Laboratory tests may identify organisms directly (eg, visually, using a microscope, growing the organism in culture) or indirectly (eg, identifying antibodies to the organism). General types of tests include
Immunologic tests Immunologic Tests for Infectious Disease Immunologic tests use one of the following: Antigen to detect antibodies to a pathogen in the patient's specimen Antibody to detect an antigen of the pathogen in the patient’s specimen Specimen... read more (agglutination tests such as latex agglutination, enzyme immunoassays, Western blot, precipitation tests, and complement fixation tests)
Nucleic acid–based identification methods Nucleic Acid–Based Identification Methods for Infectious Disease Nucleic acid–based (molecular) identification has become commonplace in clinical settings; the resulting rapid identification allows the patient to be placed on specific antimicrobial therapy... read more
Non-nucleic acid–based identification methods Non-Nucleic Acid–Based Identification Methods for Infectious Disease Once an organism has been isolated by culture, it must be identified. Identification is important in determining management (eg, drug treatment, isolation measures). Non-nucleic acid–based identification... read more
Culture is normally the gold standard for identification of organisms, but results may not be available for days or weeks, and not all pathogens can be cultured, making alternative tests useful. When a pathogen is cultured and identified, the laboratory can also assess its susceptibility Susceptibility Testing Susceptibility tests determine a microbe’s vulnerability to antimicrobial drugs by exposing a standardized concentration of organism to specific concentrations of antimicrobial drugs. Susceptibility... read more to antimicrobial drugs. Sometimes molecular methods can be used to detect specific resistance genes.
Some tests (eg, Gram stain, routine aerobic culture) can detect a large variety of pathogens and are commonly done for many suspected infectious illnesses. However, because some pathogens are missed on these tests, clinicians must be aware of the limitations of each test for each suspected pathogen. In such cases, clinicians should request tests specific for the suspected pathogen (eg, special stains or culture media) or advise the laboratory of the suspected organism(s) so that it may select more specific tests. (See table Diagnostic Tests for Common Pathogens Diagnostic Tests for Common Pathogens 
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