Lateral Flow Assay Design

Before embarking on the development of a lateral flow assay, the critical requirements of the assay you are going to produce should be outlined. Assays are designed based on these requirements which are referred to as “Design Requirements” or “Design Inputs”. Design Requirements typically describe the function, performance, usability, and safety requirements of the assay in relation to its intended use. Ultimately, these requirements will provide a framework for all future development work. Examples of design requirements include quantitative or qualitative results, limits of detection, dynamic range, and shelf life. A typical assay development pathway follows this figure:

In this module we will be focusing on the first section, “Establish General Design Requirements”, and the role it will play with the “Assay Optimization” step.

What Are Assay Design Requirements?

Assay design requirements are claims that are to be comprehensive, unambiguous, non-conflicting, and measurable. Developing design requirements necessitates a comprehensive understanding of the assay's purpose. For development of a commercial test, it is recommended that a Customer Needs Report is first drafted. This document will detail the functional needs, handling needs, as well as the diagnostic requirements of the test based on feedback from the target market. If the assay will be generating data that has medical significance, the assay is likely subject to review by organizations such as the FDA before it can be sold commercially. Once the assay is developed, the performance of the assay will be measured against these requirements in validation and verification studies. 

General Assay Design Requirements

Before any feasibility tasks begin, the first thing that needs to be done is to develop a comprehensive list of the general requirements for the assay based on the intended use. These requirements are very basic, and will serve to provide guidance during the development process. Requirements are written by listing out specific requirements (e.g. Assay Run Time), followed by the description/claim (e.g. the test will produce a reliable result for the concentration of hCG in < 25 minutes). When nanoComposix helps develop Design Requirements with our customers, we begin with the following:

Design Characteristics & Functionality

• Definition: Requirements that specify what the product is expected to do. This will be a bulk of the requirements.
• Example Requirements: Analysis method, target analyte, test format, assay run time, sensitivity limit, specificity limit, analytical range, precision, interference, limit-of-detection, clinical performance, etc.

Physical Properties

• Definition: Physical properties that affect the design. These can be associated with the test strip themselves, as well as the cartridge the strip will be placed into. These can be particularly important if you have a specific reader or analyzer your test must work on.
• Example Requirements: Cartridge dimensions, cartridge features (i.e. conjugate and sample port), overfill volume, reader requirements, etc. 

Handling Requirements

• Definition: Requirements on the environment in which the test can be used, where the test is to be stored prior to use, and how long the test can be stored.
• Example Requirements: Operating temperature/humidity (i.e. humidity does terrible things to protein stability), storage temperature, storage duration, equilibration time, etc.

Sample Requirements

• Definition: All things relating to the sample collection, preparation, transport, and storage.
• Example Requirements: Sample type, sample storage temperature, sample treatment (i.e. centrifuged).

There are many more requirement categories not listed above which should also be considered (e.g. Packaging, Shipping, Labeling, Regulatory, and Safety). It is never too early to make sure all requirements are considered to avoid future project delays.

Example Design Specifications

Assay Optimization & Feasibility

Once design requirements have been defined, this will allow for a more characterized development plan with measurable milestones. The next step is to begin optimizing the assay to meet these requirements. The sequence of modules that follow will help you through this early development phase. During the optimization process, you may find that new requirements need to be added while others will need to be adjusted or removed all together. This is very typical of the process, and important to developing the right requirements moving forward. The list of design requirements should be a living document that continuously evolves throughout the process of optimization. Optimization will eventually lead you to a point where you can effectively demonstrate that you have developed an assay that can meet all the requirements needed for a successful product.