforehand. Water is permitted. Approximately 10 minutes before collecting the sample, rinse the mouth with water for up to 5 seconds and spit it out.

This is not a preparation step. A few markers are analysed by a specialist referral laboratory rather than in house, so those results arrive after the rest of your panel. Where a test includes one, the expected turnaround is shown on that test's page and on your order, so you can plan around it.

Turnaround times are estimates and are calculated from the first working day after the laboratory receives the sample. They are not guaranteed and may occasionally be longer.

Patients should therefore allow sufficient time for these results to become available when blood testing is being undertaken in advance of a clinical consultation or treatment review.

Where an assessment of anaemia or iron status is being undertaken, patients should disclose all iron-containing medications and supplements, including the dose and timing of the most recent dose.

Samples should ideally be collected at least 24 hours after taking vitamin or mineral supplements, as serum iron is a transient marker and can be influenced by recent intake.

Accordingly, unless otherwise instructed by the prescribing clinician, do not take iron or other vitamin/mineral supplements during the 24 hours immediately before the blood draw.

Prescribed iron should not be discontinued without clinical advice.

Samples should preferably be collected between 06:00 and 10:00, and heavy exercise should be avoided for 48 hours beforehand.

Avoid vitamin D supplementation for 24 hours before test. Patients taking prescribed vitamin D or other prescribed supplements should not discontinue treatment without clinical advice. The dose and timing of the most recent dose should be documented.

Where Vitamin D is being monitored during supplementation, subsequent blood tests should ideally be performed under consistent conditions and at a comparable point relative to supplementation.

For thyroid-function testing, blood samples should preferably be collected in the morning, between 06:00 and 10:00, before the patient's daily thyroid medication.

Patients should not discontinue thyroid medication or alter their prescribed regimen without clinical advice.

For repeat thyroid monitoring, blood samples should ideally be collected at approximately the same time of day and at the same point in relation to medication administration on each occasion.

If you are currently taking HRT or an oral contraceptive, please continue taking it as prescribed and do not stop treatment solely for the purpose of blood testing.

Please inform us that you are taking HRT or an oral contraceptive and provide the name, dose and type of preparation.

Hormonal treatment can affect female sex-hormone results, so the results will be interpreted in the context of your current treatment.

If the purpose of testing is to assess your natural hormone levels, please discuss this with the clinical team before arranging the test, as HRT or hormonal contraception may need to be discontinued for an appropriate period before testing.

Patients undergoing finger-prick blood testing should remain well hydrated before and during the test.

Drink plenty of plain water before the sample is taken. Good hydration can improve peripheral blood flow and make capillary sampling easier.

Keep hands warm prior to sampling, as warmth helps to increase blood flow to the fingertips and can improve the ease and quality of the sample. Warming the hands (e.g. by washing in warm water or gently rubbing them together) may be helpful.

Avoid excessive fluid intake immediately before testing; normal hydration is sufficient.

Where fasting is required, plain water remains permitted during the fasting period.

This section is about who a PSA test is for, rather than how to prepare for one.

PSA testing should not be used as routine screening in younger men without a clinical indication.

For men at average risk, PSA testing should generally be considered from age 50, following discussion of the potential benefits and limitations of testing.

Earlier testing may be appropriate from age 45 for men at increased risk, including those with:

A significant family history of prostate cancer

A Black ethnic background

Relevant inherited genetic risk, such as a BRCA2 mutation

PSA testing in men under 40 should generally only be undertaken where there is a specific clinical indication or specialist recommendation.

There is no absolute minimum age for PSA testing where there is a genuine clinical indication. However, PSA should not be offered routinely to asymptomatic adolescents or young adults as a general health or cancer-screening test.

Where PSA testing is requested in a patient under 40, the clinical indication should be documented and the result interpreted in the appropriate clinical context.

This section is about who these tests are for, rather than how to prepare for them.

AMH

AMH can generally be measured at any point in the menstrual cycle and does not require cycle-day timing.

AMH testing should not routinely be used in patients under 18 years of age without appropriate clinical oversight.

Patients should disclose whether they are taking HRT, hormonal contraception or other hormonal medication, as this may influence AMH concentrations and interpretation.

AMH is a marker of ovarian reserve and should not be interpreted as a standalone measure of fertility or the ability to conceive naturally.

Progesterone

Progesterone testing is cycle-dependent and should be appropriately timed.

For patients with regular cycles, testing should generally be performed approximately 7 days before the expected next period. A fixed day-21 test is only appropriate for a typical 28-day cycle.

For patients with irregular cycles, progesterone should ideally be measured approximately 7 days after confirmed or suspected ovulation, where ovulation can be reliably identified.

Patients taking HRT, progesterone, progestogens or hormonal contraception should disclose their treatment, as exogenous hormones can significantly affect progesterone results and their interpretation.

Progesterone testing should not be used to assess ovulation without considering the timing of the sample in relation to the menstrual cycle and any hormonal treatment.

Work from your order rather than from memory. Your instructions stay with your order, so you can check them the night before and again on the morning.

Plain water is permitted during the fasting period, and good hydration makes a sample easier to collect. Take a note of the date and time your sample was taken, along with anything the guidance asked you to record, such as the timing of a recent dose. Several markers are read in the context of when the sample was taken, so that note is part of the result.

If you are giving your sample at a clinic or to a nurse, allow enough time that you are not arriving straight from a hard effort. Normal light activity, such as routine walking, is generally acceptable.

This happens, and telling us is always better than saying nothing. If you have broken a fast, missed a window, or taken something the guidance asked you to pause, you have two options: rearrange your collection, or collect anyway and tell us what you had and when. Both are honest answers. The difference is that one gives you a clean result now and the other gives you a result that can be read in context, or repeated if it needs to be.

Contact [email protected] before your sample is taken if you are not sure which option is better for your test. If you have booked a clinic appointment or a nurse visit, tell us as early as you can so there is time to move it.

Never start, stop or change a prescribed medicine or supplement in order to take a blood test. Speak to the clinician who prescribed it. If you have already taken something the guidance asks you to pause, tell us the name, dose and timing so your results can be read in context.