COVID-19 testing identifies an active infection, but accuracy relies heavily on proper preparation. The process involves collecting a sample, usually from the nose or mouth, to detect the SARS-CoV-2 pathogen. Contamination from outside substances can interfere with the laboratory or chemical process used to read the result. Understanding preparation guidelines is necessary to ensure the test accurately reflects whether you are currently infected.
General Rules for Diagnostic Tests
For diagnostic tests that use a nasal or oral swab, or a saliva sample, a period of fasting is required before collection. Most healthcare providers advise against eating or drinking anything for at least 30 minutes before the test is administered. This rule applies to both rapid antigen tests and the more sensitive polymerase chain reaction (PCR) tests. Following this timeframe allows the mouth and throat to clear of any recent food or beverage residue that could mix with the specimen.
The primary exception to this fasting rule is plain water, which is often permitted up until a shorter interval, such as 10 minutes before the test. However, all other liquids are strictly prohibited in the 30-minute window, including coffee, tea, juice, milk, and sugary or flavored sodas. These drinks can leave residue that disrupts the sample quality, potentially leading to an inaccurate result. This short period of restriction is designed to maximize the reliability of the collected respiratory sample.
Why Contamination Affects Sample Quality
The strict fasting requirement for oral and saliva tests exists because consuming substances can interfere with the test’s ability to detect the virus. Food and drinks can introduce foreign materials that either dilute the viral load or chemically inhibit the assay process. When a sample is diluted, the concentration of viral particles drops, making it difficult for the test to pick up a low-level infection, which can lead to a false negative result.
Certain compounds, particularly those found in highly acidic beverages like fruit juice or cola, can alter the pH balance of the sample. In rapid antigen tests, this change in acidity can chemically break down the test strip components, causing a non-specific reaction that may be mistaken for a positive result. Even with PCR tests, various substances in food and cosmetics are known to inhibit the chemical reaction used to amplify the viral genetic material. To avoid this interference, the oral cavity must return to its natural state before the sample is taken.
Non-Food Items That Interfere
Beyond food and drinks, several common non-food items can compromise the integrity of a COVID-19 test sample. Products used to clean or treat the mouth, nose, and throat are problematic because they leave behind residues that interact with the test reagents.
Oral Products
Chewing gum, mints, and cough drops often contain sugars or chemicals that interfere with the test’s function. Similarly, mouthwash and throat lozenges are prohibited because their active ingredients can directly inhibit the viral detection mechanism.
Tobacco Products
Tobacco products, whether smoked or vaped, leave chemical byproducts that contaminate the sample and lead to inaccurate results. Testing sites recommend avoiding all these items for at least 30 minutes before collection to ensure a clean, usable specimen.
Preparation for Antibody Testing
Preparation for a COVID-19 antibody test is distinctly different from a diagnostic test, as the two tests target different biological material. Diagnostic tests look for the active virus in a respiratory sample, but antibody tests look for the presence of specific proteins in the blood. These antibodies are created by the immune system in response to a past infection or vaccination.
Since the sample for an antibody test is drawn from a vein, the presence of food or drink has no bearing on the result. Standard preparation guidelines state that no special fasting or restrictions are needed for antibody tests. However, if the test is performed as part of a larger blood panel, the laboratory might request fasting, so always confirm requirements with the specific testing provider.