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The Benefits of Digital and Electrochemical Lateral Flow Assays

Lateral Flow Test Basics

Lateral flow tests (LFTs) are simple diagnostic technologies that allow rapid and low-cost detection of a given analyte at the point of care level. While LFAs have widespread uses including environmental testing they are more commonly associated with healthcare testing, for example the pregnancy or the SARS-COV-2 Antigen test.

LFT technology1 can be summarised as shown in Figure 1. (i) Some LFTs require extraction of the analyte (antigen) (e.g., SARS-CoV-2 Antigen) from the biological sample (e.g., mucus) in a buffer of a known pH (often at a physiological pH of 7.4) and composition (including salts and surfactants for sufficient flow/ clearance) while in other cases (e.g., pregnancy test) this is not required and the biological sample (urine) is instead directly applied to the test (ii). The sample is placed on the sample pad (which may contain buffer salts if a buffer solution is not used) whereupon it is drawn to the absorption pad by capillary action (iii). The solution passes to the conjugate pad where it dissolves antibody-gold nanoparticle (AuNP) conjugates. The solution continues to the test line made up of unlabelled capture antibodies where, if the antigen is present, a sandwich immunocomplex forms with a second antibody coupled with AuNPs ((iv a). AuNPs are widely used and are able to reflect light in the visible region which to the observer appears as a purple, pink or blue line when they accumulate. Conversely, if there is no antigen present, the detection antibody/AuNP cannot form a complex with the capture antibody resulting in no visible line (iv b). Next, the solution passes to the control line, where an agent specific for the detection antibody can capture it even in the absence of the analyte, so that a control line should always be visible regardless of whether the antigen is present or not. The inclusion of the control line is used to ensure that flow occurs properly, with its absence equating to a void test. (v) The solution finally passes to the absorption pad which is used to ensure consistent flow while minimising backflow.

Advantages and Disadvantages of LFTs

LFT technology is well known for affording quick development cycles if the appropriate antibodies are available. This ultimately allowed many companies (e.g. Innova, Abbot Panbio) to produce LFT strips in quick response to the Coronavirus pandemic. Additionally, LFT technology is also not only inexpensive to produce (<£1) but can be mass manufactured in large quantities. This allowed governments around the world to shoulder the costs of LFTs allowing it to be used as part of the daily routine of workers during the Coronavirus pandemic. Storage of LFTs is also trivial due to their long shelf lives without requiring refrigeration allowing easier transport and their use in less developed countries. Finally, LFTs are quick to run, only requiring sample collection before carrying out the test which can take anywhere between 3–30 minutes. ><£1) but can be mass manufactured in large quantities. This allowed governments around the world to shoulder the costs of LFTs allowing it to be used as part of the daily routine of workers during the Coronavirus pandemic. Storage of LFTs is also trivial due to their long shelf lives without requiring refrigeration allowing easier transport and their use in less developed countries. Finally, LFTs are quick to run, only requiring sample collection before carrying out the test which can take anywhere between 3–30 minutes.

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