A medical laboratory test is any examination of material derived from the human body for the purpose of providing information for the diagnosis, prevention, or treatment of any disease or impairment or for the assessment of the health of human beings. The State of California regulates two categories of laboratories where medical testing is a primary activity: clinical laboratories including hospital laboratories, and public health laboratories. Although these laboratories perform many of the same testing procedures, there is a major difference in their primary function. Clinical laboratories assist clinicians with individual patients; public health laboratories support the health officer whose patient is the community. The clinical laboratory supports primary patient care; the public health laboratory supports programs to prevent and control communicable disease and environmental pollution, and plays a key role in epidemiologic investigations of disease outbreaks. In addition, the public health laboratory serves as the local infectious disease reference laboratory for clinical laboratories in the same jurisdiction.

There are other significant distinctions between clinical and public health laboratories. As of October 12, 1995, amendments and additions to the California Health and Safety Code require that local health departments of a city or county have available the services of a public health laboratory; local health departments are not mandated to provide clinical laboratory services. Laboratory directors of clinical laboratories may be pathologists but public health lab directors must be certified public health microbiologists. Personnel reporting test results in a clinical laboratory must have a clinical technologist license; personnel reporting test results in a public health laboratory must be certified as Public Health Microbiologists. Clinical technologists cannot perform testing in a Public Health Laboratory unless they are, also, a Public Health Microbiologist. Therefore, a private (or public) clinical laboratory cannot function or substitute as a public health laboratory unless it meets all the criteria stated above.

A generalization of the workload of the Los Angeles County Public Health Laboratory is all testing necessary to support all disease control and environmental health activities within Public Health Programs and Services as well as infectious disease reference testing for all public and private clinical laboratories within Los Angeles County. The Public Health Laboratory supports epidemiologic investigations and programs to prevent and control infectious disease of humans and animals as well as pollution of air, water, and food. Organizationally, PHL is divided into the following sections: Molecular Biology (including Restriction Fragment Length Polymorphism analysis with and without Pulsed Field Gel Electrophoresis technology), General Bacteriology (includes food microbiology and botulism testing), TB and Mycology, Parasitology, Virology (includes opening and autopsy of animal heads for rabies testing), Serology (both human and animal), Environmental Microbiology, Environmental Chemistry, and Support Services.

All staff members testing and reporting laboratory results hold certificates from the state of California in public health microbiology. The remainder of the staff are laboratory assistants and support personnel.