Blood Tests: Common Types, How to Prepare, How Blood Is Drawn and What Results Mean
Updated: 2 days ago
Written and medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: November 2026 · How we review
Quick Answer: What are blood tests?
Blood tests are lab tests that measure or examine parts of the blood, such as red and white blood cells, platelets, electrolytes, proteins, hormones and minerals. They're used during routine checkups, to screen for risk factors, to look into symptoms and to see whether a treatment is working. A single abnormal number is not a diagnosis on its own. [1]
What the research says
For most people, eating before a cholesterol test barely changes the result. In 33,391 adults from the general population, normal meals changed total and LDL cholesterol by at most 0.2 mmol/L and raised triglycerides by at most 0.3 mmol/L, and non-fasting levels still predicted a 1.7- to 2.4-fold higher risk of heart events. (Langsted et al., Circulation, 2008) [2]
Fear of needles is common, so feeling nervous before a blood draw is normal. A review of 119 studies found that most children fear needles, along with 20% to 50% of teenagers and 20% to 30% of young adults; the fear generally eases with age and was frequently reported around venipuncture and blood donation. (McLenon et al., Journal of Advanced Nursing, 2019) [3]
Key takeaways
Blood tests look at blood cells, platelets, electrolytes, proteins, hormones and minerals. [1]
Common panels include the complete blood count, basic and comprehensive metabolic panels and the electrolyte panel. [1]
Most tests need no fasting, but some require several hours without food, so check with your provider. [1]
Most samples come from a vein in the arm; others use a finger stick, heel stick or an artery. [1]
Some results are ready within hours, while genetic tests can take longer. [1]
An abnormal result is a clue for your provider, not a diagnosis by itself. [1]
What is a blood test?
A blood test is a lab analysis of a small sample of your blood. Depending on the test, the lab can count your blood cells and platelets or measure electrolytes, proteins, hormones and minerals. [1]
Results give your provider important information, but they are one piece of the picture. Your provider combines them with your symptoms, history and exam. An out-of-range result does not automatically mean you have a serious problem. [1]
Why are blood tests done?
Providers order blood tests for many reasons: [1]
Routine checkups: as part of a regular physical exam
Screening: to look for risk factors for certain diseases before symptoms appear
Checking symptoms: to help explain what is causing them, or to confirm something specific such as a pregnancy
Genetic testing: to look for gene changes linked to certain conditions
Monitoring: to see how a condition is changing or whether a treatment is working
What are the most common blood tests?
Four common blood test panels and what they check.
Many routine blood tests are ordered as panels, which group several measurements from one sample. [1]
Panel | What it measures | What it helps check |
Complete blood count (CBC) | Red cells, white cells, platelets, hemoglobin, hematocrit, red cell size (MCV) and size variation (RDW) | Anemia, infection and other blood disorders |
Basic metabolic panel (BMP) | Glucose, calcium, sodium, potassium, chloride, carbon dioxide, blood urea nitrogen (BUN) and creatinine | Blood sugar, fluid and electrolyte balance, kidney function |
Comprehensive metabolic panel (CMP) | Everything in the BMP plus albumin, ALT, AST, alkaline phosphatase and bilirubin | Mainly liver and kidney health |
Electrolyte panel | The electrolytes in the BMP and CMP plus magnesium and the anion gap | Electrolyte and acid-base balance |
Sources: [1]
Our guides to the complete blood count (CBC) and the comprehensive metabolic panel (CMP) explain these two panels in more detail. [1]
What blood tests check for specific conditions?
Beyond routine panels, providers can order tests aimed at particular conditions. [1]
Condition area | Example blood tests |
Allergies | Immunoglobulin E (IgE) antibody tests for foods, pets, pollen and other allergens |
Autoimmune disease | Antinuclear antibody (ANA), complement C3, C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), peripheral blood smear |
Hormone and gland disorders | Blood glucose and A1C for diabetes, thyroid-stimulating hormone (TSH) for the thyroid, lipase and amylase for the pancreas |
Heart disease | Cardiac blood tests and arterial blood gas (oxygen and carbon dioxide levels) |
Blood disorders and clotting | D-dimer, fibrinogen, prothrombin time, reticulocyte count, free light chains |
Cancer | Tumor markers such as AFP, CA-125, calcitonin, CEA, HCG and PSA, and circulating tumor cell tests |
Other specialized tests | Ammonia, blood alcohol level, ferritin (iron stores) |
Sources: [1]
Tumor markers are substances linked to specific cancers. Circulating tumor cell tests, which look for cancer cells in the bloodstream, are relatively new and are mainly used to monitor certain cancers. For blood sugar, the A1C test reflects your average levels over the past few months, and a lipid panel checks cholesterol. [1]
How often should I get blood tests?
There is no single schedule for everyone. How often you need blood work depends on your health and your risk factors. Many providers recommend a yearly physical that includes a complete blood count, and people with a higher body mass index may be advised to have their blood sugar checked more often. [1]
How do I prepare for a blood test?
Most blood tests need no special preparation, but your provider will tell you if yours does. Helpful tips include: [1]
Fasting, if asked: some tests require you to skip food for several hours, sometimes allowing only sips of water
Ask what to avoid: check whether you should skip certain foods, drinks or activities beforehand
Drink water: being well hydrated can make your veins easier to find
Moisturize your arms: this may make it easier to insert the needle
Move a little: light activity while you wait may help your blood flow
How is blood drawn?
What happens during a routine blood draw from the arm.
Most blood samples are taken from a vein, a process called venipuncture, usually by a trained phlebotomist. [1]
The phlebotomist cleans the skin on the inside of your elbow.
They tie an elastic band around your upper arm to help the vein fill.
You may be asked to make a fist.
A thin needle goes into the vein, and blood flows into sealed collection tubes.
The band and needle are removed, and a small bandage covers the spot.
What other ways can blood be collected?
Different tests use different collection methods.
Method | How it works | Typical use |
Venipuncture | Needle into a vein, usually in the arm | Most routine blood tests |
Finger stick | A small drop from a fingertip collected on a special paper strip | Tests that need only a small sample |
Heel stick | A small sample from a baby's heel | Newborn screening |
Arterial draw | Blood taken from an artery instead of a vein | Arterial blood gas test of oxygen and carbon dioxide |
Sources: [1]
How much blood is taken?
Only a small amount. A complete blood count may need up to about 30 milliliters, while an average adult has roughly 4,500 to 5,700 milliliters of blood in their body. [1]
Do blood tests hurt?
You may feel a quick pinch or some discomfort. Draws from an artery usually hurt more than draws from a vein, and it can be more uncomfortable if your veins are hard to find. Tell the person drawing your blood if you are in pain. [1]
How can I feel less anxious about a blood draw?
Feeling nervous is common. These strategies can help: [1]
Ask what the test is for, so you know why it matters
Take slow, deep breaths
Look away from the needle
Distract yourself, for example by counting or talking
What happens after a blood test?
A bandage goes over the needle site, and you may be asked to sit and rest for a few minutes before leaving. [1]
When will I get my blood test results?
Some results come back within a few hours, while others, such as genetic tests, take longer. You may be able to see results in an online patient portal, but many providers prefer to explain them by phone or in person. [1]
If your provider asks to talk about your results, it does not necessarily mean something is wrong; it may simply be part of a routine checkup or a review of how your treatment is going. [1]
Conclusion
Blood tests are some of the most common and useful tools in medicine. A small sample can show how your blood cells, organs, hormones and metabolism are working, help screen for disease and track how well a treatment is working.
Follow any preparation instructions you are given, ask what each test is for, and review your results with your provider so you understand what the numbers mean for you.
Keep learning: Read our complete blood count (CBC) guide or our comprehensive metabolic panel (CMP) guide. For more background, see MedlinePlus on understanding your lab results.
FAQ
What can a blood test show?
It can measure blood cells, platelets, electrolytes, proteins, hormones and minerals to help screen for, diagnose and monitor conditions.
What are the most common blood tests?
The complete blood count, basic metabolic panel, comprehensive metabolic panel and electrolyte panel.
Do I need to fast before a blood test?
Most tests do not require fasting, but some need several hours without food, so ask your provider.
Can I drink water before a blood test?
Usually yes, and staying hydrated can make the draw easier, but follow any specific fasting instructions.
How is blood drawn for a test?
Most often a needle is placed in a vein on the inside of your elbow and blood is collected into tubes.
How much blood is taken for a blood test?
A small amount; a complete blood count may need up to about 30 milliliters.
Do blood tests hurt?
You may feel a brief pinch or some discomfort, and arterial draws usually hurt more than vein draws.
How long do blood test results take?
Some results are ready within hours, while genetic tests and some others take longer.
Does an abnormal blood test mean I am sick?
Not necessarily; an abnormal result is a clue your provider interprets along with your symptoms and history.
How often should I get blood work?
It depends on your health and risk factors, and many providers suggest a yearly physical with a complete blood count.
Related Reading
Iron Deficiency Anemia: Symptoms, Causes, Treatment & Complete Guide explains a condition often found with a complete blood count and ferritin test.
Diabetes: Symptoms, Types, Causes, Diagnosis & Treatment — Complete Guide covers a disease diagnosed and monitored with blood glucose and A1C tests.
Prediabetes: Symptoms, Causes, and Treatment describes raised blood sugar that routine blood tests can catch early.
High Blood Cholesterol: What Your Numbers Mean, Why It Matters, and How to Lower It explains the cholesterol results reported on a lipid panel.
References
Cleveland Clinic. Blood Tests: Types, Results & How They Work. Cleveland Clinic. Last updated December 6, 2022. https://my.clevelandclinic.org/health/diagnostics/24508-blood-tests
Langsted A, Freiberg JJ, Nordestgaard BG. Fasting and nonfasting lipid levels: influence of normal food intake on lipids, lipoproteins, apolipoproteins, and cardiovascular risk prediction. Circulation. 2008;118(20):2047-2056. PMID 18955664. https://doi.org/10.1161/CIRCULATIONAHA.108.804146
McLenon J, Rogers MAM. The fear of needles: A systematic review and meta-analysis. J Adv Nurs. 2019;75(1):30-42. PMID 30109720. https://doi.org/10.1111/jan.13818
Medical disclaimer
This article is for general education only and does not replace advice from a qualified healthcare professional. Talk to your doctor about your own diagnosis and treatment, and seek emergency care right away for severe or sudden symptoms.
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