The underlying genetic cause for your patient’s clinical features can be due to many different types of genetic variations. The genetic variations that FirstStepDx PLUS looks for include deletions (missing genetic material) and duplications (extra genetic material) on the chromosomes. The genetic variations that NextStepDx PLUS looks for are much smaller and can't be detected by FirstStepDxPLUS.
One way to look at the difference between how the two tests work is to compare them to a book. FirstStepDx PLUS looks for missing or duplicated chapters in a book. NextStepDx PLUS looks for misspellings within a single word. Neither test can do the other’s job. If FirstStepDx PLUS does not find an underlying genetic cause for your patient's clinical features, performing a NextStepDx PLUS test is a way to look at the "book" of your patient's genetic information in a totally different way.