What is a laboratory test?

A laboratory (lab) test checks a sample of your blood, urine (pee), or other body fluid or tissue to learn about your health. The sample is sent to a lab where health care professionals test it to see if it contains certain substances and, if so, how much.

Some lab tests can clearly show whether you do or don't have a specific health condition. For example, a pregnancy test can show whether or not a person is pregnant.

Other lab tests provide more general information about your health and possible problems. Test results that aren't normal give your health care provider clues about the type of health problems you may have. The results help your provider decide whether you need more tests and which ones will be most useful for diagnosing or ruling out certain conditions.

Lab tests are an important part of your health care. But they don't provide a complete picture of your health. Even though you may get your test results in your electronic health record (EHR), it's still important to talk with your provider to find out what your results really mean for you.

To get a full picture of your health, your provider will use your test results along with information from your physical exam, health history, family health history, and sometimes imaging tests, such as x-rays. In most cases, combining that information leads to a more accurate diagnosis than the results from any one lab test.

Why do I need a lab test?

Lab tests are used in different ways, including to help:

• Diagnose or rule out a specific disease or condition. Tests to diagnose health problems are done when you have signs or symptoms of a certain disease or condition. Sometimes more than one test is needed to confirm a diagnosis.

  A strep A test is an example of a diagnostic test. If you're sick with a bad sore throat and other symptoms, this test can show you whether you're infected with the bacteria that causes strep throat.

• Screen for a disease or health condition. Screening tests check for health problems when you don't have any signs or symptoms. They help find problems early, when they may be easier to treat. Screening tests are often part of a routine checkup. Different types of screening tests have different uses. They can:
  • Tell you if you have a high risk for getting a specific disease. This alerts you to take steps to lower your risk. For example, a cholesterol test measures the amount of cholesterol in a sample of your blood. The results can help you understand your risk for heart disease.

  • Suggest whether you need more testing to check for a condition or disease. These screening tests look for signs that you may have a health problem so that you can have more testing to find out for sure. For example, one type of screening test for colon cancer looks for blood in your stool (poop). If blood is found, it could be a sign of cancer. So, your provider may order a colonoscopy to find out for sure whether you have cancer.

• Monitor a disease and/or treatment. If you've already been diagnosed with a disease, you may need lab tests to see if your condition is getting better, worse, or staying the same. Tests can also show if your treatment is working.

  A blood glucose test is an example of a monitoring test. It's used to monitor diabetes and diabetes treatment. It may also be used to diagnose the disease.

• Plan your treatment. Certain tests can help show which treatments are likely to be most effective for a specific disease.

  Tumor marker tests are an example of testing to find out which treatments are likely to work to fight certain types of cancer.

What is a reference range?

Lab test results are reported in different ways. Some results are a number. But how do you know what that number means?

To answer that question, your lab report tells you whether your test result is in a normal range called a reference range or "normal values."

A reference range is a set of numbers that are the high and low ends of the range of results that's considered to be normal. The ranges are based on the test results from large groups of healthy people. A test may have different reference ranges for different groups of people. For example, there may be separate ranges for children and adults.

Reference ranges are a general guide to "normal." If your test result is higher or lower than the range that applies to you, it may be a sign of a health problem, but not always. It's common for healthy people to sometimes have results outside the reference range. And people with health problems can have results in the normal range, too.

If your results are outside of the reference range, your provider will look at other information about your health to understand what may have affected your results. You may need more testing if your result is higher or lower than the reference range, or if you have a normal result even though you have symptoms.

When looking at your lab test results, it's helpful to know that:

• Labs use different reference ranges to describe normal results. That's because they often use different testing methods. This means that:
  • To find out how your test results compare with the normal range, you need to check the reference range listed on your lab report. You can't compare your results to references ranges that you might find elsewhere. This is why our Medical Test information cannot provide normal reference ranges for most tests.
  • You can't compare test results from different labs.
  • If you're looking for trends in your test results over time, it's important to try to use the same lab for testing.
• Tests results are measured with different units of measurement. Just as you might measure ingredients for a recipe in teaspoons, cups, or ounces, different tests use different units of measurement. Examples of measurements you may see include:
  • Mcg/dL = micrograms per deciliter
  • Micromole/L = micromoles per liter
  • Pg/mL = picograms per milliliter

What do negative, positive, and inconclusive results mean?

Some test results tell you whether a certain substance, germ, type of cell, or gene was or wasn't found in your test sample. On your test report, you may see these terms:

• Negative or normal. This means "No, the test didn't find what it was looking for." So, you're unlikely to have the health problem you were tested for. But you may need more tests.
• Positive or abnormal. This means, "Yes, the test found what it was looking for." The germ, substance, or gene being tested was in your sample. So, you may have a disease or infection. In certain cases, you may need more tests to confirm a diagnosis.
• Inconclusive or uncertain. This means "not sure." Your test wasn't clearly positive or negative. There are many reasons why this may happen. If you get an uncertain result, you will probably be tested again.

Tests for the COVID-19 virus are an example of tests that tell you whether or not a specific germ was found in your sample.

What are false positive and false negative results?

Tests results are usually accurate, but no test is perfect.

• A false positive result means your test shows you have a disease or condition, but you don't really have it.
• A false negative result means your test shows you don't have a disease or condition, but you really do.

These incorrect results don't happen often, but they are more likely with certain of types tests, or if testing wasn't done right. If your provider thinks your test result may be inaccurate, you may need to have another test.

What can affect my results?

The accuracy of certain test results may be affected by what you eat, medicines you take, and even how your feel when you provide your test sample. Common things that affect tests include:

• Eating and drinking certain foods and drinks
• Taking certain medicines or supplements
• Exercising hard before your test
• Having a menstrual period at the time of your tested

Your provider will let you know if you need to prepare for your test. Follow your provider's instructions carefully. That will help make sure your test results are as accurate as possible. Before your test, let your provider know about all the prescription and over-the-counter medicines you take as well as vitamins and other supplements.

What if I do a home test?

At-home test kits are available for many types of lab tests. The kits provide everything you need to collect a sample of body fluid or cells to send to a lab. At-home tests should never replace testing that your provider orders. Ask your provider or pharmacist to recommend a test you can trust. And talk with your provider about your results, even if they're normal.