Blood Testing Made Simple: What to Know Before Your Next Test

Author: Emily Carter

Medical reviewer:

Jessica Miller, BSN, RN

Publised October 27, 2025
4 min read

If you’ve been to a hospital for treatment or a routine check-up, you most likely had to do a blood test. That’s because blood testing is the most common method health care professionals use for patient testing.

Before the test, you probably had some questions about the basics of blood testing, like: What is a blood test? When should you get one? What can be detected in a blood test, and how to overcome the fear of a blood draw if you are nervous? We’ve got you!

This article has everything you need to know about blood testing.

What Is a Blood Test?

As the name implies, a blood test is a medical procedure that involves drawing a small volume of blood from your vein or artery to facilitate testing for overall health and body function, medical issues, or the progress of treatment.

Usually, the first step in the blood test procedure is performed by a phlebotomist, a healthcare professional trained to make blood draws for medical testing.

Once your blood is drawn, it is taken for testing by a lab technician as prescribed by your physician. After the lab test, the technician writes a blood test report.

Different Types of Blood Tests

Your doctor or physician can recommend different types of blood tests based on the specific aspect of your health that they want to assess. Below are the various types of blood tests and what they mean.

Complete Blood Count (CBC)

The complete blood count test is among the most common types of blood tests. CBC focuses on your blood cells, including red blood cells, white blood cells, and platelets, to determine if they are normal in their numbers, size, color, and shape.

Abnormal blood cell results can indicate blood disorders such as anemia, infections, immune system disorders, or other underlying health issues.

You may hear some physicians refer to the CBC test as Full Blood Count (FBC) or Full Blood Examination (FBE).

Regarding what a CBC shows, the test usually produces a long report. Some key entries you’ll see on a CBC report include:

Hemoglobin: The level of protein in your red blood cells, which are responsible for carrying oxygen to your body organs.
Haemotocrit: The number of red cells that make up your blood.
Monocytes: The white blood cells that fight infections and remove dead cells to destroy germs.
Lymphocytes: Your white blood cells that make antibodies and help fight cancer.
Neutrophils: The most common white blood cells that help your body fight infections.
Platelets: The cells responsible for blood clotting to control bleeding.

Watch this video for a demonstrated explanation of the complete blood count test.

Blood Glucose Test

A blood glucose test is also called a blood sugar test or a diabetes blood test. The test checks the level of sugar in your blood and is crucial in detecting high blood sugar, which is a marker for diabetes. Doctors also use the test to monitor blood sugar levels in patients with diabetes.

Considering the growing global diabetes burden, a blood sugar test is commonly prescribed during routine tests. Your doctor may also recommend it if:

You have known risk factors, such as a family history of diabetes.
You are presenting symptoms of diabetes, such as increased thirst and frequent urination.
Your results from other tests suggest you could be having high blood sugar.

To ensure precision, physicians may request different blood sugar tests:

Fasting blood glucose test: Done after 8–12 hours of fasting.
Random blood sugar test: Done without fasting and, therefore, also called a non-fasting blood sugar test.
Oral glucose tolerance test (OGTT): Done with blood draws before and after a sugary drink.
Hemoglobin A1c (HbA1c): Used to monitor your long-term blood sugar levels for 2–3 months.

Basic Metabolic Panel (BMP)

Your physicians will prescribe a basic metabolic panel (BMP) blood test to assess your overall body metabolism and organ function. The test checks how your body processes energy from food, kidney function, and your overall body fluid balance.

Specifically, a BMP test focuses on particular substances in your body, such as glucose, potassium, calcium, sodium, carbon dioxide, and blood urea nitrogen.

Your doctor may upgrade the BMP test to a Comprehensive Metabolic Panel (CMP) to include liver function for markers such as bilirubin and Aminotransferase (AST).

Lipid Profile (Cholesterol Test)

A lipid profile or cholesterol blood test checks the levels of good and bad fats in your blood. The fats are in the form of cholesterol, lipids, and triglycerides, and are key indicators of the health status of your heart and blood vessels.

The lipid panel test shows:

Your total cholesterol levels.
The level of good cholesterol (HDL-cholesterol).
The level of bad cholesterol (LDL-cholesterol).
The level of triglycerides.

C-reactive Protein (CRP) Blood Test

The C-reactive protein is made in your liver and delivered to your bloodstream. The CRP test is one of the immune system blood tests that checks the levels of this protein in your blood as a marker of inflammation, autoimmune diseases, tissue damage, or infection.

The test will not show where the inflammation is located. So, your doctor may request follow-up tests after the CRP test.

Other Types of Blood Tests

Other less common and specialized blood tests that your doctor might recommend include:

The human chorionic gonadotropin test (hCG test): To determine a pregnancy.
Erythrocyte Sedimentation Rate (ESR): To test for inflammation.
Vitamin and Hormone blood tests: To test for endocrine balance, nutritional deficiencies, and reproductive health. The tests may focus on hormones such as insulin, thyroid, cortisol, estrogen, testosterone, and vitamins like folate, iron, calcium, magnesium, B12, and vitamin D.
Tumor markers blood tests: Including the prostate-specific antigen (PSA) test and Cancer Antigen 125 (CA-125).
Infectious disease blood tests: Such as Hepatitis, HIV, and COVID-19 antibodies.
Allergy blood test (IgE test): Tests blood for high levels of immunoglobulin E (IgE) antibodies. The test can indicate allergies to food, pollen, pet fur, and more.
Total protein test: Targets blood protein to test for liver and kidney problems.
Thyroid-stimulating hormone (TSH) test: To check for overactive thyroid (hyperthyroidism) or underactive thyroid (hypothyroidism).

It’s worth noting that your doctor can send you for more than one blood test at the same time, which also means a higher volume of blood draw.

What Conditions Can Blood Tests Help Detect?

Your doctor can order a blood test for different purposes. What a blood test checks depends on its purpose and the specific result your doctor is looking for.

From the types of blood tests explained above, blood tests can help detect:

Anemia and other blood disorders.
Infection and immune system issues.
Organ health, such as liver and kidney diseases.
Cancer markers (tumor markers).
Allergy antibodies.
Nutritional deficiencies for vitamins and minerals.
Hormonal and reproductive disorders.
Inflammation markers and autoimmune disorders.
Heart health and blood vessel issues.

When Should You Get a Blood Test?

Your doctor knows best when you should get a blood test. However, if you really want an answer to how often you should get blood work done and when you should get a blood test, the best thing is to start with the recommended routine annual checkup. Your doctor will then pick it from there, especially if you have an abnormal blood test result.

That said, there are also some specific situations when you should get a blood test:

During a routine checkup

During a preventative health screening or an overall annual health check, your physician is likely to prescribe a complete blood count and a basic or comprehensive metabolic panel.

When you have symptoms of illness

Symptoms of disease are often clear signs that you need a blood test. Such symptoms include fever, vomiting, unexplained fatigue, shortness of breath, skin issues, dizziness, hair loss, and others. Because the reasons for such symptoms are not always obvious, blood testing can reveal the underlying causes.

You have chronic medical conditions

Your doctor will recommend a blood test more frequently to monitor your progress and the effectiveness of the treatment if you have a chronic medical condition such as diabetes, heart disease, kidney disease, or thyroid disorders.

You have predisposing factors for certain health conditions

Your physician may recommend preventative blood testing or a baseline blood test to keep your health status in check if you have a family history of certain health conditions, such as diabetes and heart disease, or lifestyle predisposing factors, like smoking.

You are pregnant

Blood testing in pregnancy is crucial to test for blood type, iron levels, or health conditions such as infections and gestational diabetes that can put the fetus at risk.

You are booked for a major medical procedure

Doctors will always prescribe a thorough pre-surgery blood test before a major medical procedure, or a pre-treatment blood test before starting a rigorous treatment for chronic diseases. These blood tests help check organ function and prevent drug allergies.

What Happens During a Blood Test?

The key part of a blood test where you are actively involved is the blood draw, which happens in less than 10 minutes. These simple steps explain how a blood test is done.

The phlebotomist ushers you to the blood drawing room and asks you to sit on a chair with your arm resting on a table or arm support.
The phlebotomist assesses your arm for a noticeable vein, usually at the bend of the elbow. The clinician will ask you to make a fist or use an elastic band around the arm to control blood flow for clear vein visualization. Modern phlebotomists may also use a vein finder.
Once the vein is identified, the phlebotomist wipes the venipuncture area with a disinfectant and inserts a needle into the vein to take a blood sample. Blood flows into an attached tube or vial.
When the blood sample is drawn, the phlebotomist removes the needle and elastic band and asks you to release your fist. A piece of cotton is bandaged onto the venipuncture area to stop bleeding.
You are then free to leave, while the blood is delivered to the lab for testing.

Methods Used for Blood Collection

Phlebotomists can use different blood collection methods depending on the type of patient and the amount of blood required.

Below are the most common blood collection techniques used by healthcare professionals:

Venipuncture

This is the most common method for venous blood collection. The nurse or phlebotomist collects blood from a vein by inserting a needle that’s connected to a blood-collecting tube.

This method is used for larger blood samples required for comprehensive tests, like the CBC.

Fingerstick Test (Capillary Blood Sample)

A finger prick for blood collection involves the use of a tiny sterile pin called a lancet to puncture the skin and collect a small amount of blood. It is common in blood sugar monitoring and other rapid tests. It’s also the type of blood collection method used in home blood testing kits.

Heel Prick Test

The heel prick test is used with neonates and infants to save their tiny veins from venipuncture. The nurse or lab technician pricks the heel with a lancet to collect a few drops of blood for tests like bilirubin to assess liver health.

Central Line or Catheter Blood Sampling

This method is used with patients in a hospital setting who already have a fitted intravenous (IV) line. Central line blood collection is mostly useful for patients in critical care who need frequent blood monitoring, as it avoids multiple needle sticks for blood draws.

Arterial Blood Sampling

This method is used in complex blood testing where arterial blood is required. Such tests include the Arterial Blood Gas (ABG test). It tests blood from arteries for gases like carbon dioxide and oxygen.

Arterial blood sampling is done by a specialist and may hurt more than a venous blood draw. Blood is usually drawn from the radial artery on the wrist.

How to Prepare for a Blood Test

Generally, there is no special blood test preparation required. However, your doctor may give specific instructions on what to do before a blood test in certain cases.

Your doctor may ask you to:

Fast before a blood test.
Temporarily stop certain medications, such as anticoagulants, or reduce their dosage.
Avoid certain foods and drinks, like fatty foods, alcohol, or caffeinated drinks.
Drink plenty of water.

Staying calm and wearing clothing that you can easily roll up to reveal your arm is also a core blood collection preparation requirement. And, what if you are afraid of the needle?

Feeling Nervous? How to Overcome the Fear of Blood Tests

Some people are afraid of the needle or the sight of blood. If you are one of those who get nervous before a blood test, these tips can help you overcome the fear of blood tests.

Inform the phlebotomist about your fear of blood draws to help them take extra care or help you overcome anxiety before a blood test.
Distract yourself by looking away, closing your eyes, scrolling your phone, or talking to the phlebotomist.
Take deep breaths to help you stay calmer and prevent effects like dizziness and fainting.
Think positively by repeating to yourself the purpose of the blood draw.
Bring someone along for support if your fear of blood draws is severe. Talking to someone familiar can help with blood test anxiety management.

Quick Summary

Blood tests are a common method used by healthcare professionals to examine patients’ overall health, diagnose illnesses, and monitor treatment progress.

There are many types of blood tests. The complete blood count is one of the most commonly prescribed ones.

Medical professionals recommend blood tests during your annual health check and in specific situations, such as when you have symptoms of a disease.

If you are the kind that gets anxious about having a blood test, distracting yourself with music or closing your eyes can help you manage the fear of blood collection effectively.

Reading blood test results is usually left to your doctor, who then makes follow-up accordingly.

Sources:

Cleveland Clinic: Blood Tests.
MedlinePlus: What You Need to Know About Blood Testing.
NHS: Blood Tests.
Healthdirect: Guide to blood testing.

Meet your guide

Dr. Emily Carter

Dr. Emily Carter, MD, has over 15 years of clinical and medical education experience. She specializes in venous imaging and puncture technologies and is a Senior Medical Educator at Harvard Medical School. She is known for integrating advanced infrared technology into patient care and has authored numerous research articles.