For more specific information about what tense to use when writing your research paper please refer to section 7.4.3 of the AMA Manual.
In general, here are the rules to follow:
Past Tense for writing your Methods section or Results section.
We measured each patient’s blood pressure.
Group 1 had a seropositivity rate of 50%.
Present Tense is often used throughout the rest of your paper especially when referring to recently published material or expressing a statement of fact or general truth. Example:
He discovered enzymes—RNA polymerases—that directly copy [not copied] the messages encoded in DNA.
The present tense is often used to refer to recently published work, indicating that it is still valid. Example:
Kilgallen’s assay results demonstrate the highest recorded sensitivity and specificity to date.