How to Prepare for Blood Tests: Practical Guidance for Common Laboratory Tests

Fasting, medications, exercise, timing, and other factors that can affect blood test results.

Preparation for a blood test depends primarily on which test is being performed.

The common rule that “all blood tests must be done in the morning and strictly on an empty stomach” does not reflect modern laboratory medicine.

For fasting plasma glucose, caloric intake must be avoided for at least 8 hours. For HbA1c, fasting is not required. A standard lipid profile can now be performed without fasting in most patients. For TSH, medication use and possible biotin interference may be more important than fasting. When evaluating testosterone in men, the timing of blood collection is particularly important. For creatinine, a large cooked-meat meal shortly before testing can temporarily increase the measured concentration.

The correct approach is therefore not to impose the maximum possible number of restrictions before testing, but to follow the preparation requirements that are actually relevant to the specific laboratory investigation.

Complete Blood Count: Do You Need to Fast?

A complete blood count (CBC) generally does not require mandatory fasting.

A CBC typically includes:

• haemoglobin;
• red blood cell count;
• haematocrit;
• MCV;
• MCH;
• MCHC;
• RDW;
• white blood cell count and differential;
• platelet count.

For most of these parameters, an ordinary recent meal does not have the same diagnostic relevance as it does for tests such as fasting plasma glucose.

However, unusual or intense physical activity immediately before blood collection should be avoided when possible. Exercise and acute stress can transiently affect leukocyte counts, plasma-volume distribution, and some other haematological measurements.

If only a CBC has been ordered, prolonged fasting is usually unnecessary.

If a CBC is being performed together with fasting glucose or another test that requires fasting, the patient should follow the preparation requirements of the more restrictive test.

Glucose Testing: When Is Fasting Essential?

It is important to distinguish between different types of glucose testing.

Fasting Plasma Glucose

For fasting plasma glucose, fasting is required.

The American Diabetes Association Standards of Care in Diabetes—2026 define fasting as no caloric intake for at least 8 hours.

Plain water is generally permitted.

During this fasting period, patients should avoid:

• food;
• juice;
• milk;
• sweetened beverages;
• alcohol;
• other calorie-containing drinks.

If a test is specifically ordered as a fasting plasma glucose measurement, a sample collected after eating represents a different physiological state and should not be interpreted using fasting diagnostic thresholds.

Random Plasma Glucose

For random plasma glucose, fasting is not required.

By definition, this test may be performed regardless of the time of the last meal when clinically appropriate.

HbA1c

Fasting is not required for HbA1c.

HbA1c reflects longer-term glycaemic exposure and is much less affected by what the patient has eaten immediately before blood collection.

The absence of a fasting requirement is one of the practical advantages of HbA1c testing highlighted in current diabetes guidelines.

Cholesterol and Lipid Testing: Is Fasting Still Necessary?

For most patients, no.

A standard lipid profile usually includes:

• total cholesterol;
• LDL cholesterol;
• HDL cholesterol;
• triglycerides;
• non-HDL cholesterol;
• and sometimes ApoB or additional lipid markers.

Historically, lipid profiles were routinely performed after 8–12 hours of fasting. That approach has changed substantially.

A joint consensus statement from the European Atherosclerosis Society (EAS) and the European Federation of Clinical Chemistry and Laboratory Medicine (EFLM) recommends non-fasting lipid profiles for routine assessment in most patients.

After a typical meal, average changes in total cholesterol and LDL cholesterol are generally small, while HDL cholesterol and ApoB show little clinically relevant difference between fasting and non-fasting states.

Fasting may still be useful in selected circumstances, including:

• markedly elevated triglycerides;
• situations in which the physician specifically requires fasting triglycerides;
• simultaneous testing that includes another laboratory test requiring fasting.

EAS/EFLM recommendations suggest considering a repeat fasting measurement when non-fasting triglycerides are above approximately 5 mmol/L (440 mg/dL).

Therefore, the statement that “cholesterol must always be measured fasting” is no longer universally correct.

Liver Blood Tests: ALT, AST, GGT, ALP, and Bilirubin

Common biochemical markers used in liver assessment include:

• ALT;
• AST;
• GGT;
• alkaline phosphatase;
• bilirubin;
• albumin;
• and other tests depending on the clinical context.

For most of these investigations, 8–12 hours of fasting is not a universal requirement.

Exercise

AST is found not only in the liver but also in skeletal muscle.

After strenuous physical exercise, several markers may increase, including:

• AST;
• creatine kinase (CK);
• sometimes ALT;
• other muscle-associated biomarkers.

A raised AST concentration after heavy exercise should therefore not automatically be interpreted as evidence of liver disease.

Alcohol

Recent alcohol consumption can influence hepatic metabolism and several laboratory measurements.

If the aim is to assess the patient's usual baseline state, unusually heavy alcohol intake shortly before testing is best avoided.

At the same time, deliberately adopting an artificially “perfect” lifestyle for several days purely to improve laboratory values is not clinically useful. The purpose of testing is to assess the patient's actual physiological state.

Creatinine and Kidney Function: Does Food Matter?

Routine fasting is generally not mandatory for measurements such as:

• creatinine;
• eGFR;
• urea;
• cystatin C.

However, serum creatinine has an important dietary consideration.

Cooked Meat Can Temporarily Raise Creatinine

During cooking, some of the creatine naturally present in muscle tissue is converted into creatinine.

A large cooked-meat meal can therefore transiently increase serum creatinine and, as a consequence, lower calculated eGFR.

Clinical studies have shown that this effect can be large enough in some patients to temporarily alter CKD staging.

For this reason, when small changes in creatinine or eGFR are being monitored over time, repeat tests should ideally be performed under comparable conditions.

For example, it is not ideal to compare a fasting morning creatinine measurement with a creatinine measurement obtained shortly after a large meat meal and interpret the difference automatically as deterioration in kidney function.

Ferritin: Do You Need to Fast?

For ferritin, strict fasting is usually not a major requirement.

A much more important issue is that ferritin is both:

• a marker of iron stores;
• and an acute-phase protein.

Ferritin concentrations may rise in association with:

• inflammation;
• infection;
• liver disease;
• some chronic disorders.

For that reason, ferritin often needs to be interpreted together with:

• complete blood count;
• CRP;
• transferrin;
• transferrin saturation;
• serum iron.

A low ferritin concentration is generally more informative for identifying absolute iron deficiency than an isolated serum iron measurement.

Serum Iron and Transferrin Saturation

The situation is different for serum iron.

Serum iron is a relatively variable laboratory parameter.

Its concentration can be influenced by:

• biological variability;
• recent food intake;
• timing of blood collection;
• inflammation;
• medications;
• iron supplementation.

Studies also demonstrate substantial within-person variation in serum iron over the course of the day. The simple assumption that “serum iron is always higher in the morning” does not hold true for every individual.

If serum iron or TSAT is being monitored over time, repeat measurements should ideally be performed under similar conditions.

TSH, Free T4, and Other Thyroid Tests

For standard thyroid function tests such as:

• TSH;
• free T4;
• free T3;

fasting is usually not the principal requirement.

A more important issue is potential biotin interference.

Biotin Can Produce Misleading Thyroid Results

Biotin, or vitamin B7, is found in many dietary supplements, especially products marketed for hair, skin, and nails.

High-dose biotin can interfere with some laboratory immunoassays.

Depending on the analytical platform, this may create a laboratory pattern of falsely low TSH combined with falsely elevated T4 or T3.

In other words, the laboratory profile may mimic hyperthyroidism even when thyroid function is not actually increased.

The American Thyroid Association advises patients and clinicians to consider biotin use when thyroid laboratory results are being interpreted.

Should Levothyroxine Be Taken Before TSH and Free T4 Testing?

For patients taking thyroid hormone replacement, consistency in the timing of blood collection relative to medication is important.

Levothyroxine can temporarily influence circulating free T4 after a dose.

For serial monitoring, it is therefore preferable to follow a consistent approach each time blood is collected.

Whether blood should be taken before or after the morning dose should be agreed with the treating clinician, particularly when TSH and free T4 are being interpreted together.

Patients should not stop thyroid treatment on their own before testing.

Vitamin D: Is Fasting Required?

For routine measurement of 25-hydroxyvitamin D [25(OH)D], fasting is generally unnecessary.

Blood can usually be collected regardless of recent food intake unless the laboratory provides specific instructions.

It is more important to ensure that the correct vitamin D marker is being measured.

25(OH)D is the standard biomarker used to assess vitamin D status.

It should not be confused with 1,25-dihydroxyvitamin D, the active metabolite, which is measured for different clinical indications.

Vitamin B12 and Folate

Preparation requirements for vitamin B12 and folate may vary between laboratories and analytical methods.

It is particularly important to disclose the use of:

• vitamin B12 supplements;
• folic acid;
• multivitamins;
• other relevant nutritional supplements.

Recent supplementation can make the measured concentration less representative of the patient's untreated baseline status.

When the purpose of testing is to diagnose deficiency before treatment, blood should ideally be collected before supplementation is started, provided this is clinically appropriate.

Testosterone in Men: Timing Matters

When a man is being evaluated for possible testosterone deficiency, a random testosterone measurement taken later in the day may be insufficiently reliable.

Testosterone shows significant diurnal variation.

The Endocrine Society recommends confirming suspected hypogonadism with repeat measurements of early-morning fasting total testosterone.

Current guidance also emphasises that a diagnosis should not be based on a single low result.

Low testosterone should be confirmed with at least two properly obtained morning measurements in a patient with compatible symptoms and signs.

In practice, early-morning sampling is commonly performed around 07:00–10:00, although local protocols may vary.

Why Is One Testosterone Measurement Not Enough?

Testosterone can be temporarily reduced by:

• acute illness;
• substantial calorie restriction;
• excessive physical exercise;
• some medications;
• other physiological stressors.

A single borderline or low result should therefore not automatically establish a diagnosis of hypogonadism.

Cortisol: Time of Day Is More Important Than Fasting

Cortisol is one of the clearest examples of a laboratory marker with a strong circadian rhythm.

Concentrations are typically higher in the early period after waking and decline over the day.

Therefore, a cortisol concentration of 400 nmol/L at 08:00 and a cortisol concentration of 400 nmol/L in the evening do not have the same physiological meaning.

If a clinician orders an 8 AM cortisol, the sample should not simply be collected at 2 PM and interpreted against the same clinical criteria.

Endocrine evaluation may use different protocols depending on the suspected disorder, including:

• morning serum cortisol;
• late-night salivary cortisol;
• dexamethasone suppression testing;
• ACTH stimulation testing;
• other dynamic endocrine tests.

These protocols are not interchangeable.

Medications Before Blood Tests: Should You Take Them or Stop Them?

Do not stop prescribed medication on your own simply because you are having a blood test.

Medicines can interact with laboratory testing in three fundamentally different ways.

1. A Real Biological Effect

A medicine may genuinely alter the parameter being measured.

For example, lipid-lowering therapy is expected to lower LDL cholesterol. This is not laboratory interference. It is the intended biological effect of treatment.

2. Analytical Interference

A drug or supplement may interfere with the laboratory assay itself and create a falsely high or falsely low result.

Biotin interference with certain immunoassays is a well-known example.

3. Therapeutic Drug Monitoring

Sometimes the drug itself is being measured.

Examples include certain:

• antiepileptic drugs;
• immunosuppressants;
• lithium;
• digoxin;
• selected antibiotics.

In these situations, the timing of blood collection relative to the last dose can be essential.

For some medications, clinicians measure a trough concentration, meaning the blood sample is collected shortly before the next scheduled dose.

The laboratory may therefore need to know the medication name, dose, time of the last dose, and time of blood collection.

Can You Exercise Before a Blood Test?

When the purpose of testing is to assess the body's ordinary baseline state, unusually intense exercise immediately beforehand should generally be avoided.

Exercise can affect:

• CK;
• AST;
• lactate;
• white blood cell count;
• selected hormones;
• metabolic markers.

However, the simplistic rule “Do not exercise for exactly 24 hours before any blood test” is not scientifically appropriate.

The duration of exercise-related changes depends on the biomarker, exercise intensity, duration, and the patient's training status.

After particularly strenuous exercise, CK and some other markers may remain elevated for considerably longer than 24 hours.

Ordinary light activity or walking does not require the same restrictions.

Can You Drink Water Before a Blood Test?

For most blood tests, yes.

Plain water is generally permitted even when fasting is required.

Patients should not deliberately dehydrate themselves before testing.

They also do not need to drink excessive amounts of water immediately before blood collection.

The most appropriate approach is normal physiological hydration.

Can You Drink Coffee Before a Blood Test?

If the test does not require fasting, the significance of coffee depends on the particular analyte.

If a test has been ordered strictly fasting, the most standardised approach is to consume only water before blood collection.

Coffee is not physiologically identical to water, even without sugar or milk. Caffeine and other components can have metabolic and neuroendocrine effects.

Can You Have a Blood Test After Drinking Alcohol?

Alcohol can influence:

• triglycerides;
• glucose metabolism;
• hepatic metabolism;
• hydration;
• other biochemical parameters.

If the aim is to assess a patient's usual baseline state, significant alcohol consumption shortly before planned testing should generally be avoided.

However, artificially adopting an unusually restrictive lifestyle for several days purely to improve laboratory results is also inappropriate.

The objective of laboratory testing is to understand the patient's real physiological condition.

What If Several Blood Tests Are Ordered at the Same Time?

This is extremely common.

For example, a physician may order:

CBC + fasting glucose + HbA1c + lipid profile + ALT/AST + creatinine + TSH + ferritin + vitamin D.

Individually, many of these tests do not require strict fasting.

However, because fasting plasma glucose is included, it is reasonable for the entire blood draw to be performed after the required fasting period.

When several tests are collected at the same time, preparation is usually determined by the most restrictive test in the panel.

Practical Examples

How to Prepare When You Are Unsure About the Test Requirements

1. Check the exact names of the tests that have been ordered.
2. Determine whether any of them specifically require fasting.
3. Check whether time of day or timing relative to medication is important.
4. Inform the laboratory or clinician about regularly used medications and supplements.
5. Do not stop prescribed treatment without medical advice.
6. Avoid unusually strenuous exercise before routine baseline testing.
7. For repeat testing, try to reproduce the conditions of previous blood draws.

Why Consistent Conditions Matter for Repeat Blood Tests

When laboratory tests are used to monitor changes over time, the goal is not only to obtain a valid individual result but also to ensure comparability between measurements.

For example:

Ferritin: 32 → 41 µg/L

or

Creatinine: 98 → 108 µmol/L

should not be interpreted solely from the numerical difference.

Relevant questions include:

• Was the same laboratory used?
• Were the samples collected at a similar time of day?
• Was fasting status comparable?
• Was there recent strenuous exercise?
• Had medication changed?
• Was there infection or inflammation?
• Were collection conditions similar?

Every laboratory result contains a combination of true biological information, normal within-person biological variation, and analytical variation.

For this reason, small changes are most meaningful when testing conditions are reasonably consistent.

The Main Principle: There Is No Single Preparation Protocol for Every Blood Test

One of the most common mistakes is applying the same preparation rules to fundamentally different laboratory investigations.

For fasting plasma glucose, at least 8 hours without caloric intake is required.

For HbA1c, fasting is not required.

For a routine lipid profile, most patients do not need to fast.

For a CBC, special fasting is usually unnecessary.

For ferritin, inflammation is generally more important than the last meal.

For creatinine, a large cooked-meat meal may transiently increase the result.

For TSH and free T4, fasting is usually not the main concern, whereas biotin may substantially interfere with some laboratory assays.

For testosterone, appropriate early-morning repeat measurements are essential when diagnosing male hypogonadism.

For cortisol, timing of the sample is a fundamental part of interpretation.

Do not prepare for every blood test in the same way. Prepare according to the specific test that has actually been ordered.

The purpose of preparation is not to make laboratory values look “better.”

It is to obtain measurements that:

• accurately address the clinical question;
• can be correctly compared with diagnostic thresholds and reference intervals;
• minimise avoidable preanalytical variation;
• can be meaningfully compared with previous and future results.

Scientific and Clinical References

1. American Diabetes Association Professional Practice Committee. Diagnosis and Classification of Diabetes: Standards of Care in Diabetes—2026. Diabetes Care. 2026.
2. Nordestgaard BG, Langsted A, Mora S, et al. Fasting is not routinely required for determination of a lipid profile: clinical and laboratory implications. Joint consensus statement from the European Atherosclerosis Society and European Federation of Clinical Chemistry and Laboratory Medicine. European Heart Journal. 2016;37:1944–1958. doi:10.1093/eurheartj/ehw152.
3. Simundic AM, Bölenius K, Cadamuro J, et al. Joint EFLM-COLABIOCLI Recommendation for venous blood sampling. Clinical Chemistry and Laboratory Medicine. 2018;56(12):2015–2038.
4. Bhasin S, Brito JP, Cunningham GR, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. Journal of Clinical Endocrinology & Metabolism. 2018.
5. Endocrine Society. Statement on Testosterone Replacement Therapy. July 2026.
6. American Thyroid Association. Clinical guidance on biotin interference with thyroid laboratory testing.
7. Nair S, O'Brien SV, Hayden K, et al. Effect of a cooked meat meal on serum creatinine and estimated glomerular filtration rate in diabetes-related kidney disease. Diabetes Care. 2014;37:483–487. doi:10.2337/dc13-1770.
8. Dale JC, Burritt MF, Zinsmeister AR. Diurnal variation of serum iron, iron-binding capacity, transferrin saturation, and ferritin levels. American Journal of Clinical Pathology. 2002;117:802–808. doi:10.1309/2YT4-CMP3-KYW7-9RK1.
9. KDIGO. Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. 2024.
10. Clinical and Laboratory Standards Institute (CLSI). Collection of Diagnostic Venous Blood Specimens.