Flow-through test

Flow-through tests are a type of diagnostic assay that allows users to test for the presence of a biomarker, usually a specific antibody, in a sample such as blood. Detection of a specific biomarker aids in the diagnosis of and allows therapeutic action to be initiated quickly. The field of applications for this technology is wide, whereas generally all applications applying lateral flow tests can be replaced by flow-through assays. Commercial applications mainly target testing for infectious disease, both by viruses (e.g. HIV, Hepatitis C, Hepatitis B ) or bacteria (e.g. Syphilis or Helicobacter pylori ).

Applications & History

Flow-through tests are one of two commonly used types of rapid tests, the second being lateral flow tests. Both flow-through and lateral flow are typically performed closer to the patient, and can be completed by users with minimal training. These tests are also sometimes called Point of care (POC) tests. In contrast to other established clinical diagnostic methods, such as enzyme-linked immunosorbent assay (ELISA) or polymerase chain reaction (PCR), Point of Care tests can be performed independent of laboratory equipment and deliver instant results. Of the two types of POC tests, flow-through is the more mature technology.

In the mid to late 1990’s, a significant number of manufactures switched to newly developed lateral flow tests. However that switch did cause problems as the technology was owned by a company who took an aggressive stance to protect its IP, and have done so successfully. Other companies who did not make the switch further developed flow-through. Some of those companies such as Bio-Rad continued to use original version of the technology that dated back to the late 1980s. Other companies have evolved the technology to varying degrees.

The evolved flow-through tests have benefits that rival or exceed lateral flow technology: they can be transported and stored at room temperature for prolonged periods time and allow rapid diagnostic of multiple diseases on-site with results within minutes. This makes them the preferred method in e.g. large screening campaigns but also in situations of accidental blood contact or during labour when immediate treatment of a child exposed to HIV during birth by a HIV- positive mother can significantly reduce mother-to-child transmission.

Principle of flow-through tests

Flow-through assays are by principle binding-assays. In practice they are mostly applied to detect the interaction of an antibody,from e.g. the test-subjects blood-sample, with immobilizedAntigens resulting in the formation of an antigen-antibody complex. However, other types of capture-assays are technically feasible; this also includes small molecule capture-assays or antigen tests.

“The test-principle involves a flow of fluid containing the analyte through a porous membrane and into an absorbent pad. A second layer, or submembrane, inhibits the immediate backflow of fluids, which can obscure results. The analyte is captured on the surface of the membrane by analyte capture molecules and then visualized by the addition of analyte detection molecules. These tests can be used to detect both antibodies and antigens. To perform the test, a sample is applied to the membrane and allowed to wick through by capillary action.”. The principle is comparable to lateral flow diagnostic tests, however offers significant technical advantages. Flow-through tests are the assay of choice for point-of-care applications where besides accuracy, speed and testing for several analytes at once are beneficial.

Advantages of flow-through tests

Advantages of flow-through tests:

• Detection of multiple pathogens in one test – combinations of up to three pathogens e.g. HIV with Hepatitis C and B are commercially available.
• Instant results within minutes - up to 5x faster result compared to e.g. lateral flow diagnostic assays.
• Speed and independence of laboratory equipment decrease costs and e.g. enable easy mass-testing.
• Sensitivity of up to 100% was reported for HIV flow-through tests.

To test for combinations of often co-occurring infections such as HIV and Hepatitis offers significant benefits; especially in large-scale screening campaigns or other field applications. In a recent evaluation, the US Army has chosen flow-through rapid tests as the best solution to test for transfusion transmitted such as HIV and hepatitis viruses in blood, in adverse settings where they do not have access to standard diagnostics. The main reasons for this choice have been the multiplexing capability, reliability and the near-instant assay-time; all essential factors for a (field) point-of-care test. For the military it also allows medics to carry less materials, thus saving weight and space in their backpacks. The speed allows decisions to be made more quickly and action to be taken.

Today flow-through tests are mainly applied in screening campaigns where their speed and multiple results allows the product to have greater impacts.. However the advantages in assay-time and multiplexing make it likely that also primary healthcare providers will switch to flow through assays in increasing numbers over the next years. Multiple results can be already obtained while the physician is still counseling the patient.