Question

How can you change a sandwich ELISA into a quantitative lateral flow design in order to obtain a test which can be carried out faster?

Answer 1

Immunochromatography or lateral flow tests involve the migration of an antigen, or antigen–antibody complexes, through a support, for example, nitrocellulose film, filter paper, or agarose. In a typical format, a labeled antibody is allowed to react with the unknown antigen, the antigen–antibody complex migrates by capillary action through the solid support, and a second antibody embedded in the solid support allows detection of the complexes if antigen is present. Positive and negative controls are included to ensure that individual tests are valid. Immunochromatography tests are available for measuring viral antigens such as HIV p24, dengue NS1, influenza A and B, RSV, etc. and are of particular value for rapid point-of-care testing where rapid results are needed and access to equipment is limited.

In many point-of-care and home-use tests the sample is transported by chromatography, examples are Clearblue and Persona test kits. In immunochromatography tests, the sample moves along an absorbent matrix such as a porous membrane. The capture antibody is fixed in a band and immobilizes analyte and bound labeled antibody (attached to colored latex particles), as the sample moves past. The unbound material is carried away from the capture antibody band by the chromatographic effect. This type of system is known as lateral flow immunoassay.

An interesting test from a design perspective is Persona™, which measures luteinizing hormone and estriol-3-glucuronide in urine, and applies algorithms to estimate the time of ovulation from multiple data points from both analytes. This alone makes it very advanced compared to the typical single-analyte/single-time application of immunoassay. Unlike many lateral flow tests, the measurement is quantitative, rather than qualitative. It uses a miniaturized, low-power spectrophotometer in a pocket-size unit. To initiate the device, the user presses a button at the time of a menstrual period. Here, the user is prompted to run a test on appropriate days by showing a yellow light. A green light indicates that the user is unlikely to conceive on those days, and a red light indicates that contraception is advised. Over a period of up to six cycles, the device “learns” about the owner’s menstrual cycle and becomes more accurate, reducing the number of days on which a test is required. All the data from the six cycles are available for investigation purposes via an RS-232 interface hidden under a panel on the back of the unit.

Compared with ELISAs, lateral flow assays offer advantages in terms of detection time and required equipment: it takes only ~5–20 minutes to obtain assay results and no supporting instrumentation is needed. Although lateral flow assays mainly provide qualitative results, their simple design and operation compared with quantitative ELISAs make them the most user-friendly for the public, allowing rapid adoption in rural countries.