How to Prepare for Blood Tests: Fasting, Medicines, Exercise, Sleep and Common Mistakes

Laboratories estimate that most errors affecting test results occur not in the analyser but before sample collection: because of food, coffee, exercise, supplements, a sleepless night or poorly chosen timing. A patient who has not prepared correctly may receive a result that appears to indicate a medical condition but is actually an artefact, or conversely, a result that masks a genuine health problem. Below, we explain how to prepare for the most common blood and urine tests, focusing on factors that actually affect the results. These general guidelines apply at ALAB, Synevo and ETER-MED laboratories; individual laboratories may specify additional requirements for specialised tests.

What does fasting mean?

Fasting means going without food and drinks other than water for 8–12 hours. Ideally, have your last meal by 19:00. It should be light, without excessive fat or alcohol. You may drink a glass of still water in the morning; in fact, this is advisable because dehydration concentrates the blood and makes sample collection more difficult. Avoid coffee and tea (even without sugar), juices, chewing gum, cigarettes and lozenges. Fasting for longer than 14–16 hours can also affect results: it increases bilirubin and uric acid levels while lowering glucose.

Tests requiring fasting: Glucose – laboratory test, Insulin – laboratory test, Lipid profile (cholesterol) – laboratory test (cholesterol, LDL, HDL, triglycerides), Iron – laboratory test, liver function tests, phosphorus, uric acid, homocysteine, Vitamin B12 – laboratory test and most hormone tests. Tests that do not require fasting, although consistent preparation is advisable for comparison: full blood count, C-reactive protein (CRP), ESR, Thyroid-stimulating hormone (TSH) – laboratory test, Creatinine – laboratory test, electrolytes, 25-hydroxyvitamin D (25 OH) – vitamin D test, Glycated haemoglobin (HbA1c), antibody tests and serological tests.

Time of day

Many blood parameters follow a circadian rhythm. Iron, cortisol, testosterone, TSH and prolactin levels are highest in the morning; testing in the afternoon may produce values below the reference range even in a healthy person. The standard blood collection window is between 7:00 and 10:00. If tests are being repeated for comparison, samples should be collected at the same time of day.

Physical exercise

Intense exercise within 24–48 hours before testing increases CK, Aspartate aminotransferase (AST) – laboratory test, Alanine aminotransferase (ALT) – laboratory test, LDH, Creatinine – laboratory test, uric acid, white blood cells, Ferritin – laboratory test and C-reactive protein (CRP), while lowering haemoglobin through plasma dilution. It can also affect urinary protein levels. The day before testing, walking is fine, but avoid gym workouts and long-distance running. On the day of your test, do not arrive straight from a bicycle ride; sitting quietly for 15 minutes before blood collection helps stabilise the results.

Alcohol

Alcohol consumed within 2–3 days before testing increases triglycerides, Gamma-glutamyl transferase (GGT) – laboratory test, Alanine aminotransferase (ALT) – laboratory test, uric acid and MCV, lowers glucose and disrupts electrolyte balance. Avoid alcohol for at least 3 days before a lipid profile or liver function tests; even a single night of heavy drinking the day before can double triglyceride levels.

Medicines and supplements

Do not stop regular medication (for high blood pressure, thyroid disease, diabetes or heart conditions) without consulting your doctor. However, it is worth asking whether to take your medication before or after blood collection. Levothyroxine should be taken after blood has been collected for a TSH test. Guidelines for supplements:

• Biotin (hair and nail supplements, high-dose B vitamins) can interfere with thyroid hormone tests, troponin, vitamin D and many other immunoassays: stop taking it for 2–3 days before testing, or for a week if taking very high doses.
• Iron: stop supplementation for 24–48 hours before measuring iron levels and transferrin saturation.
• Vitamin D, B12, magnesium and zinc: take your morning supplements after blood collection; when assessing the effectiveness of supplementation, your doctor may recommend a 1–2 day break.
• Creatine can increase creatinine levels: mention its use when providing your medical history.
• Aspirin, ibuprofen and anticoagulants: do not stop taking them without medical advice, but inform the healthcare professional before coagulation and platelet tests.

Sleep, stress and infections

A sleepless night increases cortisol, glucose and white blood cell levels. Stress before blood collection (fear of needles or rushing to an appointment) increases prolactin, cortisol, glucose and blood pressure. An infection within the previous 2 weeks, even a mild one, can increase CRP, ESR, ferritin and white blood cell levels, and may lower TSH, iron and platelet counts. Routine preventive tests are best postponed until 2 weeks after an infection; monitoring tests during an illness should be performed as instructed by your doctor.

Hormone tests: additional preparation guidelines

Prolactin – laboratory test: in the morning, after sitting quietly for 30 minutes, avoiding sexual intercourse and breast stimulation on the previous day. Serum cortisol – laboratory test: between 7:00 and 9:00, after 15 minutes of rest. Total testosterone – laboratory test in men: between 7:00 and 10:00, after a good night’s sleep; a low result should always be confirmed by a second measurement. FSH, LH and Oestradiol – laboratory test in women: days 2–5 of the menstrual cycle; progesterone: approximately 7 days before the expected period. Thyroid-stimulating hormone (TSH) – laboratory test: on any day of the menstrual cycle, in the morning.

Urinalysis and urine culture

Collect the first morning urine sample from the midstream flow, after washing the area around the urethra with water (without disinfectants), into a sterile container purchased from a pharmacy. Deliver the sample to the laboratory within 2 hours (or refrigerate it for up to 4 hours). Avoid testing during menstruation and for 2 days before and after it. The day before testing, avoid strenuous exercise, beetroot (which can discolour urine) and high doses of vitamin C (which can interfere with urine dipstick results). A urine culture requires the same collection technique, but the sample should be obtained before starting antibiotic treatment.

Oral glucose tolerance test (OGTT)

For 3 days before the test, follow your normal diet, including at least 150 g of carbohydrates daily (a low-carbohydrate diet can produce falsely elevated results suggestive of diabetes). Fast for 8–12 hours, attend in the morning and avoid smoking and coffee. The test takes approximately 2 hours; during this time, you must not eat, drink, walk around the building or smoke. Remain seated in the waiting area. Anyone who vomits after drinking the glucose solution should inform the staff, as this invalidates the test.

Children and infants

For infants and young children, a full fasting period is neither necessary nor safe; a break of 3–4 hours since the last feed is sufficient (or as advised by the doctor). Make sure your child is well hydrated and reassured. Blood collection centres usually have designated appointment times or facilities for children.

Checklist for the evening before your test

• have your last meal by 19:00, keep it light and avoid alcohol for 3 days,
• avoid exercise today and tomorrow morning,
• stop taking biotin and iron supplements according to the guidelines above,
• schedule blood collection between 7:00 and 10:00, allowing an extra 15 minutes to rest,
• prepare your identity document and proof of purchase for your tests (confirmation code or email),
• for urine testing: prepare a container from the pharmacy and collect a morning sample,
• in the morning: drink a glass of water, take regular medication as agreed with your doctor and take your thyroid tablet after blood collection.

After blood collection, it is advisable to have breakfast and avoid strenuous physical activity immediately afterwards. Results for most routine tests are available online on the same or the following day; hormone and specialised test results usually take several days.

Frequently asked questions

Can I drink coffee without sugar before a blood test?

No. Caffeine increases glucose, cortisol and fatty acid levels, and coffee also affects the lipid profile. Water is the only permitted drink.

I had a fatty dinner last night. Should I reschedule my test?

Yes, postpone lipid profile and liver function tests by one day. A full blood count, Thyroid-stimulating hormone (TSH) – laboratory test or Creatinine – laboratory test can still be performed.

Do I need to see a doctor to have blood tests without a referral?

No. Tests purchased online can be performed at a blood collection centre by presenting an identity document. No referral (a formal request from a doctor) or doctor’s appointment is required.

Sources

• Simundic AM et al., EFLM Working Group for Preanalytical Phase, Clin Chim Acta, Standardization of collection requirements for fasting samples, 2014. eflm.eu
• Simundic AM et al., Clin Chem Lab Med, Joint EFLM-COLABIOCLI Recommendation for venous blood sampling, 2018. degruyterbrill.com