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What’s the difference between screening and diagnostic testing in pregnancy?

Pregnancy can be a time of great anticipation, excitement, and sometimes, anxiety. Navigating through the many different tests that are available can be confusing and overwhelming. Understanding what information a test can provide may help when deciding which tests to choose, and this is especially important for genetic screening and diagnostic tests.

What is the difference between a “screening” and a “diagnostic” test?

Screening tests are typically done on individuals without symptoms. They are meant to evaluate if an individual is at an increased risk for a disease or condition, or in the case of a pregnancy, if the baby is at an increased risk. They do not give a definite “yes” or “no”. Some examples of screening tests are routine mammograms for breast cancer or blood glucose tests for diabetes. Ideally, a good screening test is positive for a high percentage of individuals who truly have the condition in question, and negative in those individuals who do not. This is referred to as the test sensitivity and specificity.

Screening may lead to the need for a diagnostic test. Diagnostic tests are for individuals with an increased risk because these tests can provide a “yes” or “no” answer. For example, a breast biopsy is a diagnostic test because it can determine if cancer is present. Likewise, a chromosome analysis is a diagnostic test that can determine if an individual has Down syndrome. Diagnostic tests are used by healthcare providers to decide the best options for care for a person with a certain disease or condition.

What is “cell-free DNA screening” offered during pregnancy?

Cell-free DNA screening is a blood test performed on a pregnant woman that looks for small pieces of DNA (cell-free DNA) from both the mother and her baby’s placenta. Labs can use cell-free DNA to determine if a baby may have too many or too few chromosomes (which are made of DNA). For example, if more DNA from chromosome 21 is found than expected, the test result will screen positive for Down syndrome because Down syndrome is caused by an extra copy of chromosome 21. Historically, cell-free DNA screening was only recommended for women who were considered to be high-risk for having a baby with Down syndrome or another chromosomal condition. This included women 35 years old or older, those found to have certain findings on ultrasound, or those with a family history of a chromosome disorder. However, because of the information this screening test can provide, some healthcare providers now give women the option of having a cell-free DNA test performed even if they are younger than 35 years or not considered to be at high-risk.

What are the options after an “increased risk” prenatal screening result?

A good screening test will evaluate one’s risk but cannot diagnose a genetic condition. That is why a positive result is followed by a recommendation for diagnostic testing. This is true for Quest’s prenatal cell-free DNA screening, QNatal® Advanced. When prenatal screening identifies an increased risk of a baby having a genetic condition, the woman and her healthcare provider will discuss the results and decide if further invasive testing is needed. These are procedures that can look at some of the baby’s cells, so a chromosome analysis can be performed and an accurate diagnosis of what the baby may or may not have can be made.

Understanding the difference between “screening tests” and “diagnostic tests” can help to clarify the purpose, benefits, and limitations of the many tests recommended during pregnancy. A screening test tells a provider and a pregnant woman more testing may be needed, and a diagnostic test gives more definite information. Each test helps a woman decide how to proceed with the pregnancy. More information about Quest’s prenatal cell-free DNA screening, QNatal® Advanced, can be found here