The liquid in a COVID-19 test kit is a buffer solution designed to maintain a stable chemical environment, particularly a controlled pH level, for the collected biological material. Without this medium, the viral components collected during swabbing would quickly degrade, making the test useless. The specific composition of the liquid differs significantly depending on the type of test, primarily distinguishing between laboratory-based molecular tests and at-home rapid tests.
The Standard Transport Medium
For COVID-19 tests requiring laboratory processing, such as Polymerase Chain Reaction (PCR) testing, the liquid is called Viral Transport Medium (VTM) or Universal Transport Medium (UTM). The medium’s purpose is to keep the collected sample stable and viable during transport to the testing facility. VTM is primarily a buffered saline solution fortified with various components to prevent degradation. It includes protein stabilizers, such as bovine serum albumin, to shield the viral particles and their genetic material. Antimicrobial agents, like antibiotics, are also added to prevent the overgrowth of bacteria or fungi collected from the body, which could otherwise interfere with sensitive laboratory analysis.
How the Transport Medium Stabilizes the Sample
Sample stability relies on the medium maintaining a precise pH balance, usually around the neutral level of 7.3. Biological molecules, including viral RNA, are sensitive to changes in acidity and will quickly break down if the pH shifts. Buffer salts, such as HEPES, are dissolved in the medium to actively resist these fluctuations, ensuring viral components remain intact until analysis.
Preserving the viral RNA is essential because PCR tests detect and amplify this specific genetic material. If the RNA degrades before reaching the lab, the PCR test may fail to detect the virus, potentially causing a false negative result. Protein stabilizers further assist by coating the viral envelope, protecting it from environmental stresses like temperature variations during transport.
The Liquid Used in Rapid Antigen Tests
The liquid in at-home rapid antigen tests is known as an Extraction Buffer or Lysis Buffer. Its function is not to preserve the virus but to chemically process the sample immediately to release specific viral components. This extraction process is necessary because rapid tests detect viral proteins, called antigens, which are initially trapped within the viral structure. The extraction buffer contains detergents designed to quickly break down (or lyse) the viral envelope and any cellular debris collected on the swab. Once the virus is broken open, the antigens are immediately released into the solution. These released proteins then flow onto the test strip, binding to immobilized antibodies that produce the visible test line, enabling the rapid, typically 15-minute, turnaround time.
Safety and Handling Precautions
The chemical composition of the extraction buffer often includes preservatives or inactivating agents that require careful handling. A common preservative is sodium azide, which is present in very small, low-concentration amounts. While the overall toxicity risk is low due to the minute quantities, this chemical is a potent poison in larger doses. Ingestion, particularly by small children, can lead to symptoms like low blood pressure and dizziness, and eye contact can cause irritation. Other chemicals in the extraction buffer, such as guanidinium salts in some formulations, are strong irritants and are not meant for contact with skin or eyes. Users should strictly follow kit instructions, keeping the vials out of reach of children and avoiding contact with the skin or eyes. In the event of a spill or ingestion, washing the affected area thoroughly or contacting a poison control center immediately is the appropriate action.