Overview

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 decisions.

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
• To 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 specific disease.
• To help rule out a disease or condition
• To assess the severity of a disease
• To 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 drug use)

Many conditions can change your lab results. Your doctor will talk with you about any abnormal results as they relate to you.

Results

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.
• A 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 level of 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?

A 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.

A 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 test.

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 blood sugar, pregnancy, 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 results.

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.
• Drinking caffeinated beverages or alcohol.
• Eating meat, especially beef.
• Taking vitamins, especially vitamin C.
• Stress.
• Strenuous exercise.
• Your occupation.
• Pregnancy.

Follow your doctor’s instructions to make sure that your test results are as accurate as possible.