How to prepare for your blood test

Accurate interpretation begins before the sample is collected. Food, exercise, the time of day, medicines and supplements can all influence particular blood measurements. A temporary change can otherwise be mistaken for your usual baseline.

Preparation is intended to make the result more meaningful. It cannot guarantee a particular number, and the objective is not to produce a report with no flags.

Some tests need very little preparation. Others depend on a specific fast, sampling window or interval from treatment. Follow the requirements attached to your own test rather than assuming every instruction on this page applies to you.

This guide provides educational information. It does not replace advice from your own clinician, and it is not a reason to start, stop or change a prescribed medicine without clinical advice.

The preparation requirements appear on the test page before you buy, continue through the cart and checkout, and are repeated with your order. Use the order as your practical checklist.

The sections below explain the requirements, including exact timing where relevant. Some describe collection or eligibility rather than preparation. Referral turnaround explains why particular results may arrive later.

Where several tests are taken from one sample, follow the most restrictive applicable requirement. If two instructions appear to conflict, or a requirement is difficult or unsafe for you to follow, contact the provider before collection. People using medicines that can cause hypoglycaemia may need an individual fasting plan.

Where lipids, fasting insulin or fasting glucose are being assessed, the published Praetor preparation requirement is a 12-hour fast before the sample.

Plain water is permitted. Avoid food and caloric drinks. Under this protocol, avoid coffee and tea as well as milk, juice and other beverages, and avoid alcohol throughout the fasting period. Follow the plain-water instruction rather than trying to decide which alternative drink contains few enough calories.

HbA1c does not require fasting. If it is being measured alongside a test that does require a fast, the stricter requirement still applies to the combined sample.

Continue prescribed medicines unless your clinician has specifically advised otherwise. Record relevant medicines and supplements taken during the fasting period. If fasting may create a problem with your health or treatment, obtain advice in advance rather than altering medication yourself.

Avoid alcohol for at least 24 hours before blood testing.

Where Liver Function, GGT or a Lipid Profile is being assessed, the published guidance prefers 48 hours without alcohol, particularly when establishing a baseline or investigating previous abnormalities.

Alcohol can affect triglycerides and some liver-related measurements. Repeated tests are easier to compare when the period of avoidance is similar. A brief period without alcohol does not necessarily reverse a persistent alcohol-related abnormality.

Tell the provider about significant drinking in the days before collection. The information helps interpretation; concealing it can make the result less useful.

Timing depends on the hormone, menstrual pattern and use of hormonal treatment. The first day of menstrual bleeding is generally counted as day 1 when cycle-day instructions are used.

For baseline Oestradiol, FSH and LH assessment in regular cycles, the published guidance generally uses the early follicular phase, typically days 1–3. An irregular or absent cycle may make a calendar-based instruction inappropriate; the indication for testing should guide the plan.

Progesterone is generally measured approximately seven days before the expected next period. Day 21 is appropriate only for a typical 28-day cycle. Longer or shorter cycles require different timing.

Where ovulation can be identified reliably, a sample approximately seven days afterwards may be appropriate. An incorrectly timed low result should not be treated as proof that ovulation did not occur.

Cortisol

The preferred window is 08:00–09:00, when Cortisol is normally near its daily peak. Repeat measurements should be taken at approximately the same time.

Cortisol sampling does not need to be arranged for a particular menstrual-cycle day, including where cycles are irregular. The actual sample time should be recorded. A morning result is not a complete measure of chronic stress or daily cortisol production.

Testosterone

For assessment of natural testosterone status, collect in the morning, ideally before 11:00, when concentrations are generally highest. Where possible, test fasted and when clinically well.

A low or borderline result generally needs a second morning measurement before Testosterone Deficiency is established. Symptoms, medical history and other possible causes still need assessment.

If you are receiving Testosterone Replacement Therapy, timing relative to administration is important. Ask the prescribing clinician which interval is appropriate for your formulation and treatment plan. A generic instruction to test in the morning does not replace that advice.

Record the date and time of collection, the date and time of the most recent testosterone dose, the dose administered and the route or formulation where relevant.

DHEA-S

DHEA-S can be measured at any time of day and does not require menstrual-cycle timing. Where it forms part of a broader hormone panel, morning sampling is preferred for consistency with the other markers. Repeat tests should ideally be at a similar time.

It does not require the specific 08:00–09:00 window used for Cortisol.

Disclose DHEA and other androgenic supplements, including dose, most recent dose time and duration of use. These can substantially alter the result.

Age, sex, symptoms and other androgen measurements guide interpretation. In women being assessed for androgen excess, relevant details include menstrual history, hormonal contraception or HRT, and features such as hirsutism, acne or androgen-related hair loss. DHEA-S is not an isolated measure of overall androgen health.

The salivary Cortisol day curve requires four samples:

On waking, within 30 minutes of getting up.

At midday, 12:00.

In the afternoon, 16:00.

Immediately before going to sleep.

Record the exact collection time for every sample. Cortisol changes through the day, and timing is part of the information the laboratory needs. If the schedule cannot be followed, contact the provider rather than substituting an unrecorded time.

Avoid sexual intercourse or ejaculation for at least 48 hours before PSA testing. Recent ejaculation can temporarily raise PSA and complicate interpretation.

This is a test-specific preparation requirement. It is not an instruction that applies automatically to every blood test.

For PSA testing, avoid vigorous exercise, including heavy resistance or weight training, for 48 hours before collection under the published protocol.

Do not assume the shorter general exercise interval below overrides this requirement when PSA is included in the sample.

Avoid cycling and other activities that put significant pressure on the perineum for at least 48 hours before the test.

If PSA is unexpectedly raised, the clinician may consider repeat testing after potential transient causes have resolved. Preparation alone does not determine whether a raised result needs investigation.

Avoid strenuous or unusually intense exercise for at least 24 hours before testing, particularly where CRP, Full Blood Count or liver-related markers are being assessed.

Hard exercise can alter inflammatory and blood-cell measurements and raise muscle-derived enzymes, particularly CK and AST. ALT and other biochemical measurements may also change. These effects can make a sample less representative of your usual resting state.

For an accurate baseline or treatment-monitoring sample, 48 hours without strenuous exercise may be preferable, especially after an exceptionally hard or prolonged session. Follow any longer instruction specific to the tests you are having.

Normal light activity, including routine walking, is generally acceptable. The aim is to avoid taking a baseline sample immediately after an unusually demanding effort.

Insulin and Homocysteine require a suitable venous sample under the Praetor laboratory protocol. Do not select a capillary or finger-prick collection method for these markers.

Sample type, processing and the laboratory method affect reliability. Attend a venous blood draw where those markers are required, and use the collection options shown for the actual product.

The saliva kit is included for the relevant test. For each sample, avoid food, drink other than permitted water, smoking and tooth brushing for at least 60 minutes beforehand.

Water is permitted. Approximately ten minutes before collection, rinse the mouth with water for up to five seconds, then spit it out.

Follow the supplied kit instructions as well as the timing schedule. A saliva protocol is different from a blood-test fasting instruction; use the guidance for the sample you are collecting.

Some markers are analysed by a specialist referral laboratory. Those results may arrive after the rest of the panel.

The expected turnaround is shown on the test page and order. It is an estimate, counted from the first working day after the laboratory receives the sample, and is not guaranteed. Delays can occur.

Allow sufficient time before a consultation or treatment review. This is an explanation of the reporting process rather than a preparation step.

Biotin can interfere with some laboratory immunoassays, producing misleading results. Thyroid tests, AMH and other hormone assays can be affected, depending on the method.

The published requirement is to avoid non-prescribed Biotin supplements for 48 hours, or two days, before collection. Check multivitamins and products marketed for hair, skin and nails, which may contain it.

If Biotin is prescribed, do not stop it without discussing the plan with the prescribing clinician. Tell the provider about all supplements, including the dose and most recent use. If the pause has not been possible, disclose that before relying on the result.

For anaemia or iron-status assessment, disclose all iron-containing medicines and supplements, including dose and the time of the most recent dose.

The published protocol prefers sampling at least 24 hours after vitamin or mineral supplements because recent intake can influence serum Iron. Unless the prescribing clinician instructs otherwise, avoid non-prescribed iron and other vitamin or mineral supplements during the 24 hours immediately before collection.

Do not discontinue prescribed Iron without clinical advice. If a dose cannot appropriately be delayed, the clinician should determine the sampling plan.

The preferred collection window for this assessment is 06:00–10:00, with heavy exercise avoided for 48 hours beforehand. Where several instructions apply, use the strictest applicable interval and seek advice if they cannot be reconciled.

The published requirement is to avoid non-prescribed Vitamin D supplements for 24 hours before the test. Do not discontinue prescribed Vitamin D or another prescribed supplement without clinical advice.

Record the dose and the time of the most recent dose. Where the purpose is to monitor supplementation, repeat testing should be carried out under comparable conditions and at a similar point relative to dosing.

A short pause does not remove the effect of a long-term regimen. The result still needs to be interpreted in the context of ongoing intake.

For Thyroid Function testing, the published preference is morning collection between 06:00 and 10:00, before the daily thyroid medicine.

Do not discontinue treatment or change the prescribed regimen without advice. If the preferred sampling time does not fit the way you are prescribed to take the medicine, ask the prescriber how to arrange the sample.

For repeat monitoring, use a similar time of day and the same interval relative to medication where possible. The formulation, dose and recent administration may affect interpretation.

Continue HRT or oral contraception as prescribed. Do not stop treatment solely to arrange a blood test.

Tell the provider the name, dose and type of preparation. Hormonal treatment affects female sex-hormone results, which need to be interpreted as measurements during treatment rather than assumed to describe natural production.

If the purpose is specifically to assess natural hormone levels, discuss this with the GP or prescriber before booking. A supervised period off treatment may sometimes be needed, but that decision and interval must come from the clinician. Do not create your own washout period.

Remain normally hydrated before and during finger-prick testing. Plain water is allowed during the fasting period.

Keep your hands warm to support peripheral blood flow. Washing them in warm water or gently rubbing them together may help make collection easier. Follow the kit's collection instructions.

Avoid excessive drinking immediately before testing. Normal hydration is sufficient; forcing down water can alter concentrations and does not guarantee a better sample.

The website's “male only” label concerns the intended use of PSA testing, rather than a preparation step. Clinical relevance depends on the presence of a prostate and the individual's history.

PSA should not be used as routine screening in younger people without a clinical indication. It has potential benefits and limitations, including false-positive results and the possibility of finding a cancer that would not have caused harm. Testing requires an informed discussion.

The published guidance generally considers testing from age 50 for average-risk men, with earlier discussion from age 45 where risk is increased. Relevant factors include a significant family history of prostate cancer, Black ethnicity and inherited risk such as a BRCA2 mutation.

In men under 40, PSA should generally be reserved for a specific clinical indication or specialist recommendation. There is no absolute minimum age where a genuine indication exists, but routine testing of asymptomatic adolescents or young adults is inappropriate. The reason for testing should be documented.

The website's “female only” section explains the intended clinical use of these tests. Anatomy, treatment and the reason for testing remain relevant to suitability.

AMH can generally be measured at any point in the cycle. Testing under the age of 18 should not be undertaken routinely without appropriate clinical oversight. HRT, hormonal contraception and other hormonal medicines should be disclosed because they may affect interpretation.

AMH contributes information about ovarian reserve. It is not a stand-alone measure of fertility or the ability to conceive naturally.

Progesterone used to assess ovulation requires appropriate timing: generally about seven days before the expected next period, or approximately seven days after reliably identified ovulation where appropriate. A fixed day-21 sample only fits a typical 28-day cycle.

Disclose HRT, Progesterone, other progestogens and hormonal contraception. Exogenous hormones can substantially affect the result. A Progesterone concentration cannot be used to assess ovulation without considering sample timing and treatment.

Check the instructions attached to your order the night before and again in the morning. Confirm the fasting interval, sampling window, collection method and any information you have been asked to record.

Bring or record relevant medication and supplement details. Note the actual collection date and time and any recent doses that matter to interpretation. Arrive with enough time to avoid taking the sample immediately after a hard physical effort.

Plain water remains permitted during a required fast. Routine light activity is generally acceptable unless your individual instructions say otherwise.

Tell the provider. A broken fast, missed timing window or recent supplement dose is useful information, and concealing it makes interpretation harder.

Depending on the test, the options may be to rearrange collection or proceed with the deviation documented. The provider can advise whether the result will still answer the intended question or may require repeating. Rearranging improves standardisation; it does not guarantee a particular result.

If a clinic appointment or nurse visit needs to move, make contact as early as possible through the details on the order or the Praetor website. State what happened, when it happened, and the name and dose of any relevant medicine or supplement.

Never start, stop or change prescribed treatment to obtain a preferred blood result. A representative sample, interpreted with an honest account of the circumstances, is more useful than a result obtained by changing your normal treatment without advice.