A diagnosis of a food or waterborne illness may be initially made clinically by documenting a person's signs and symptoms, their severity and duration, what the person has consumed before the symptoms began, and where the person may have traveled.
- Are severe and/or persist beyond a few days
- Appear to be part of a larger outbreak
- Are thought to be due to a parasite
- Are thought to be caused by a different condition with similar symptoms
Testing may be performed on those affected to diagnose a condition and sometimes to monitor the effectiveness of treatment.
Laboratory testing on samples of the food or water that are suspected of causing illness may be performed to help determine the source of contamination, especially with outbreaks. In these cases, testing of both people and food and water may be performed at the state, national, or international public health level to determine which strains of microorganisms are causing illness, to monitor their locations, and to manage and contain outbreaks.
Initial testing for an individual suspected of having a food or waterborne illness often begins with an examination of stool. This may include one or more of the following:
- Stool culture – detects several commonly encountered bacteria
- O&P (Ova and Parasite) – ordered to detect and identify parasites
- Stool antigen tests – individual tests that can detect Shiga toxin-producing E. coli (STEC), Giardia, Cryptosporidium, or Entamoeba histolytica
- Stool white blood cells (WBC)—the presence of WBCs in stool may indicate a more severe infection with bacteria or some parasites
Other tests may be performed in conjunction with or in follow up to stool testing. Some of these include:
- CBC (Complete Blood Count) – sometimes ordered to look for increased white blood cells as a sign of bacterial infection
- Blood culture – sometimes ordered to detect bacterial infections that have spread into the blood (septicemia)
- Antibody testing on a blood sample for specific bacteria or for hepatitis A; occasionally ordered to help confirm a diagnosis
- Special staining tests may be ordered to identify specific parasites in a stool sample.
- Molecular testing (reverse transcriptase polymerase chain reaction, RT-PCR) – sometimes performed to identify norovirus
- Rotavirus antigen testing – sometimes ordered to detect rotavirus
In addition, specific nutrient media for culture and special techniques may be needed to detect and identify some microorganisms. The doctor must recognize potential exposures and request specialized testing.
Public health laboratory testing
Healthcare practitioners and clinical laboratories report suspected cases of foodborne illness to local or state public health agencies and investigations are undertaken to identify the likely source of the illnesses. Stool samples from affected people and/or samples of the microorganism causing the illnesses are often sent to public health laboratories so that special testing can be done. If bacteria are suspected to be the cause, these labs can perform molecular tests that can determine a "DNA fingerprint," also known as a pulsed field gel electrophoresis (PFGE) pattern, of the pathogen that is present. This information is entered into PulseNet, a database used by local and state public health agencies, federal food safety regulatory labs, and the Centers for Disease Control and Prevention (CDC) to track illnesses. It allows for rapid comparison of DNA fingerprints in order to determine if illnesses and groups of illnesses are related. PulseNet also has an international component that allows for tracking of illnesses that may cross borders, an important activity given the increasing globalization of the food supply.