A nucleic acid test, often called a "NAT" test, (or nucleic acid amplification test - "NAAT") is a biochemical

Biochemistry

Biochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...

 technique used to detect a virus

Virus

A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...

 or a bacterium

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

. These tests were developed to shorten the window period

Window period

In medicine, the window period for a test designed to detect a specific disease is the time between first infection and when the test can reliably detect that infection...

, a time between when a patient has been infected and when they show up as positive by antibody tests

ELISA

Enzyme-linked immunosorbent assay , is a popular format of a "wet-lab" type analytic biochemistry assay that uses one sub-type of heterogeneous, solid-phase enzyme immunoassay to detect the presence of a substance in a liquid sample."Wet lab" analytic biochemistry assays involves detection of an...

.

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
  Polymerase chain reaction
  The polymerase chain reaction is a scientific technique in molecular biology to amplify a single or a few copies of a piece of DNA across several orders of magnitude, generating thousands to millions of copies of a particular DNA sequence....
  
  . These tests use a primer to rapidly make copies of the genetic material.
  • A reverse transcriptase
    Reverse transcriptase
    In the fields of molecular biology and biochemistry, a reverse transcriptase, also known as RNA-dependent DNA polymerase, is a DNA polymerase enzyme that transcribes single-stranded RNA into single-stranded DNA. It also helps in the formation of a double helix DNA once the RNA has been reverse...
    
     PCR (RT-PCR) is used for HIV
    HIV
    Human immunodeficiency virus is a lentivirus that causes acquired immunodeficiency syndrome , a condition in humans in which progressive failure of the immune system allows life-threatening opportunistic infections and cancers to thrive...
    
     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.
• Ligase chain reaction
  Ligase chain reaction
  The ligase chain reaction is a method of DNA amplification. While the better-known PCR carries out the amplification by polymerizing nucleotides, LCR instead amplifies the nucleic acid used as the probe. For each of the two DNA strands, two partial probes are ligated to form the actual one; thus,...