The issue you are discussing is not so much an issue of precedent but of statutory law. Missouri has a statute regulating post-conviction requests for DNA testing.
The movant bears the burden of producing facts that meet the statutory requirements. And the standard for the court order is very high, meaning that the court does not have to order DNA testing unless a reasonable probability exists that the DNA evidence will disturb the conviction.
There have been a few cases that have addressed the law more in depth, and what the requirements of the court are:
Basically, a court is given wide discretion to determine if the moving party has met the requirements. The court is required to issue findings and conclusions that support its decision. The appellate court is limited to setting aside the court's ruling on the DNA issue if "the motion court's findings and conclusions are clearly erroneous," which is a high standard.
The other problem is that at this point, you are not eligible to have the issue considered again. You filed the request, and it was denied, and then you could have appealed it. At this point, the issue has been fully and finally litigated. It also seems that from your question, you are no longer in custody and have served your sentence
. The DNA testing is only available for individuals who are currently incarcerated (see the law above). So because you have waited too long since your original petition to take the case to the highest court that you can, and because you are not eligible under state law for testing any more, there is nothing you can do about the retest unfortunately.
While DNA testing is not available to you, there are other routes you can take to try to address the conviction. A writ of error coram nobis is a petition that requests that an earlier conviction be set aside because of some fundamental error with the conviction (such as evidence not known at the time, which may include DNA testing). You can also try to obtain a pardon, which is official government forgiveness of the conviction:
Sorry I can't give you better news about the DNA testing issue, but it is too late due to the fact that you already filed the request and too much time has passed to request review of the denial.