Lab tests play one role in your health
care. But while it is an important role, in most cases lab tests don't provide
all the information your doctor needs to make a diagnosis or treatment
Unless the test results are clear—either you are
pregnant or you're not—your doctor will rarely make a decision or diagnosis
based only on the results of a lab test. Instead, he or she will use the
results of your tests along with information about your health, gender, age,
and other factors.
Making sense of your lab test involves more
than just knowing why the test is done. It is also important to understand what
the results mean and what factors can affect results. Sometimes test results
can be affected by when you last ate or exercised, your age, and medicines or
herbal supplements you're taking.
Although lab test results may
not provide all of the information that your doctor needs, the tests help him
or her make a diagnosis. Understanding your results will help you and your
doctor discuss the next step in your diagnosis or treatment.
Why It Is Done
Lab tests are generally done for one
of the following reasons:
To find the cause of symptoms
confirm a diagnosis
To screen for a disease. Screening tests are
often done for people of a specific age or those who have a high risk for a
To help rule out a disease or
To assess the severity of a disease
monitor the progression of a disease, how well an organ is working, or if
treatment is helping
To verify specific events (for example,
DNA testing after a rape, or urine testing to look for
Many conditions can change your lab results. Your doctor
will talk with you about any abnormal results as they relate to you.
Lab test results may be positive, negative,
or inconclusive. Your doctor will discuss what your test results mean for you
and your health.
A positive test result means that the substance or condition
being tested for was found. Positive test results also can mean that the amount
of a substance being tested for is higher or lower than normal.
negative test result means that the substance or condition being tested for was
not found. Negative results can also mean that the substance being tested for
was present in a normal amount.
Inconclusive test results are those
that are not clearly positive or negative. For example, some tests measure the
antibodies to some bacteria or viruses in blood or
other bodily fluid to look for an infection. It is not always clear if the
level of antibodies is high enough to indicate an infection.
What are false-positive and false-negative test results?
false-positive test result is one that
shows a disease or condition is present when it is not present. A
false-positive test result may suggest that a person has the disease or
condition when he or she does not have it. For example, a false-positive
pregnancy test result would appear to detect the substance that confirms
pregnancy, when in reality the woman is not pregnant.
false-negative test result is one that
does not detect what is being tested for even though it is present. A
false-negative test result may suggest that a person does not have a disease or
condition being tested for when he or she does have it. For example, a
false-negative pregnancy test result would be one that does not detect the
substance that confirms pregnancy, when the woman really is pregnant.
Some lab tests can give you specific information. For
example, your doctor may suspect you have
strep throat and order a throat
culture to see if streptococcus bacteria are present.
A positive lab test confirms that you have strep throat and helps your doctor
choose the right treatment for you.
But some tests give only a
clue that must be considered with other information to support a diagnosis,
identify a risk, or help choose a treatment. For example, your doctor uses your cholesterol levels plus other things, such as blood pressure and age, to check your risk of a heart attack.
What do the units mean?
Lab test results usually
contain a number followed by a
unit of measurement, such as 37 milligrams per deciliter (mg/dL). The units
provide a way to report results so that they can be compared. Usually, but not
always, the same test is reported in the same units no matter which lab did the
What is a reference range?
Many lab test results
are expressed as a number that falls within a reference range. A reference
range is determined by testing large groups of healthy people to find what is
normal for that group. For example, a group of 30- to 40-year-old men would be
given a specific test and the results averaged in order to create the reference
range for that group.
Each reference range is different because it
is created from information from a specific group. For example, the following
table shows reference ranges for a sedimentation rate test. This test helps
determine whether inflammation, infection, or an
autoimmune disease may be present.
What if your results are different than the reference range?
It is possible to have a result that is different than the
reference range even though nothing is wrong with you. Sometimes certain
factors can affect your test results, such as pregnancy, a medicine you are
taking, eating right before a test, smoking, or being under stress.
When your lab numbers are lower or higher than the numbers in the
reference range, further testing may be needed. Your doctor may want to repeat
the test or order another test to confirm the results.
Why do values or reference ranges vary from lab to lab?
Labs may use different types of equipment and tests, and sometimes they
set their own reference ranges. Your lab report will contain the reference
ranges your lab uses. Do not compare results from different labs.
Only a handful of tests, such as
blood sugar, have standardized reference ranges that
all labs use. This means that no matter where these tests are done, the results
are compared to the same reference ranges.
Home Test Kits
You can do some types of tests at
home, such as testing for
urinary tract infections, and
HIV infection. Some home tests give you results right
away, such as a pregnancy test. Others provide a way
for you to send a sample to a lab for testing. The lab then reports results
back to you.
The quality and reliability of home tests vary greatly. Ask your doctor or pharmacist to recommend a reliable brand. Follow
the instructions, and check with your doctor if you are concerned about the
results. Your doctor will usually do further testing to confirm your
What Affects the Test
Things that may interfere with
the accuracy of a lab test include:
Not following the instructions for how to
prepare for the test. Some tests, such as a
cholesterol and triglycerides analysis, might require you to
stop eating for at least 12 hours before the test. Other tests, such as a
2-hour postprandial blood sugar, require that you eat
exactly 2 hours before the test.
Some medicines or herbal
treatments. For example, many medicines raise or lower blood sugar levels and
could interfere with blood sugar tests. Birth control pills and some steroids
can raise blood sugar, while some medicines to treat
high blood pressure and
depression can lower it.
caffeinated beverages or alcohol.
Eating meat, especially
Taking vitamins, especially vitamin C.
Follow your doctor's instructions to make sure that your
test results are as accurate as possible.
Fischbach FT, Dunning MB III, eds. (2009).
Manual of Laboratory and Diagnostic Tests, 8th ed.
Philadelphia: Lippincott Williams and Wilkins.
How this information was developed to help you make better health decisions.