What Are the Most Common Blood Tests, and Why Are They Done?

Blood tests are commonly used in healthcare to evaluate overall health and identify possible health issues at an early stage^[1]. Whether you are visiting for a routine check-up or investigating specific symptoms, understanding what these tests measure can help you feel more prepared and informed. Your blood can provide important insights into the functioning of your organs, immune system, and metabolism^[2]. This article explains what blood tests are, how they are performed, and what the most common types can reveal about your health. Note that this is general educational information, not a guide for interpreting your individual results.

What is a blood test, and why are they done?

A blood test is a medical procedure where a small sample of blood is collected from your body and analysed in a laboratory. Blood tests provide crucial insights into the health and functioning of your organs and body systems^[1].

Healthcare providers order blood tests for several important reasons^[1],^[3]:

• Assessing your overall health during routine check-ups
• Checking for specific diseases and conditions such as anaemia and infections
• Assessing the function of your organs (such as liver, kidneys, heart)
• Monitoring the effectiveness of ongoing treatments
• Identifying individuals at increased risk for health conditions

Your blood contains three main types of cells: red blood cells, white blood cells, and platelets. Each component serves a specific function, and measuring their levels can reveal important information about conditions ranging from infections to clotting disorders^[2].

Blood test results should always be interpreted by a qualified healthcare provider who can consider your complete medical history.

How are blood tests done?

The blood collection process, called venipuncture, is straightforward and typically takes about 10 minutes. Here is what you can expect^[4]:

1. A healthcare professional will ask you to sit or lie down comfortably
2. An elastic band (tourniquet) is wrapped around your upper arm to make your veins more visible
3. The collection site, usually the inside of your elbow, is cleaned with an antiseptic
4. A sterile needle is inserted into a vein
5. Blood is collected into one or more small tubes
6. The needle is withdrawn, followed by gentle pressure on the site to control any bleeding.

The volume of blood collected during a routine blood test depends on the number and type of tests required. For example, a purple bottle holds about 3 ml for a full blood count (FBC) and erythrocyte sedimentation rate (ESR), while a yellow bottle holds about 2.5 ml and is usually filled completely for chemical tests. If several tests are needed, up to around 30 ml of blood may be collected^[5].

Sometimes blood may be taken from your wrist, or a finger-prick test may be used for certain quick screenings^[4].

How many types of blood tests are there?

There are dozens of different blood tests available, each designed to measure specific components or substances in a blood sample. The types of blood tests your doctor orders depend on what information they need about your health.

The complete blood count (CBC) is a commonly requested blood test among clinicians. Research suggests that CBC results may help assess risk for various conditions, including cardiovascular disease and metabolic syndrome^[6].

Blood tests can be broadly categorised into^[1],^[2]:

• Tests that count and evaluate blood cells (CBC, blood typing)
• Tests that measure chemical substances (metabolic panels, glucose tests)
• Tests that check organ function (liver panel, kidney function tests)
• Tests that assess disease risk (lipid panels, inflammatory markers)

The most common types of blood tests

Healthcare providers frequently order several standard blood tests to evaluate different aspects of your health. Here are the most commonly requested tests and what they measure.

Complete blood count (CBC)

A complete blood count (CBC) consists of several tests that assess the quantity and size of various blood cells. This test evaluates^[7]:

• Red blood cells: Carry oxygen from the lungs to the rest of the body.
• White blood cells: Protect the body by fighting infections and diseases.
• Platelets: Help the blood clot and prevent excessive bleeding.
• Haemoglobin: An iron-containing protein in red blood cells that transports oxygen throughout the body.

The CBC can help diagnose anaemia, infections, clotting problems, blood cancers, and immune system disorders^[2].

Comprehensive metabolic panel (CMP)

A comprehensive metabolic panel (CMP) is a routine blood test that measures various substances in the blood, providing insights into metabolic function and overall chemical balance in the body. The CMP consists of tests that evaluate^[8]:

Electrolytes are minerals with an electrical charge that help regulate fluid levels in the body ^[8].

Lipid panel (cholesterol test)

A cholesterol test evaluates your potential risk of heart disease by measuring specific types of cholesterol and fats in the blood. It measures^[9]:

• LDL cholesterol: Often known as “bad” cholesterol, LDL can build up in the arteries and cause blockages
• HDL cholesterol: Known as "good" cholesterol, HDL helps remove LDL cholesterol from your bloodstream
• Total cholesterol: The combined measurement of all cholesterol types
• Triglycerides: Fats in the blood that provide the body with energy

Higher HDL levels may help reduce your risk of heart attack or stroke^[9].

Other commonly ordered tests

Beyond these three major panels, doctors frequently order additional tests based on individual patient needs^[1],^[3],^[8]:

• Blood glucose test: Measures blood sugar levels to screen for diabetes
• Thyroid function tests: Evaluate thyroid hormone levels
• Iron studies: Check for iron deficiency or overload
• Vitamin D and B12 levels: Assess nutritional status
• Liver function tests: Evaluate liver health through enzyme levels
• Kidney function tests: Measure creatinine and BUN levels

The specific tests your doctor orders will depend on your symptoms, medical history, and risk factors. Always discuss your test results with your doctor.

Understanding your blood test results

When you receive your blood test results, you will notice numbers alongside reference ranges. Understanding these ranges can help you make sense of your results.

What are reference ranges?

Reference ranges represent the expected values for a particular measurement in a healthy population. They indicate the span within which approximately 95% of healthy individuals' results will fall^[10].

Why results may fall outside normal ranges

Because reference ranges include 95% of healthy people, this means 1 in 20 healthy individuals may have a result outside the reference range^[10]. An abnormal result does not automatically mean you have a health problem.

Several factors can temporarily affect your results^[11]:

• Recent food intake
• Fasting status
• Medications, supplements and herbal remedies
• Time of day the sample is collected
• Body position/posture before or during blood collection
• Dehydration

Why ranges vary between laboratories

Different laboratories use different equipment and methodologies to analyse blood samples^[10]. This means reference ranges may vary slightly between testing facilities. Your results should always be compared against the specific ranges provided by the laboratory that processed your sample.

Your doctor will consider your complete clinical picture, including symptoms, medical history, and other test results, before reaching any conclusions.

When to talk to your doctor about your results

Understanding when to discuss your blood test results with your doctor can help ensure you receive appropriate care.

You may want to speak with your doctor when^[12]:

• A result is outside the reference range
• You are uncertain about the meaning of your test results
• Your results have changed significantly from previous tests
• You have symptoms despite results being within the reference range
• Results are abnormal, positive, or unclear and may require further testing
• You have concerns or questions about how your test results may affect your health

Contact a healthcare professional if you need help interpreting your test results.

Conclusion

Blood tests are valuable diagnostic tools that provide important insights into your overall health. From the commonly ordered CBC that evaluates your blood cells to comprehensive metabolic panels that assess organ function, these tests help doctors detect conditions early and monitor treatment effectiveness.

While understanding what different blood tests measure can help you feel more informed, always remember that interpreting results requires professional expertise. If you receive results that concern you or that you do not understand, schedule a conversation with your doctor. They can explain what your results mean in the context of your individual health and recommend any necessary follow-up care.

Medical Disclaimer

This article is for informational purposes only and does not constitute medical advice. The information provided should not be used for diagnosing or treating health conditions. Always consult a qualified healthcare provider for diagnosis, treatment, and personalised medical advice. Do not disregard professional medical advice or delay seeking it because of information found in this article. If you have a medical emergency, contact your doctor or emergency services immediately.