If you haven’t read it yet, you may find the VirusTotal FAQ informative. Especially the section:
“BAD IDEA: VirusTotal for antivirus/URL scanner testing”
Excerpt:
“At VirusTotal we are tired of repeating that the service was not designed as a tool to
perform antivirus comparative analyses, but as a tool that checks suspicious samples
with several antivirus solutions and helps antivirus labs by forwarding them the malware
they fail to detect. Those who use VirusTotal to perform antivirus comparative analyses
should know that they are making many implicit errors in their methodology, the most
obvious being: …”
Correction: Sorry … the quote is from the “About” page at VirusTotal not the FAQ.