infections
Diagnosis of Infectious Disease
Tests That Detect Genetic Material in Microorganisms
Tests that detect genetic material from microorganisms are called Nucleic acid–based tests If a microorganism is difficult to culture or identify by other methods, doctors can do tests to identify pieces of the microorganism’s genetic material. This genetic material consists of nucleic acids: deoxyribonucleic acid (DNA) or ribonucleic acid (RNA). Some of the DNA and RNA in each organism are unique to that organism. Thus, finding some of that DNA or RNA means that specific microorganism is present. The polymerase chain reaction (PCR) is an example of this type of test. The PCR technique is used to produce many copies of a gene from a microorganism, making the microorganism much easier to identify. Each genetic test is specific to only one specific microorganism. That is, a genetic test for hepatitis C virus detects only that virus and not any other. Thus, these tests are done only when a doctor already suspects a particular disease. Most nucleic acid–based tests are designed to identify the presence of a microorganism (called qualitative testing). However, for certain infections, such as HIV and hepatitis C, tests also can measure how much of the microorganism's genetic material is present (called quantitative testing) and thus determine how severe the infection is. Quantitative tests can also be used to monitor how well treatment is working. Nucleic acid–based tests can sometimes be used to check the microorganisms for genes or gene mutations that make the microorganism resistant to a drug. However, these tests are not completely accurate because not all resistance mutations are known. Thus, tests cannot check for all the genes for resistance that may be present. Also, these tests are expensive, not widely available, and available for only a few microorganisms. 