Diagnostic vs Screening Tests

Diagnostic Test

A diagnostic test identifies individuals who have a disease or condition. The most common method used to diagnose a disease requires the physician to obtain tissue, blood, or fluid for analysis to determine if the condition is present or absent. For this reason, genetic amniocentesis or chorionic villus sampling have been used to obtain fluid or tissue for analysis of fetal chromosomes. Unfortunately, the methods used to obtain these tissues have potential complications associated with them; the most serious resulting in unexpected loss of the pregnancy. This complication rate varies. It may be as high as 1 in 100, or as low as 1 in 300. This rate depends upon the experience of the physician performing the procedure.

Screening Test

A screening test is designed to identify patients at increased risk for a certain disease. An example of this type of testing is the PAP smear, performed for women to identify cancer of the cervix. Another example is mammography to identify breast cancer. Screening tests do not identify all patients who may have the disease, but are effective enough that they can be applied to an entire population of patients to identify those who are at high-risk for a particular disease. When a screening test is positive, or abnormal, then a diagnostic test is required to identify pathology. In the example of the PAP smear, tissue is obtained from the cervix for analysis to determine if cancer is present. Similarly, a screening test that is POSITIVE for detecting Down syndrome would require amniocentesis or chorionic villus sampling to obtain fetal cells for analysis to determine if there is an abnormal number of chromosomes. JUST BECAUSE A SCREENING TEST IS POSITIVE DOES NOT MEAN THAT THE PATIENT OR FETUS HAS THE DISEASE.

To understand this concept, examine the pie chart below. In this example, 10% of screened patients test positive for Down syndrome. This group will contain 80% of the Down syndrome fetuses. In the 90% of women who were screen negative, only 20% of the Down syndrome fetuses will be present. For this reason, a screening test does not identify 100% of Down syndrome fetuses. If the screened group represented women over 35, then only 10% would undergo amniocentesis to find 80% of the Down syndrome fetuses. However, if all women underwent amniocentesis, (diagnostic test) then all of the Down syndrome fetuses would be identified. If all women underwent genetic amniocentesis, more normal fetuses would be lost from complications from the procedure. However, if only 10% underwent amniocentesis (women identified as high-risk from the positive screen group) then 80% of Down syndrome fetuses would be identified and the loss rate from amniocentesis of normal fetuses would be decreased by 90%. The reasons for this is because the number of patients undergoing amniocentesis would be decreased by 90%.