About all

Blood tests at doctors: What They Show, Why They’re Done, More

Blood Test | Johns Hopkins Medicine

A blood test is a lab analysis of things that may be found in your blood. You may have blood tests to keep track of how well you are managing a condition such as diabetes or high cholesterol. You may also have them for routine checkups or when you are ill.

Blood tests are very common. They are ordered by healthcare providers to:

  • Find out how well organs such as your kidneys, liver, heart, or thyroid are working

  • Help diagnose diseases such as cancer, diabetes, coronary heart disease, and HIV/AIDS

  • Find out if your medicine is working to make you better

  • Diagnose bleeding or clotting disorders

  • Find out if your immune system is having problems fighting infections

  • Diagnose anemia, such as iron-deficiency anemia, pernicious anemia, aplastic anemia, or hemolytic anemia

  • Find variations in hemoglobin such as hemoglobin S, C, or E, which are common in people of African, Mediterranean, or Southeast Asian background

  • Monitor chronic health conditions and diseases

  • Find health problems in their early stages

You have the right to know why a blood test has been ordered. Ask your healthcare provider if you are not sure why he or she wants you to have the test.

Types of blood tests

These are common blood tests:

  • Complete blood count, also called a CBC

  • Blood chemistry tests

  • Blood enzyme tests

  • Blood tests for heart disease risk

Blood tests can give your healthcare provider a lot of information. He or she can see if certain elements in your blood are in a normal range. But in many cases, blood tests are only part of the information your healthcare provider needs to make a diagnosis of a health condition. You might need to have some other types of tests as well.

Preparing for a blood test

For most kinds of blood tests, you don’t need to prepare. These tests are to see what your blood is like under normal conditions. 

For some blood tests, you will have to not eat (fast) for a certain amount of time before the blood test. This usually means no eating or drinking anything after midnight before the test. These tests are often scheduled for early in the morning.

Your healthcare provider will let you know if you need to fast before a blood test.

The procedure

In order to test your blood, a technician called a phlebotomist will use a needle to take a sample of blood. Tell the technician if the sight of needles makes you nervous. He or she can help you feel more at ease. You can also look away during the procedure, and bring a family member or friend to help distract you.

In most cases, the sample is taken from a vein in your arm. You will be seated or lying down. You may be asked to make a fist. The technician will tie a rubber band around your arm. Once he or she sees a vein, the technician will clean the area and then insert the needle. You might feel a small prick. Once the technician has drawn enough blood, he or she will take the needle out and put an adhesive bandage over the site. You may be asked to press firmly on the site to stop any bleeding. 

After the procedure

Your blood sample will be sent to a lab. Trained technicians then look for the information the healthcare provider has ordered. This may take a day or up to a week or more. Check back with your healthcare provider’s office to find out about the results. 

What happens during a blood test and what can the results show?

Blood tests can be a simple way of getting more information about what’s going on inside your body. But what’s involved? Senior Cardiac Nurse Philippa Hobson explains.

Why would I have a blood test?

Blood tests are usually done to check how your body copes with illness, injury, inflammation, infection and some types of medication. Blood needs a very precise balance in order for your body to work well. If blood test results are abnormal, it gives the doctors a good indication of how to treat you or prevent problems occurring in the future.

Want to get fit and healthy?

Sign up to our fortnightly Heart Matters newsletter to receive healthy recipes, new activity ideas, and expert tips for managing your health. Joining is free and takes two minutes.

I’d like to sign up

Where would I have a blood test?

This may be done in hospital or by your GP or practice nurse. If you’re having an NHS health check, your blood test is usually a quick finger prick test that looks at the blood’s glucose and cholesterol levels. If the results are higher than recommended, you may be asked to go to your hospital for a full blood test.

How will my blood be taken?

A rubber strap called a tourniquet is wrapped tightly around your upper arm – this helps to bring the vein that runs inside your elbow to the surface, so that it is easier to find.

The skin around the vein is cleaned with a sterile alcohol wipe. A small needle is inserted into the vein and a small blood bottle is then attached.

The blood will flow into the bottle and when the bottle (or bottles) is full the needle is removed. If your veins are fragile or difficult to find, sometimes a different type of needle called a ‘butterfly’ (because it has plastic ‘wings’ on either side) is inserted into a small vein in the back of your hand.

After the needle is removed, a piece of gauze will be put over the puncture site and you will be asked to hold it there for a few minutes. Bleeding from a vein usually stops very quickly, but may take a few minutes if you take blood-thinning medication.

What happens to my blood after the test?

Usually, the blood bottles contain small amounts of a chemical to stop your blood clotting in the tube, so it can be measured accurately in the lab. Each bottle is labelled with your name, date of birth and hospital number, when the blood was taken, and has a different coloured top according to the type of test. Then they are taken to be analysed.

What will my blood be tested for?

Blood tests can be used for many different things, including to check cholesterol and blood glucose levels. These help monitor your risk of heart and circulatory diseases and diabetes, or how your condition is being managed.

Tests for different chemicals and proteins can indicate how your liver or kidneys are working. A test for troponin can help to diagnose a heart attack, and a test for brain natriuretic peptide (BNP) can help diagnose heart failure.

If you take warfarin, your INR level (a measure of how quickly your blood will clot) will be tested regularly to make sure that you are prescribed the correct dose.

Do I need to do anything to prepare for a blood test?

Occasionally you may need to fast (not eat) before your blood test, but your doctor will let you know if this is required. Unless you have been told not to, have a drink of water before your test. If you’re dehydrated, it can be more difficult to find your vein.

How long does it take to get blood test results?

This partly depends on the urgency of the test. Most tests done at your GP surgery are ‘routine’, meaning there is no urgency, so it may take a few days to get the results. If the people reporting on the blood results see anything they are worried about, they will contact your doctor or nurse and the surgery will get in touch.

Will I need other tests too?

Sometimes the results of blood tests mean that the doctor may want to do other tests, such as a scan on your heart or kidneys, or an electrocardiogram (ECG).

I am scared of having a blood test. What can I do?

Many people feel like this and it is nothing to be ashamed of. Tell the person who is doing your blood test – this will help them to look after you better.

You could also ask someone to come with you and distract you during the test. Some people also find breathing for relaxation helpful. If the problem is that you faint at the sight of blood, this is caused by a fall in blood pressure: you can learn techniques to stop this from happening.

Some clinics can use a cream that numbs the area where the needle goes in. You should let them know in advance if this would help, as the cream may need to be applied at home in order to work properly. Unfortunately, there is no ‘needle-free’ way of obtaining blood.

  • Find out what to expect from giving blood.
  • Learn more about other common tests for heart conditions, including SPECT scans and echocardiogram.

How to decipher a blood test: a diagnostician explains

  • Health

To make a correct diagnosis, you need to consult a specialist, but the basic information will help you understand if you need additional examinations.

September 6, 20215

Getty Images

Theater begins with a hanger, and the examination of almost any patient begins with a complete blood count. It is he who will help the doctor to navigate in the further examination. For example, to suspect the development of anemia in the early stages, to notice inflammatory manifestations – viral or bacterial. It will help to suspect the development of allergies or helminth infestation. And even for the initial assessment of hemostasis – the number of platelets – this analysis is necessary.

How to read a blood test


Red blood cells resembling concave discs. In the forms of the results of a general blood test, they are referred to as RBC-red blood cells. They are our “transport horses”; contain hemoglobin – Hb , which is capable of easily binding oxygen and transporting it to all organs and tissues.

Norm – the average hemoglobin level is 120-140 g/l for women, and 130-160 g/l for men.

Reduced level. When, for one reason or another, anemia develops, that is, iron deficiency is formed in the body (iron depot is depleted), in the general blood test we observe a decrease in the level of erythrocytes and hemoglobin.

Elevated levels of red blood cells and hemoglobin may be a sign of lack of fluid in the body. In this case, it is necessary to establish a drinking regimen, using a sufficient amount of liquid. Such a process can be observed while taking diuretics, prolonged diarrhea, vomiting, high fever and other conditions when we lose fluid.

An increase in hemoglobin and red blood cells may not always be associated with a disease. This is possible in people who are in places with low oxygen levels, for example, highlands. These may include climbers, skiers, frequent flyers and aircraft crew members.

Elevated levels of hemoglobin and erythrocytes (polycythemia) may indicate an increase in blood viscosity and a risk of thrombosis . Such a long-term condition is especially dangerous in patients who are carriers of mutations in the genes of hemostasis – the blood coagulation system. Also, polycythemia can be observed in other diseases, such as diabetes mellitus (especially against the background of polyuria), chronic heart failure, lack of vitamin B and folic acid, and even with lung pathology.

Polycythemia is not a reason to be afraid and make all kinds of diagnoses. You need to consult with your doctor promptly.

One of the widespread diseases characterized by a decrease in the level of hemoglobin and erythrocytes is iron deficiency anemia . Its development is preceded by a latent iron deficiency. At this stage, the level of hemoglobin in the general blood test still remains within the normal range, but the iron depot is already slowly exhausting its resources. Patients are concerned about weakness, brittle nails, hair loss and dryness. Therefore, if you have similar symptoms, and in the results you see a normal level of hemoglobin, you need to contact a specialist and undergo an examination.

See also


WBC white blood cells. It is leukocytes that are one of the foremost fighters of our immunity against viruses, bacteria and other pathogens. As part of the leukocyte formula, basophils, neutrophils, lymphocytes, monocytes and eosinophils are isolated. All of them differ in appearance and functions.

Normally average level of leukocytes is 4-9×10 9 .

Level Up white blood cells is called leukocytosis. It is observed against the background of inflammatory and infectious diseases, but not only. To put forward an assumption about the nature of the pathology, it is important to understand how the level of other indicators of the leukocyte formula changes.

Getty Images

If you don’t have an empty stomach for a complete blood count, it’s possible to get leukocytosis as a result of the test. It’s called postprandial. This is a short-term increase in the level of leukocytes, not associated with pathology.

With bacterial infection , not only the level of leukocytes, but also neutrophils, increases. But the level of lymphocytes is reduced. In the case of virus infection , on the contrary, leukocytes can be not only normal, but even decrease. But the level of lymphocytes and monocytes increases. By the way, for differential diagnosis, it is useful to determine the level of C-reactive protein, which increases with bacterial infections.

Normal CBC results may contain few or even no cells such as eosinophils. Their increased content – eosinophilia , a sign that further testing is needed. An increase in the number of eosinophils can be observed against the background of allergic or parasitic diseases.

Read also


PLT platelets, these are platelets, the average number of which is in the norm 150-400×10 9 . They have the ability to stick together with each other, participating together with other factors of the blood coagulation system in the formation of a blood clot, which protects against bleeding when a blood vessel is damaged. But in the case of pathology, platelets are involved in the formation of blood clots.

An increase in the level of is called thrombocytosis. However, this does not always indicate pathology, and to assess the state of hemostasis, determining the level of platelets is not enough. For clarification, it is necessary to examine the coagulogram.

Physiological thrombocytosis can be observed against the background of lack of fluid, excessive physical activity, after surgical interventions, severe injuries, while taking medications , as well as in patients with oncological diseases . And, of course, in order to understand the true causes and choose a further course of action, it is not enough to independently interpret the results of the analyzes. A doctor’s appointment is required!

Read also


An indicator that is one of the first to respond to a decrease in the level of iron in the body. It is the main form of the depot of this element. Its concentration in the blood is proportional to our total supply of iron. Therefore, if the study shows a decrease in ferritin is the developmental marker of the iron deficiency state .

In turn, an increase in the level of can indicate not only excess accumulation of iron, but also the presence of inflammatory diseases , since ferritin belongs to acute inflammatory proteins. And if you evaluate the indicator against the background of an infectious or inflammatory process, the result may be interpreted incorrectly. In such cases, the most informative will be the determination of soluble transferrin receptors . The same study is recommended when it is necessary to differentiate between iron deficiency anemia and anemia observed in chronic diseases.

The project was implemented with a grant from St. Petersburg.

Text author:Anna Gerasimenko


Home / Directions / Tests, injections / CBC

What is CBC?

Whichever doctor you visit for the first appointment, in most cases, for the initial diagnosis, you will first of all be prescribed take a complete blood count (CBC). A complete blood count is the main and most common clinical study. However, it must be understood that the UAC allows the doctor to draw general conclusions about the state of the blood, and primary conclusions about the patient’s health status. But for the final diagnosis of a complete blood count is often not enough and additional tests or diagnostic procedures may be required.

General analysis allows you to evaluate the main blood parameters, such as:

  • Red blood cell count;
  • Leukocyte count;
  • Hemoglobin level;
  • Hematocrit;
  • ESR (erythrocyte sedimentation rate).

When is a general blood test prescribed?

A complete blood count (CBC) is taken at almost every visit to health care. This analysis can be prescribed according to indications, with patient complaints about health, before hospitalization, during routine examinations in children, pregnant women, and the elderly.

How is a general blood test taken?

Complete blood count can be taken in two ways: from a finger and from a vein. Children are most often prescribed a sample sampling from a finger to minimize injuries. Very young children (up to a year old) take venous blood, but if there are problems with this, biological material can be taken from:
– Forearms;
– Back of the hand;
– Calf area;
– Heads.

A blood test from a vein gives a slightly more extended result, however, a finger sample can also be indicated for an adult if there are problems with the veins. In any case, the doctor prescribes the method of taking a general blood test based on the patient’s existing data.

How to prepare for a general blood test?

Complete blood count is given in the morning, strictly on an empty stomach (it is not recommended to eat 8-10 hours before the test). During the day, doctors do not recommend eating fatty, smoked, heavy foods, alcohol. One hour before the test, you should stop smoking.

A separate issue concerns taking medications. Most often, the doctor will advise you to stop taking any medications 48 hours before the test. However, there are situations where this is not possible due to the health of the patient. In this case, it is necessary to warn your doctor about this situation in advance.

What should I do if there are abnormalities in the blood test?

Complete blood count examines more than 10 blood parameters in total. Today, on the Internet you can find a lot of resources that offer tables for decoding tests, including a general blood test. However, it must be understood that such information can be used for general information purposes only. Because, , only the attending physician will be able to correctly interpret the results of a general blood test, focusing on the patient’s condition and history.

The cost of a general blood test and other tests

Please note that prices may vary in different AlfaMed centers.