Steps to remember for Laboratory diagnosis
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 (agglutination tests such as latex agglutination, enzyme immunoassays, Western blot, precipitation tests, and complement fixation tests)
- Nucleic acid–based identification methods
- Non-nucleic acid–based identification methods
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 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.)
Laboratories use a variety of methodologies to test the countless analytes that are of interest to the medical community. Understanding the method used for a test provides a broader context for understanding your test results. Below are explanations of several common laboratory methods mentioned on this site.
Laboratory methods are based on established scientific principles involving biology, chemistry, and physics, and encompass all aspects of the clinical laboratory from testing the amount of cholesterol in your blood to analyzing your DNA to growing microscopic organisms that may be causing an infection. Such methods are much like the recipes in a cookbook, defining the procedures or processes that are used to test biological samples for particular analytes or substances. The laboratory scientist follows step-by-step procedures until the end product, a test result, is achieved.
Journal of Clinical chemistry and Laboratory Medicine