What Are Blood Tests?
Blood tests help doctors check for certain diseases
and conditions. They also help check the function of your organs and show how
well treatments are working.
Specifically, blood tests can help doctors:
- Evaluate how well organs, like the kidneys,
liver, and heart, are working
- Diagnose diseases like cancer, HIV/AIDS,
diabetes,
anemia
(uh-NEE-me-eh), and heart disease
- Learn whether you have risk factors for heart
disease
- Check whether medicines youre taking are
working
Overview
Blood tests are very common. When you have routine
checkups, your doctor often orders blood tests to see how your body is
working.
Many blood tests dont require any special
preparations. For some, you may need to fast (not eat any food) for 8 to 12
hours before the test. Your doctor will let you know whether this is
necessary.
During a blood test, a small amount of blood is
taken from your body. Its usually drawn from a vein in your arm using a
thin needle. A finger prick also may be used. The procedure is usually quick
and easy, although it may cause some short-term discomfort. Most people
dont have serious reactions to having blood drawn.
Lab workers draw the blood and analyze it. They use
either whole blood to count blood cells, or they separate the blood cells from
the fluid that contains them. This fluid is called plasma or serum.
The fluid is used to measure different substances in
the blood. The results can help detect health problems in early stages, when
treatments or lifestyle changes may work best.
However, blood tests alone cant be used to
diagnose or treat many diseases or medical problems. Your doctor may consider
other factors, such as your signs and symptoms, your medical history, and
results from other tests and procedures, to confirm a diagnosis.
Outlook
Blood tests have few risks. Most complications are
minor and go away shortly after the tests are done.
Types of Blood Tests
Some of the most common blood tests that doctors
order are:
- Complete blood count (CBC)
- Blood chemistry tests
- Blood enzyme tests
- Blood tests to assess heart disease risk
Complete Blood Count
The CBC is one of the most common
types of blood test. It's often done as part of a routine checkup.
A CBC measures many different parts of your blood
(as described below). This test can help detect blood diseases and disorders.
These include
anemia,
infection, clotting problems, blood cancers, and immune system disorders.
Red Blood Cells
Red blood cells carry oxygen from your lungs to the
rest of your body. Abnormal red blood cell levels may be a sign of anemia,
dehydration (too little fluid in the body), bleeding, or another disorder.
White Blood Cells
White blood cells are part of your immune system,
which fights infections and disease. Abnormal white blood cell levels may be a
sign of infection, blood cancer, or an immune system disorder.
A CBC measures the overall number of white blood
cells in your blood. A differential count looks at the amounts of different
types of white blood cells in your blood.
Platelets
Platelets (PLATE-lets) are blood cells that help
your blood clot. They stick together to seal cuts or breaks and stop bleeding.
Abnormal platelet levels may be a sign of a bleeding disorder (not enough
clotting) or a thrombotic disorder (too much clotting).
Hemoglobin
Hemoglobin (HEE-muh-glow-bin) is an iron-rich
protein in red blood cells that carries oxygen. Abnormal hemoglobin levels may
be a sign of anemia,
sickle
cell anemia,
thalassemia
(thal-a-SE-me-ah), or other blood disorders.
If you have diabetes, excess glucose in your blood
can attach to hemoglobin and raise the level of hemoglobin A1c.
Hematocrit
Hematocrit (hee-MAT-oh-crit) is a measure of how
much space red blood cells take up in your blood. A high hematocrit level might
mean you're dehydrated. A low hematocrit level might mean you have anemia.
Abnormal hematocrit levels also may be a sign of a blood or bone marrow
disorder.
Mean Corpuscular Volume
Mean corpuscular (kor-PUS-kyu-lar) volume (MCV) is a
measure of the average size of your red blood cells. Abnormal MCV levels may be
a sign of anemia or thalassemia.
Blood Chemistry Tests/Basic Metabolic Panel
The basic metabolic panel (BMP) is a group of tests
that measure different chemicals in the blood. These tests usually are done on
the fluid (plasma) part of blood. The tests can give doctors information about
your muscles, including the heart; bones; and organs, such as the kidneys and
liver.
The BMP includes blood glucose, calcium,
electrolyte, and kidney tests. Some of these tests require you to fast (not eat
any food) before the test, and others don't.
Blood Glucose
Glucose is a type of sugar that the body uses for
energy. Abnormal glucose levels in your blood may be a sign of diabetes.
For some blood glucose tests, you have to fast
before your blood is drawn. Other blood glucose tests are done after a meal or
at any time with no preparation.
Calcium
Calcium is one of the most important minerals in the
body. Abnormal calcium levels in the blood may be a sign of kidney problems,
bone disease, thyroid disease, cancer, malnutrition, or another disorder.
Electrolytes
Electrolytes are minerals that help maintain fluid
levels and acid-base balance in the body. They include sodium, potassium,
bicarbonate, and chloride.
Abnormal electrolyte levels may be a sign of
dehydration, kidney disease, liver disease,
heart
failure,
high
blood pressure, or other disorders.
Kidneys
Kidney tests measure levels of blood urea nitrogen
(BUN) and creatinine. Both of these are waste products that the kidneys filter
out of the body. Abnormal BUN and creatinine levels may be signs of a kidney
disease or disorder.
Blood Enzyme Tests
Enzymes are chemicals that help control different
reactions in your body. There are many blood enzyme tests. This section focuses
on blood enzyme tests used to check for
heart
attack.
These include creatine kinase (CK) and troponin
tests.
Creatine Kinase
When muscle or heart cells are injured, CK (a blood
product) leaks out, and its levels in your blood rise. There are different
types of CK. CK-MB is released when the heart muscle is damaged.
High CK levels can mean that you've had muscle
damage in your body. High levels of CK-MB can mean that you've had a heart
attack. Doctors order CK tests (such as CK-MB) when patients have chest
pain or other
heart
attack signs and symptoms.
Troponin
This is a muscle protein that helps your muscles
contract. Blood levels of troponin rise when you have a heart attack. For this
reason, doctors often order troponin tests along with CK-MB tests when patients
have chest pain or other heart attack signs and symptoms.
Blood Tests To Assess Heart Disease Risk
Abnormal levels of certain chemicals in the blood
may mean that youre at higher risk for heart disease. Your doctor may
want to test the levels of these chemicals to assess your risk and to suggest
ways to reduce it.
Lipoprotein Panel
This test can help show how high your risk is for
coronary
heart disease. A lipoprotein panel looks at substances in your blood that
carry cholesterol.
The test gives information about your:
- Total cholesterol.
- LDL ("bad") cholesterol. This is the main source
of cholesterol buildup and blockages in the arteries.
- HDL ("good") cholesterol. This type of
cholesterol helps decrease blockages in the arteries.
- Triglycerides. These are another form of fat in
your blood.
A lipoprotein panel measures the levels of HDL and
LDL cholesterol and triglycerides in your blood. Abnormal cholesterol and
triglyceride levels may be signs of increased risk for coronary heart
disease.
Most people will need to fast for 9 to 12 hours
before a lipoprotein panel.
High-Sensitivity C-Reactive Protein
This is a fairly new test for heart disease risk. It
looks at blood levels of C-reactive protein (CRP). High CRP blood levels can be
a sign of inflammation.
Doctors use standard CRP tests to check for
inflammation and autoimmune diseases. Your doctor may order an hs-CRP test,
along with other tests, to see whether youre at increased risk for heart
disease.
However, CRP tests arent routinely done,
because its still unclear how useful they are for showing heart disease
risk.
Homocysteine
High levels of this chemical in the blood can mean
that youre at higher risk for heart attack or stroke. This isnt a
routine test for heart disease risk. But some doctors may use it, a long with
other tests, if they think youre at increased risk.
What To Expect With Blood Tests
What To Expect Before Blood Tests
Many blood tests dont require any special
preparation and take only a few minutes.
Other blood tests require fasting (not eating any
food) anywhere from 8 to 12 hours before the test. Your doctor will let you
know whether you need to fast for your blood test(s).
What To Expect During Blood Tests
Blood usually is drawn from a vein in your arm or
other part of your body using a thin needle. It also can be drawn using a
finger prick.
The person who draws your blood might tie a band
around the upper part of your arm or ask you to make a fist. These things can
make the veins in your arm stick out more. This makes it easier to insert the
needle.
The needle that goes into your vein is attached to a
small test tube. The person who draws your blood removes the tube when it's
full, and the tube seals on its own. The needle is then removed from your vein.
If you're getting a few different blood tests, more than one test tube may be
attached to the needle before its withdrawn.
Some people get nervous about blood tests because
theyre afraid of the needle. Others may not want to see blood leaving
their bodies.
If youre nervous or scared, it can help to
look away or talk to someone to distract yourself. You might feel a slight
sting when the needle goes in or comes out.
Drawing blood usually takes less than 3 minutes.
What To Expect After Blood Tests
Once the needle is withdrawn, youll be asked
to apply gentle pressure with a piece of gauze or bandage to the place where
the needle went in. This helps stop bleeding. It also helps prevent swelling
and bruising.
After a minute or two, you can remove the pressure.
You may want to keep a bandage on for a few hours.
Usually, you dont need to do anything else
after a blood test, except wait for the results. They can take anywhere from a
few minutes to a few weeks to come back. Your doctor should get the results.
Its important that you follow up with your doctor to discuss your test
results.
What Are the Risks of Blood Tests?
The main risks with blood tests are discomfort or
bruising at the site where the needle goes in. These complications usually are
minor and go away shortly after the tests are done.
What Do Blood Tests Show?
Blood tests show whether the levels of different
substances in your blood fall within a normal range.
For many blood substances, the normal range is the
range of levels seen in 95 percent of healthy people in a particular group. For
many tests, normal ranges are different depending on your age, gender, race,
and other factors.
Many factors can cause your blood test levels to
fall outside the normal range. Abnormal levels may be a sign of a disorder or
disease. Other factorssuch as diet, menstrual cycle, how much physical
activity you do, how much alcohol you drink, and the medicines you take (both
prescription and over-the-counter)also can cause abnormal levels.
Your doctor should discuss any unusual or abnormal
blood tests results with you. These results may or may not suggest a health
problem.
Many diseases or medical problems cant be
diagnosed with blood tests alone. However, they can help you and your doctor
learn more about your health. Blood tests also can help find potential problems
early, when treatments or lifestyle changes may work best.
Result Ranges for Common Blood Tests
This section presents the result ranges for some of
the most common blood tests.
NOTE: All values in this section are for adults
only. They dont apply to children. Talk to your childs doctor about
values on blood tests for children.
Complete Blood Count
The table below shows some normal ranges for
different components of the complete blood count (CBC). Some of the normal
ranges are different for men and women. Other factors, such as age and race,
also may affect normal ranges.
Your doctor should discuss your results with you. He
or she will advise you further if your results are outside the normal range for
your group.
Test |
Normal Range Results* |
Red blood cell (varies
with altitude) |
Male: 5 to 6 million
cells/mcL Female: 4 to 5 million cells/mcL |
White
blood cell |
4,500 to 10,000
cells/mcL |
Platelets
|
140,000 to 450,000
cells/mcL |
Hemoglobin (varies with
altitude) |
Male: 14 to 17 gm/dL
Female: 12 to 15 gm/dL |
Hematocrit (varies with
altitude) |
Male: 41 to 50%
Female: 36 to 44% |
Mean
corpuscular volume |
80 to 95
femtoliter |
* Cells/mcL = cells per microliter; gm/dL
= grams per deciliter
Blood Glucose
This table shows the ranges for blood glucose levels
after 8 to 12 hours of fasting (not eating). It shows the normal range and also
the abnormal ranges that are a sign of prediabetes or diabetes.
Plasma Glucose Results (mg/dL)*
|
Diagnosis |
99 and below |
Normal |
100 to 125 |
Prediabetes |
126 and above |
Diabetes† |
* mg/dL = milligrams per deciliter.
† The test is repeated on another day to confirm the results.
Lipoprotein Panel
The table below shows ranges for total cholesterol,
LDL ("bad") cholesterol, and HDL ("good") cholesterol levels after 9 to 12
hours of fasting.
High
blood cholesterol is a risk factor for
coronary
heart disease.
Your doctor should discuss your results with you. He
or she will advise you further if your results are outside the desirable range.
Total Cholesterol
Level |
Total Cholesterol
Category |
Less than
200 mg/dL |
Desirable |
200–239 mg/dL |
Borderline high |
240 mg/dL
and above |
High |
LDL Cholesterol Level |
LDL Cholesterol
Category |
Less than
100 mg/dL |
Optimal |
100–129 mg/dL |
Near optimal/above
optimal |
130–159 mg/dL |
Borderline high |
160–189 mg/dL |
High |
190 mg/dL
and above |
Very high |
HDL Cholesterol Level |
HDL Cholesterol
Category |
Less than
40 mg/dL |
A major risk factor
for heart disease |
40–59 mg/dL |
The higher, the
better |
60 mg/dL
and above |
Considered protective
against heart disease |
Key Points
- Blood tests help doctors check for certain
diseases and conditions. They also help check the function of your organs and
show how well treatments are working.
- Blood tests are very common. When you have
routine checkups, your doctor often orders blood tests to see how your body is
working.
- Some of the most common types of blood tests that
doctors order are complete blood count (CBC), blood chemistry tests, blood
enzyme tests, and blood tests to assess heart disease risk.
- A CBC can detect blood diseases and
disorders.
- Blood chemistry tests measure different
chemicals in the blood. These tests give doctors information about nerves,
muscles (including the heart), bones, and organs such as the kidneys and liver.
- Blood enzyme tests measure the amounts of
enzymes in your blood. These tests can help diagnose a
heart
attack.
- Blood tests to assess heart disease risk
measure chemicals in your blood that show whether you're at increased risk for
heart disease.
- Many blood tests don't require any special
preparation and take only a few minutes. Other blood tests require fasting (not
eating any food) anywhere from 8 to 12 hours before the test.
- During a blood test, blood usually is drawn from
a vein in your arm or other part of your body using a thin needle. It also can
be drawn using a finger prick. Drawing blood usually takes less than 3 minutes.
- Once the needle is withdrawn, you'll be asked to
apply gentle pressure with a piece of gauze or bandage to the place where the
needle went in. This helps stop bleeding. After a minute or two, you can remove
the pressure.
- The main risks with blood tests are discomfort or
bruising at the site where the needle goes in. These complications usually are
minor and go away shortly after the tests are done.
- Your doctor should get the results of your blood
test(s) and discuss them with you. Blood tests show whether the levels of
different substances in your blood fall within a normal range. Your doctor
should discuss any unusual or abnormal results with you.
Links to Other Information About Blood Tests
NHLBI Resources
Non-NHLBI Resources
|