Nucleic acid test

A nucleic acid test (NAT) is a technique used to detect a particular nucleic acid sequence and thus usually to detect and identify a particular species or subspecies of organism, often a virus or bacterium that acts as a pathogen in blood, tissue, urine, etc. NATs differ from other tests in that they detect genetic materials (RNA or DNA) rather than antigens or antibodies. Detection of genetic materials allows an early diagnosis of a disease because the detection of antigens and/or antibodies requires time for them to start appearing in the bloodstream.^[1] Since the amount of a certain genetic material is usually very small, many NATs include a step that amplifies the genetic material—that is, makes many copies of it. Such NATs are called nucleic acid amplification tests (NAATs). There are several ways of amplification, including polymerase chain reaction (PCR), strand displacement assay (SDA), or transcription mediated assay (TMA).^[2]

Virtually all nucleic acid amplification methods and detection technologies use the specificity of Watson-Crick base pairing; single-stranded probe or primer molecules capture DNA or RNA target molecules of complementary strands. Therefore, the design of probe strands is highly significant to raise the sensitivity and specificity of the detection. However, the mutants which form the genetic basis for a variety of human diseases are usually slightly different from the normal nucleic acids. Often, they are only different in a single base, e.g., insertions, deletions, and single-nucleotide polymorphisms (SNPs). In this case, imperfect probe-target binding can easily occur, resulting in false-positive outcomes such as mistaking a strain that is commensal for one that is pathogenic. Much research has been dedicated to achieving single-base specificity.