A nucleic acid test, often called a "NAT test," is a biochemical technique used to detect a virus or a bacterium. These tests were developed to shorten the window period, a time when a patient has been infected and when they show up as positive by antibody tests.

The term includes any test that directly detects the genetic material of the infecting organism or virus. There are multiple methods that fall in this group, including:

• Methods based on the Polymerase chain reaction. These tests use a primer to rapidly make copies of the genetic material.
• A reverse transcriptase PCR (RT-PCR) is used for HIV and other RNA viruses.
• Transcription mediated amplification uses a slightly different molecular method than PCR but has the same basic principle.
• Branched DNA (quantiplex bDNA) tests use a molecule that links to the specific genetic material.